Microfabrication of PPy microactuators and other conjugated polymer

J. Micromech. Microeng. 9 (1999) 1–18. Printed in the UK

PII: S0960-1317(99)97590-3

Microfabrication of PPy microactuators and other conjugated polymer devices Elisabeth Smela† Condensed Matter Physics and Chemistry Department, Risø National Laboratory, FYS-124, PO Box 49, DK-4000 Roskilde, Denmark Received 16 September 1998, in final form 21 October 1998 Abstract. Conjugated polymers have a number of interesting properties that can be exploited

in microfabricated sensors and actuators. For example, polypyrrole is a conjugated polymer that can change volume to deliver significant stresses and strains. These materials can be patterned using conventional microfabrication techniques. The procedures for doing this are described in this paper, focusing on the microfabrication of polypyrrole microactuators. In addition, other methods for the deposition and patterning of conjugated polymers are reviewed. A special technique for releasing actuators, differential adhesion, is also detailed.

1. Introduction

New materials can be used to realize innovative microfabricated devices. One class of materials that is receiving growing attention for this purpose is conducting, or conjugated, polymers. Conducting polymers are distinguished by conjugation (alternating single and double bonds between carbon atoms) along the polymer backbone; the conjugation results in a band gap, which makes the polymers semiconducting. This gives them a number of interesting properties and makes them potentially quite beneficial to use together with silicon [1, 2]. For example, they emit light in the visible region of the electromagnetic spectrum, and several microfabricated light emitting diodes (LEDs) have already been demonstrated [3–7]. A microfabricated light sensing array has also been made [8]. Electronic devices like thin film transistors [9] and Schottky diodes [10–15] are another important class of applications, as well as chemical sensors [2, 8, 16–21]. (An ‘electronic nose’ based on polypyrrole [22] is now a commercial product [23].) Conjugated polymers are also used in batteries [24] and are being developed for solar energy conversion, which might some day allow on-chip energy storage or generation. Based on their mechanical properties, their use as low-friction coatings for micromachined components has been proposed [2]. Of particular interest to us is their use for microactuators [25–28]. In addition to a wide range of applications, conjugated polymers have some advantages over materials traditionally used in microfabrication. For instance, they can easily be coated onto arbitrary surfaces, such as large-area or flexible substrates. Furthermore, their molecular structure can be altered to tailor their properties (such as the color of emitted light) or they can be blended with other polymers † E-mail address: [email protected].

0960-1317/99/010001+18$19.50

© 1999 IOP Publishing Ltd

for improved characteristics (such as mechanical strength). A number of good introductory articles about conducting polymers and their applications are available in the literature [24, 29–35]. For more advanced reviews, see [36–38] and the books [39–41]. Conjugated polymers are also called conducting polymers because in the ‘doped’ state they conduct electricity. The word doping is used because the process is analogous to the doping of silicon; however, the doping level in conjugated polymers is much higher, typically 25%. P-doping can be accomplished either electrochemically, electrically (such as in field effect transistors [42]), chemically (by the introduction of electron-withdrawing molecules like iodine) or by ion implantation [15]. Some of these polymers (including polyacetylene) can also be n-doped by electron donors [43]. The charge on the polymer in the doped state is delocalized along the backbone: because of the π electron system (conjugation) the charge is easily shared among these carbon atoms [36]. The charge carriers are called solitons, polarons and bipolarons and are associated with ‘defects’ in the order of the single and double bonds [44]. The conductivity of conjugated polymers spans 12–15 orders of magnitude between the doped and undoped states, with the conductivity of polyacetylene going up to 105 S cm−1 . Unlike in silicon, in conjugated polymers the doping level is reversible and can be controlled. In some of these materials, there is a volume change associated with the change in doping level, and this can be exploited to make actuators [25, 26, 45–48]. Several other properties of these polymers also change with doping level, including color (electrochromism), so the doped material can be considered to be a distinctly different compound. The electrochemical doping/undoping process can be represented by:
P+ A− + C+ + e− ↔ P0 (AC) (1) 1

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Figure 1. Chemical structures of pyrrole and polypyrrole (PPy) in neutral and oxidized forms.


P + A − + C + + e − ↔ P 0 + A − + C+

(2)

where P+ represents the doped (oxidized) state of the polymer and P0 the undoped (reduced, neutral) state. P+ (A− ) indicates that the anion A− is incorporated in the polymer as a dopant. Electrochemistry is further discussed below. In the case of an immobile anion A− , the cation C+ enters the film to maintain charge neutrality upon undoping (reduction), resulting in a volume expansion (equation (1)). If the anion is mobile, it is expelled and a volume contraction results upon reduction (equation (2)). Both processes may occur simultaneously. For additional information, see [38, 49–55]. There are many different kinds of conjugated polymer, but we will focus on polypyrrole (PPy), which is one of the most stable [37]. Its structure is shown in figure 1. In this paper, the procedures for fabricating PPy bilayer microactuators will be given in detail. In these devices, the conducting polymer film is the active, volume-changing element, or ‘artificial muscle’. Bilayers of gold and polypyrrole undergo a large bending and can act as hinges to rotate rigid plates. Self-folding boxes have been made [28], and the hinges can also lift bulk micromachined silicon plates out of the plane of the wafer [56]. Although these devices have been described previously [28] the fabrication sequence, the various fabrication options that are available and the pitfalls have not been elaborated. In addition, the literature on other microfabrication techniques that have been applied to conjugated polymers is herein reviewed. We begin by describing methods for depositing conjugated polymer films, starting with a survey of various techniques. Electrochemistry is reviewed next, followed by a general description of the electrochemical deposition of polypyrrole, and finally the specific procedures we use to deposit polypyrrole films for microactuators. Methods for patterning conjugated polymer films are reviewed next. A discussion of some of the processing considerations follows. The fabrication sequence for gold/PPy bilayers is then illustrated together with specific recipes, including a description of the differential adhesion method and the methods we use for patterning PPy. In the subsequent section, the procedures 2

Figure 2. Some of the ways that conjugated polymer films can be deposited. (a) Directly from solution. (b) From a precursor or monomer. (c) Chemical vapor deposition. (d) Electrochemical deposition.

for actuating these bilayers are given. Finally, the fabrication and actuation of polyaniline bilayers is described. 2. Deposition of conjugated polymer films

2.1. Review There has been a large effort to develop conjugated polymers for applications in microelectronics manufacturing as charge dissipation layers, as conducting photoresists, and for corrosion protection [57]. Therefore, a good deal is already known about how to handle these materials in the microfabrication environment. Conjugated polymer films can be applied to substrates using a number of methods; some of these are schematically illustrated in figure 2. If the polymer can be dissolved or dispersed in a solvent, then it can be cast or spin-coated directly. This is the case for polyaniline (PANI) [57, 58] and many polythiophenes, and it is the simplest method. Articles describing spin-coatable forms of PPy have also recently appeared [59–62], but we have been unable to reproduce this work in our laboratory. The polymer can likewise be applied directly to a substrate if it is melt-processable. If the polymer has a soluble precursor or precursor monomer, then that can be applied to the substrate and cured, such as by heating, to form the conjugated polymer (figure 2(b)). An example is the pyrrole-2-carboxylic acid precursor monomer, which can be converted to

Microfabrication with conjugated polymers

PPy by heating [63]. (The monomer was dissolved in tetrahydrofuran (THF) with FeCl3 , and a polyethylene (PE) film impregnated with the solution and dried. The film was placed in an oven at 100 ◦ C to induce polymerization in the PE matrix. The mechanical properties of the resulting film were determined by the PE, but the combination was conducting.) This process could probably be adapted for use with silicon substrates. Alternatively, it is possible to UV-crosslink a precursor with a photo-initiator or photoacid generator. These methods have been used for poly(pphenylenevinylene) (PPV) [4, 12]. There are several ways to deposit films from an unmodified monomer solution. Chemical vapor deposition has been used for PPV [64] and for PPy [65]. Alternatively, a solution of the monomer, an oxidizer and an inhibitor can be applied to the substrate, and the mixture heated to drive off the inhibitor, allowing the polymerization to take place on the wafer. We have used this method to deposit poly(3,4ethylenedioxythiophene) (PEDOT), a member of the polythiophene family commonly used in LEDs. (A solution of the EDOT monomer, the oxidant salt iron(III)tris-ptoluenesulfonate and the polymerization inhibitor imidazole was prepared in 1-butanol [66]. To form films, the solution was spin-coated, baked at 110 ◦ C for 5 min, which caused polymerization, and rinsed to remove excess salt.) If the polymer is intractable (insoluble, cannot be melted) and none of the above methods can be applied, then electropolymerization (akin to electroplating of metal layers [67]) is an easy alternative. This is the usual method for depositing PPy. Electropolymerization has one disadvantage, however: it requires an electrode. Direct coating, as is done for instance with polyaniline, is preferable because it gives one more freedom in device design since film deposition can be done on any surface. 2.2. Electrochemical deposition Electrochemistry is a method of carrying out chemical reactions with the aid of electrodes that donate or remove electrons [68–70]. This is represented by: O + ne− ↔ R where O and R stand for oxidized and reduced species and ne− is the number of electrons. The applied potential affects reaction rates and their directions; over a 1 V range, the reaction rate may change by a factor of 108 [71]. The potentials are applied using a potentiostat, which has three electrodes: a working electrode (WE), a reference electrode (RE) and a counter-electrode (CE) (also called the auxiliary electrode). The working electrode is the substrate onto which the film is deposited. The reference electrode, typically Ag/AgCl or calomel (mercury/mercury chloride), provides an equilibrium reaction that determines the reference level in the electrochemical cell. No current flows through the reference electrode. Potentials are always specified versus a certain reference, and these equilibrium potentials are given in electrochemical series tables with respect to the standard hydrogen electrode so that they can be inter-converted. The current for the reaction is provided by the counter-electrode. The counter-electrode

can be, for example, a piece of platinum foil, a gold-covered wafer or an indium/tin-oxide- (ITO-) covered glass plate. Electrochemistry is done in an electrolyte (salt solution), which is usually a liquid. To deposit a film electrochemically, a monomer-containing electrolyte is used. To complete the electrochemical circuit, if electrons are removed at one electrode, an equal number must be injected at the other. If a positive potential is applied to the WE, then species are oxidized at that electrode. Simultaneously, at the CE species are reduced. The species that are reduced may be the oxidized products from the WE, so that the reactions are simply reversed, or they may be different. It is important to remember that there is a compensating reaction on the CE, even though it is not usually discussed. The area of the CE should be large enough to accommodate these reactions at the other half-cell, ideally at least 10 times larger then the WE [39]. Pt gauze makes a good counter-electrode. A sign that the electrode area is too small is the noticeable generation of bubbles. In aqueous solutions, for example, if high anodic potentials are applied to the WE, or large currents drawn, the CE reduces water to evolve hydrogen and the WE may evolve oxygen. The deposition process can be controlled either by potential or by current. In thefirst case the voltage can be constant over time (potentiostatic) or it can be time varying (potentiodynamic). In the second case, a constant current is supplied (galvanostatic), and the voltage varies to accommodate it. For potentiostatic deposition, the current is measured against time in a chronoamperogram; for potentiodynamic deposition the current is recorded against the potential in a cyclic voltammogram. 2.3. Electrochemical deposition of PPy A typical chronoamperogram for the electrodeposition of PPy(DBS) in an aqueous solution of 0.1 M pyrrole and 0.1 M sodium dodecylbenzenesulfonate (NaDBS) is shown in figure 3. When the potential is first applied, there is a large current spike due to double layer (capacitive) charging. This transient current dies out quickly. After the minimum, the current rises when polymerization begins. This is associated with nucleation and growth. If the current instead simply decays, then the applied potential is too low or there is another problem. After the nucleation period, the current reaches a plateau; PPy is doped and conducting when deposited, so the deposition rate remains more or less constant, and the film grows linearly with time [72]. If the electrodeposited material were insulating, the current would fall. For a discussion of the PPy polymerization reaction, see the excellent review [39] and also [37, 38, 73, 74]. Briefly, the removal of an electron from pyrrole creates a reactive radical cation. This reacts with a second radical to form a dimer, and this grows by the addition of more such units. The pH near the WE decreases over time because protons are generated during this process. The pyrrole units link primarily at the α positions, but PPy does not form perfect straight chains as shown in figure 1; instead it is crosslinked at some of the β positions. The oxidation potentials of the dimer, trimer and oligomers are lower than that of the monomer, so it is easier to add units to a growing chain. When the chain reaches a certain length, which depends on the electrolyte, it becomes 3

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Figure 3. Chronoamperogram showing a typical potentiostatic deposition of PPy in aqueous 0.1 M pyrrole, 0.1 M NaDBS. The current spike at t = 0 is due to double-layer capacitive charging, which decays quickly. The polymerization current increases after film nucleation on the electrode surface until it reaches a steady state value.

insoluble and starts to precipitate onto the working electrode. Once the electrode is covered with oligomers, growth on the surface is easier and the current increases. The current also increases because at this stage there are more oligomers, which have a lower oxidation potential. Oligomers continue to form in solution and also to add to the chains on the surface. This reaches a steady state, and the current plateaus. PPy is stable in its oxidized form because of its low oxidation potential. During polymerization, therefore, for each monomer approximately 2.25 electrons are withdrawn: one from each of the two α positions of the monomer to form the polymer, and an additional one for every ∼ four monomer units to oxidize (dope) the polymer. The same number of anions, ∼ one per four monomer units, is incorporated into the film to maintain charge neutrality [74]. PPy is very sensitive to the growth conditions, including the anion, the solvent, the temperature and the applied potentials [74]. These affect the conductivity, morphology, mechanical strength, doping behavior, degree of crosslinking, film uniformity and other properties [39]. In fact, for practical purposes, PPy films grown in different electrolytes (solvent + salt, where the salt is also called the supporting electrolyte) can be considered to be different materials. Dopant anions such as DBS− that are aromatic, amphiphilic and/or bulky have been found to give films with the highest conductivity [75, 76] and the best mechanical or device properties [50, 51, 53, 77, 78]. DBS has also been found to be beneficial in polyaniline [79]. Such dopants are only soluble in water. For some applications, an organic solvent such as propylene carbonate (PC) or acetonitrile (ACN) is preferred. In these cases, supporting electrolytes with small anions like ClO− 4 are commonly used. At higher potentials, the film grows faster. However, it also becomes more non-uniform, growing thicker at the edges and corners of electrodes than in the center. This is shown in figure 4. Also, if the potential is too high, the polymer will be overoxidized and will degrade [80–82]. A lower potential gives a more uniform thickness. Deposition takes longer, but the polymer also tends to be more perfect because the proportion of linkages at the α positions is higher and other, damaging side reactions are reduced. This may or may not be beneficial for a particular application. 4

Figure 4. Appearance of PPy(DBS) films grown to a fixed charge (0.1 C) at various potentials. The films become increasingly non-uniform in thickness as the potential is increased from left to right: 0.50 V, 0.55 V, 0.60 V, 0.70 V, 0.80 V against Ag/AgCl.

Because the deposition rate for thin films is constant, film thickness depends linearly on time (figure 3). Once the thickness against consumed charge per unit area is known, it is possible to produce films with repeatable thickness by controlling the consumed charge, which is recorded by some potentiostats or can be found by integrating the current. Note, however, that PPy growth on the back side of the substrate will make charge measurements inaccurate. Alternatively, if the surface area is unknown but the thickness against time at a given potential has been established for a given electrochemical cell geometry, one can grow for a fixed time. Lastly, the PPy film is not opaque when it is thin (< 1 µm); thus, on a gold electrode there are interference colors. If neither area nor growth rate is known, one can use these interference colors, which change in a predictable way (as for SiO2 on Si) while the film is deposited, as a gage of film thickness. For PPy(DBS), we have established the relationship between charge, color and thickness (determined using a Dektak profilometer) given in figure 5. When wet, some of the colors change; for instance, the violet appears to be salmon. As the films dry, they shrink somewhat, causing the colors to shift slightly. The 150 mC offset is due to capacitive and other currents that are consumed before polymerization begins. A rule of thumb is that it takes 200 mC cm−2 to obtain 1 µm. An example of potentiodynamic deposition in the same electrolyte is shown in figure 6. A cyclic voltammogram is a helpful diagnostic tool because it reveals what happens at various potentials [83, 84]. In the first scan, initially little current flows as the potential is increased. When the potential reaches the value necessary to drive the polymerization reaction (near 0.5 V against Ag/AgCl), anodic current (+I ) starts to flow. The deposition rate depends exponentially on applied potential after this point†. After the scan † The current is related exponentially to the so-called overpotential, η. This is expressed by the Tafel and Butler–Volmer equations [68].

Microfabrication with conjugated polymers

Figure 5. Interference colors of PPy(DBS) on gold and their approximate correspondence to charge and thickness. These samples were deposited at 0.52 V (against Ag/AgCl) and were of uniform thickness. Above 50 mC cm−2 , the film cycles between green and pink, becoming gradually darker until at ∼1 µm the film is black.

percent (typically 0.5 to 3%) [51–53, 85]. In subsequent excursions to the upper potential limit, the anodic current due to polymerization is approximately the same in every scan; the same thickness is deposited in each cycle. However, the current associated with redox grows with every scan, because this corresponds to the total amount of material deposited. The ratio of the polymerization charge (Qp ) to the redox charge (Qox or Qred ) is a measure of the efficiency of the polymerization reaction. (The polymerization current may include contributions from the generation of soluble oligomers that do not precipitate onto the electrode, the formation of electro-inactive PPy and other parallel reactions in the cell.) The redox charge reflects the amount of good, electroactive PPy in the film. Figure 6. Cyclic voltammogram showing the potentiodynamic deposition of PPy in aqueous 0.1 M pyrrole, 0.1 M NaDBS. The scan rate was 100 mV s−1 . Every fourth scan is shown, the first as a dashed line.

direction is reversed, the current falls again. However, the polymerization current on the first return scan is typically a bit higher than it was on the outgoing scan, making what has been called a nucleation loop. It is, as explained above, easier to deposit PPy on an existing PPy layer. When the potential is scanned to a sufficiently reducing potential, −0.6 V, a small cathodic current (−I ) peak is observed. This arises from undoping the polymer, converting it to its neutral state. There is a corresponding movement of ions (and possibly solvent): either anions exit the polymer or cations enter, or both, depending on the supporting electrolyte [49–52]. When scanned in the positive direction again, the polymer is again oxidized (doped) near −0.35 V. The charge consumed by these processes can be found by integrating the peak currents, and the oxidation and reduction charges (Qox and Qred , respectively) will, of course, be almost the same but opposite in sign. These reduction–oxidation (redox) reactions are accompanied by a volume change of a few

2.4. Deposition of PPy(DBS) for microactuators For our microactuators, we usually use 0.1 M pyrrole (Aldrich) in 0.1 M aqueous NaDBS (Aldrich) for the deposition solution. The pyrrole is distilled when we receive it, then stored at −30 ◦ C or colder; as long as it is frozen solid, pyrrole does not degrade, but in the refrigerator at +4 ◦ C it gradually oxidizes and turns yellow. The sodium salt of DBS is a powder and can be stored on the shelf. We normally use deionized or Millipore water. Pre- mixed 0.1 M NaDBS solutions can be stored indefinitely; we prepare 1 liter amounts. The pyrrole-containing polymerization solutions are made just prior to use by adding pyrrole to a quantity of premixed NaDBS solution, and they are quickly mixed by shaking. (Pyrrole-containing solutions can be stored for a couple of days in the refrigerator, but not longer.) Because DBS− is amphiphilic (soapy), this produces some foam which disappears after several minutes. The solution is allowed to reach room temperature before use. We do not stir the solution during electrodeposition, since this inhibits polymer deposition [39]. It may be helpful to say a few words about the DBS− anion, shown in figure 7(a). It is large, approximately as 5

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(a)

(b) Figure 7. (a) The idealized (linear) and actual structures of the commercially available dopant dodecylbenzenesulfonate (DBS− ). These

molecules are amphiphilic, aromatic and bulky. During redox they remain in the PPy. (b) HPLC of commercially available NaDBS (Aldrich). The fraction of molecules having total chain lengths of 10, 11, 12 or 13 carbons is ∼ 15%, 30%, 30%, and 25%, respectively. Within each weight group, the peaks represent different attachment points, with R1 = CH3 in the rightmost position and R1 increasing with each peak to the left.

large as the approximately four pyrrole units it compensates. DBS− is a surfactant, or amphiphilic molecule, which means it has hydrophobic and hydrophilic ends. It is used commercially as a detergent. In solution it forms micelles at high enough concentration (above its critical micelle concentration, or CMC, which is 1.2–1.5 mM in pure water [86]). The Na+ stays outside the micelles in the water phase, but the pyrrole, which is hydrophobic, is probably found inside the micelles. Surfactants strongly influence PPy growth and morphology [78, 87]. Unless specifically stated otherwise, the commercially available DBS (from Aldrich, for example) is not, in fact, a linear 12-carbon alkyl chain attached to a benzene ring on one side, as the name would imply and as is shown in the top of figure 7(a). Rather, it consists of a mixture of structural isomers with chains ranging from, in the sample we measured, 10 to 13 carbons in total length, each approximately 25% of the mixture. The rings are not attached at the end of the chains, but somewhere in the center. The light yellow color of the powder indicates that there are also some double bonds on the alkyl chains. A high pressure liquid chromatogram (HPLC) is shown in figure 7(b). The rightmost peaks of each group represent R1 = CH3 , the next R1 = CH2 CH3 , etc, with some peaks overlapping on the left sides. We have also found that the behavior of different batches of DBS varies slightly. The effect of the various isomers on film and actuation properties is unknown, but we 6

plan to study it in the near future. In our laboratory, we typically use potentiostatic rather than galvanostatic deposition in order to prevent possible overoxidation; we have obtained electro-inactive PPy(DBS) films with potentiodynamic deposition if the film is cycled to reducing potentials (as in figure 6). (With PPy(ClO4 ), potentiodynamic deposition has been reported to give superior results [88]). A constant potential of ∼0.52 V against Ag/AgCl gives fairly good uniformity over a 4 in wafer. This results in a current density of ∼0.1 mA cm−2 and a deposition rate of ∼5–6 Å s−1 . To grow a one micrometer thick film takes ∼30 minutes, and the consumed charge is ∼0.2 C cm−2 . These numbers are approximate and depend on electrode geometry and placement. For example, if the sample has small [21], widely spaced electrode areas, the growth rate can be much faster (double or more) because there is more pyrrole available at those points; it is not consumed by the adjacent areas. We employ a gold-coated wafer as a CE oriented parallel to the sample in order to ensure the most uniform electric fields; others use a large area Pt gauze. We use a separation between the CE and WE wafers of 1–2 cm and place the reference electrode (available from Bioanalytical Systems) between them to one side. The electrochemistry is done in an inert glass or plastic cell with grooves in the side walls to hold the wafers in position. The electrochemical cell is shown in figure 8. The wafers are connected to the potentiostat with

Microfabrication with conjugated polymers

Figure 8. Cell for electrochemical deposition: (a) overhead view; (b) side view.

alligator clamps. It is important to keep those out of the liquid, or they will corrode; this contaminates the solution, generates substantial current and interferes with polymerization. (Gold or Pt-plated clamps, or other inert materials, can be immersed in the solution.) Exposed Si and Cr are not a problem; PPy may grow on those areas, which must be considered when calculating the necessary charge, but it peels off easily. At the counter-electrode, species are reduced. It is best to separate the CE a good distance from the WE in order to prevent interference from the reduced species in the polymerization reaction [39]. A large separation between the WE and CE results in more uniform growth. This brings us to a discussion of the materials that can perform as a WE. Inert metals such as gold or platinum are normally used because they do not react in the potential window of interest. Other metals form an oxide or undergo other electrochemical reactions at potentials less than or equal to that necessary to deposit PPy, and those reactions can interfere with the deposition. (The reactions can be found in the electrochemical series tables.) For example, metals like Ag and Cu etch or dissolve at those potentials [89]. Evaporated Al forms an oxide, turning white and foaming. PPy can be deposited on Si and Cr, which also form oxides, but it will not adhere. On other metals, such as iron [90–92], special conditions must be used to deposit PPy. Results on various metals are strongly dependent on the electrolyte and polymerization conditions [90, 91, 93, 94]. There is also the question of adhesion. For many applications, including actuators, the film must adhere strongly to the metal. Adhesion depends, among other things, on the metal’s morphology. Rougher surfaces allow greater mechanical interlocking. In our experience, thermally evaporated gold films from different batches have a significant variation in morphology. Roughening the surface by etching in gold etch (1 g I2 , 2 g KI, 50 ml H2 O) can sometimes improve the adhesion. It has also been found that PPy adheres strongly to titanium that has been oxidized by dipping for ∼5–10 seconds in the RCA SC1 solution (for the removal of particles and organic contamination: boiling ammonia (25%), hydrogen peroxide (30%) and water, 1:1:5 by volume) [95]. The mechanism for this adhesion is unclear, but it is probably due to the substantially increased roughness. Another method to increase adhesion is the use of self-assembled monolayers on gold [96–98], but these treatments have not been tested during electrochemical

cycling. Likewise, silanization of surfaces has been used to adhere PPy films to Si [99]. There are some conditions under which the growth of PPy(DBS) is difficult or unsuccessful. One of these is highly alkaline pH. The pH of commercially available NaDBS varies, but can be ∼pH 10 at 0.1 M, which still results in nice films [100]. Above pH 11 or 12, however, films have trouble nucleating. At high enough potentials, one can deposit electroactive material, but it grows in spots. A similar result can arise from contaminated deionized water; a house supply of DI water may not be clean of organics. In that case, tap water or high purity water should be used instead. PPy may also have trouble nucleating on some electrode materials, like indium tin oxide (ITO). In that case, one can try beginning the polymerization by applying a higher potential, such as 0.85 V, for a few seconds in order to start the deposition uniformly on this surface; then switch to a lower potential to deposit the rest of the film. 3. Patterning conjugated polymers

3.1. Review A considerable number of strategies, illustrated in figure 9, can be used to pattern a conjugated polymer film. The best method will depend on the polymer and the rest of the fabrication sequence, which will be discussed in the next section. Some polymers can be patterned directly using light or electron beams (figure 9(a)). For example, poly(alkylthiophenes) cross-link under UV irradiation [101, 102], although that may also cause them to degrade [103]. An improvement results by adding an acid-sensitive group to the polymer and blending it with a molecule that releases a proton under irradiation. The acid-sensitive group can be catalytically eliminated, leaving the chains insoluble, and the unexposed polymer can be rinsed away [104]. Similarly, poly(3-octylthiophene) films have been patterned using electron beam lithography, and line widths down to 50 nm demonstrated [105]. A polymer precursor film can also be selectively cross-linked or converted by shining UV light through a mask. This has been used to micropattern PPV [4]. Again, a different PPV precursor was patterned using electron beams [106]. UV light has also been used to pattern polyaniline indirectly through the use of onium salts, which decompose upon exposure to generate an acid [57]. 7

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Figure 9. Some ways to pattern conjugated polymer films. For details, see the text.

The acid dopes the polymer, rendering it insoluble. This work was done with a view toward developing a conducting photoresist. A cross-linking functionality was added to the backbone in another procedure for patterning polyaniline; ebeam irradiation caused cross-linking, and the un-irradiated portions could be rinsed away with water [57]. This method has also been used with polythiophenes [57]. Light can also be used to initiate polymerization of monomers. For example, an He–Ne laser has been reported to induce light-localized polymerization of pyrrole in an Alternatively, the acetonitrile/AgClO4 solution [107]. addition of a photosensitizer to a solution of the monomer and an oxidant restricts the chemical polymerization to areas exposed to light. This approach has been used to deposit PPy with linewidths of 10 µm on a Nafion membrane (490 nm wavelength, no exposure time specified) [108]. Another approach used the photochemical generation of the oxidant to form polypyrrole patterns on paper and on glassy carbon electrodes (150 W halogen lamp, 20 min exposure) [109]. The inverse approach has been used by another group. It involves the incorporation of a photosensitive oxidant in a host polymer. Exposure to UV light inactivates the oxidant; thereafter, the surface is exposed to the monomer, which only polymerizes in non-exposed areas [110]. Lastly, photoelectrochemical polymerization has also been used to define areas for film growth. When semiconducting electrodes, such as TiO2 , were irradiated, polymerization occurred selectively in the illuminated areas [111, 112]. Aqueous solutions were used, and line widths of 45 µm achieved. Additional methods for patterning conjugated 8

polymers by photopolymerization and photocross-linking are reviewed in [113], and further examples of irradiation techniques can be found in [114–124]. Patterned electrodes can be used to determine the area on which a polymer is deposited electrochemically (figure 9(b)). The electrodes are defined using the usual photolithographic methods, then the polymer is grown over them. This has the advantage of simplicity and avoids exposing the polymer to solvents and potentially harmful species. However, it also has some disadvantages. Foremost among them is that the films are of non-uniform thickness. This is most pronounced in an array of structures, for which the innermost devices will have the thinnest coating, and at all sharp corners, which will be thickest. For some applications, however, this may not be critical. An additional consideration is that the polymer has a certain lateral growth rate which depends, among other things, on the characteristics of the surface [125, 126]. Electrodes separated by only some microns can thus connect after some time [10, 11]. Another method for obtaining thin patterned films of PPy on previously patterned metal electrodes is chemical polymerization from the vapor phase [127]. Sputtered copper exposed to chlorine vapor reacts to form the chlorinated salt CuCl2 . The chemical vapor deposition of PPy can then be performed. However, the resulting PPy is disordered, and care must be taken with the chlorination and polymerization conditions [127]. Chlorinated Cu nanoparticles entrapped in a commercial photoresist pattern have also been used to direct the polymerization of PPy from the vapor phase [128], as has a solid oxidizing salt patterned on an insulating substrate

Microfabrication with conjugated polymers

[129]. A final example of the use of patterned electrodes for the electrochemical deposition of PPy is found in [130]; the electrodes were coated with PVC to produce patterned PVC/PPy alloy films. A photoresist pattern can be used to define the electrodeposition of the polymer (figure 9(c)). The entire wafer is covered by a metal film and coated with positive resist, and the areas in which the polymer is wanted are exposed using a positive mask. This is somewhat akin to patterning by lift-off. (Other materials that are insulating, water-resistant and patternable could be used instead of resist. See for example [131, 132].) The polymer is deposited on the exposed metal areas, and the resist is then removed. This is one of the methods we use and will be described in more detail below. A variation of this method is to use a micropatterned self-assembled monolayer (SAM) to define areas favorable or unfavorable for polymer deposition. This was shown in [133– 135]. A pattern of alkanethiol (for use with PPy) or 12-amino1-dodecanethiol (for PANI) was defined by microcontact printing on gold, then the substrate was immersed in a solution containing short-chain alkanethiols. The polymers were preferentially deposited over the regions covered by the short chains. Films as thick as 1 µm with linewidths as thin as 6 µm could be defined [133]. Similar work has been described in [136, 137], and area-selective deposition on Si or glass is described in [138]. Controlling the hydrophobicity or charge on regions of the surface is also effective [139–141]. Conjugated polymers can also be patterned by removal of unwanted material, whether by reactive ion etching (RIE) or laser ablation [142–146] (figure 9(d)). These methods work well for intractable polymers. PPy, for example, can be etched in an oxygen plasma. This is another method that we use in the fabrication of our bilayers. A general method for patterning organic materials is the Top-CARL technique, in which photoresist is silylated to make it resistant to RIE [147]. If the microfabricated device does not require the complete removal of the polymer film between active areas, then it can be selectively ‘killed’ outside them (figure 9(e)). For example, we have not yet found a satisfactory way to pattern PEDOT, but exposure to Cr etchant drains its light blue color and results in a loss of conductivity. Photoresist can be used as a mask for this process. It is difficult to define features less than 50 µm, but for larger ones this can be an acceptable patterning method. Material is also removed slowly, but the lateral destruction rate under the resist mask is significant enough that one cannot wait for complete etching if the features are small. A related but inverse method involves the selective doping of certain regions (figure 9(f)). If the film is insulating, then it may be possible to dope those regions using a mask, either by exposure to a gas or an acid [148] or by electrochemical doping [149]. Like the previous method, this leaves a polymer layer covering the substrate and therefore would not be appropriate for applications such as actuators. Ink-jet printing has also been used for conducting polymer device fabrication: the polymer was dissolved in a volatile solvent and ejected dropwise onto a substrate [150]. This simple technique allows the deposition of polymeric patterns onto a wide variety of different surfaces with the

resolution allowed by the print head. It also enables threedimensional structures to be built up. Finally, a few more exotic methods for patterning should be mentioned. Scanning electrochemical microscopy has been used to deposit PPy lines with a diameter of 10 µm [151] and polyaniline with line widths of 2 µm [152]. Near-field scanning optical microscopy was used to change the doping level of poly(3,4-diphenyl-2,5- thienylenevinylene) through photo-oxidation with a resolution of 0.1 µm, breaking the diffraction limit [153]. In a so-called template synthesis, PPy nanotubes have been formed inside porous membranes [154]. 3.2. Processing considerations While there is great flexibility in patterning conjugated polymer films, these materials are nevertheless quite sensitive to some chemicals. High temperatures can also be a problem, although some are fairly tough: PPV can withstand 300 ◦ C and the plasma enhanced chemical vapor deposition (PECVD) of a silicon nitride (Six Ny ) insulating layer [155]. PPy, however, should not be heated over 150 ◦ C. Therefore, care must be taken to devise an appropriate process sequence. Table 1 summarizes some of the known hazards to PPy and some of the treatments that we have found to be harmless. There are, fortunately, a number of instances when some of the hazards can be avoided. For example, developer damages PPy, leaving it insulating. (It is not clear whether this can be completely reversed by exposure to acid.) However, those areas that are covered by resist are undamaged, and the damage does not propagate laterally, underneath the resist. Thus, patterning the polymer by RIE as outlined above is straightforward. This might not be the case with other polymers; in that event an intermediate layer below the resist would be necessary, such as a metal whose etchant does not damage the polymer, or another polymer. Resist also protects PPy from attack by some acids, including hydrofluoric acid (HF), at least for short times. However, Cr etchants destroy PPy (recipe 1: 2.7 g Ce(SO4 )2 · H2 O (3.5 g H4 Ce(SO4 )4 can be substituted), 17.5 ml HNO3 (65%) and 32.5 ml H2 O; recipe 2: 200 g (NH4 )2 Ce(NO3 )6 , 35 ml CH3 COOH and 700 ml H2 O). The few seconds required to remove a thin adhesion layer of Cr can be tolerated, provided there is resist over the PPy, but longer times cannot. If it is necessary to perform a longer Cr etch, a layer of resist that overlaps all the edges of the PPy areas should be used to completely seal them. It is important to ensure that all resist has been removed from over the conjugated polymer film, or ion transport in and out of it will be impossible. It is difficult to see a thin layer of resist, however. The resist can usually be removed using acetone or ethanol. However, acetone dehydrates PPy, which creates tensile stresses. Ethanol is not as good a solvent for the resist, but thorough rinsing will remove it, and this method is less dehydrating. Commercial remover (stripper) destroys PPy immediately and must be strictly avoided. After resist has been partially etched by RIE, the remainder can only be adequately eliminated by further RIE etching. Patterning is difficult if the polymer is applied by spin-coating and remains soluble. Fortunately, polyaniline undergoes some sort of structural or chemical change, 9

E Smela Table 1. Processing hazards for PPy.

Damaging

Use care, test first

Harmless

Cr etchant high temperatures developer (KOH), bases resist stripper/remover

acids

Au etchant hot plate at 100 ◦ C photoresist UV light in mask aligner

solvents

Figure 10. Comparison of (a) sacrificial layer and (b) differential adhesion method of microstructure release.

believed to be due to hydrogen bonding [156], after it is deposited from solution and dried. Thereafter, the PANI is mostly insoluble and can be patterned using the same RIE method as for PPy [57]. However, soluble poly(alkylthiophenes), deposited for example from chloroform, can re-dissolve. Thus, when resist is spun over them, the two layers mix together. 4. Fabrication sequence for PPy microactuators

4.1. Differential adhesion for microstructure release Microactuators require a method to partially release them from the substrate. A sacrificial layer is normally used in micromachining [157], and has also been used for PPy bilayer actuators [158, 159]. However, that approach was problematic for a number of reasons, primarily because the step over the sacrificial layer was too large (see figure 10). The gold layer is typically only 1000 Å thick or less, the PPy layer 1 µm or less. In order to ensure that the gold layer is continuous over the step, the sacrificial layer has to be kept thin. Even then, the gold on the vertical part of the step will be thinner, and there is a danger that most of the bending will occur right at that edge. A second drawback is that for large areas, underetching takes a long time. The risk of damaging the PPy during this process increases. We therefore devised another method, which we call the differential adhesion method [28, 160]. This approach eliminates the need to protect layers from chemical attack during a final etch release step and allows one to free arbitrarily large areas. The differential adhesion method is based on (a) the cohesion of a gold film and (b) a pattern of stickiness on the surface under it. The stress generated by the activated PPy film achieves the final release. This method is analogous to gluing down a piece of paper by applying adhesive to the four edges, then later cutting three sides free. The method relies on the mechanical integrity of the ‘paper’ layer, which must remain intact and attached to the substrate even though it is only partially affixed. 10

It is well known that gold adheres weakly to bare silicon, silicon dioxide or glass, and therefore a thin film of a metal such as chromium or titanium is normally deposited first as an adhesion-promoting layer. In the differential adhesion method, the adhesion layer is patterned. Because gold coheres to itself, a chromium layer patterned with small openings can still be used to ‘glue’ it down. Additional layers can be deposited over the gold as long as they are almost stress-free; otherwise the gold layer will be pulled off the bare silicon areas prematurely. We usually use silicon wafers as substrates, but glass can also be used with success. The first step in the fabrication is the deposition and patterning of an adhesion layer. (Refer to figure 11(a).) We have found that the combination of a thin layer of Cr followed by a layer of Au works more consistently than a layer of Cr alone. Cr is oxidized upon exposure to air, and that oxide is not always sufficiently adhesive. Au sticks very well to Cr + Au. Therefore, Cr is thermally evaporated, typically to a thickness of 30 Å, and then Au is evaporated immediately afterward, typically to 200–300 Å. Where the final structures should be released, windows are opened in these layers using standard photolithography and wet etching. (The pattern is essentially the inverse of what would be used to pattern a sacrificial layer.) This results in areas of bare silicon to which a second layer of gold will not adhere. The choice of material combinations that can be used to achieve the same effect is large. In addition to metal films, a patterned monolayer of silane-modified molecules on silicon or thiolmodified molecules on gold could be employed to generate an adhesion difference, as could a single material with areas of roughness and smoothness. The exact thickness of the Cr is not important, as long as there is enough to guarantee adhesion to the silicon substrate. Likewise, the thickness of the first Au layer is not critical, either. However, the etched pattern must be visible through the second layer of Au, deposited next. Thus, the total thickness of the pair must be enough to result in a significant step, one that is easily visible in the mask aligner after being covered by 1000 Å of additional Au. Some Cr etchants (particularly recipe 1, above) contain Ce in such a form that it remains on the Si surface after rinsing, and this can cause the second Au layer to adhere to the bare Si or SiO2 . It is therefore critical to remove it, which can be done by rinsing in a dilute solution of nitric acid (∼10% HNO3 works well) for ∼30 s. Likewise, exposure to hydrofluoric acid can cause unwanted adhesion to the substrate. The adhesion layer must be clean. It is best to remove the photoresist with commercial stripper; this step is best done twice to guarantee that all the resist has really been completely eliminated. If the resist has been recently stripped, then the metal will be sufficiently clean for the next evaporation because commercial remover

Microfabrication with conjugated polymers

of force and bending, however, the Au thickness should be 2000–3000 Å [164]. The surface cannot be stressed after this stage, for example by cleaning in SC1 or the deposition of highly stressed layers like Cr; this causes the unattached Au to come off. However, procedures such as spin-coating resist, rinsing with a spray of water and drying under nitrogen pose no problems. 4.3. Rigid plates

Figure 11. Differential adhesion process sequence for making a simple PPy/Au bilayer. (a) Deposit and pattern adhesion layer. (b) Deposit structural Au layer. (c) Deposit and pattern BCB. (d) Deposit and pattern PPy. (e) Etch Au.

contains surfactants and is thus an excellent cleaning agent. Otherwise, the samples should be cleaned just prior to the evaporation using, for example, SC1. The nitric acid rinse and the resist removal can be done in either order. It is important that the windows in the adhesion layer are not too large: otherwise there is a risk that internal stresses induced during processing will cause the second Au layer to come off. It is also important that the windows are completely surrounded by the adhesion layer. If there is an opportunity for liquid to creep between the structural Au layer and the Si, the Au will lift and tear off. 4.2. Au for bilayer The next step is the deposition of the second, structural Au layer (figure 11(b)), which must be free of Cr contamination. In the microactuator, this gold layer functions not only as the working electrode, but also as the passive mechanical layer of the bimorph. Its thickness is typically 1000 Å, which we found empirically to give good actuator results for a 1 µm thickness of PPy. We also calculated that, for the Young’s moduli of Au and PPy, a thickness ratio of 1:10 gives the greatest bending† [164]. To obtain the best combination † As an approximation, the greatest bending occurs for E1 h21 = E2 h22 , where E is the Young’s modulus, h is the layer thickness and 1 and 2 are

To incorporate rigid parts (figure 11(c)), an ordinary, nonconducting polymer like polyimide or benzocyclobutene (BCB) can be used. Both come in UV-curable versions that can be patterned directly. (Photo-BCB and the developer are available from Dow under the trade name Cyclotene. Photo-BCB lasts about 6 months under refrigeration, but if the solution is too old the solids start to precipitate and patterns no longer develop cleanly.) The reason for using such a polymeric rigid layer is that it has almost no internal stress. Another constraint on this layer is that it should not form an alloy with the Au so that it can be separately etched. Lift-off cannot be used to pattern the rigid layer because the structural gold layer would also be removed by that process. To make a rigid plate that can be rotated by a bilayer hinge [28], we spin-coat a solution of photo-BCB over the structural Au layer. Normally we use the solution directly and spin at a rate of 4000 rpm for 90 seconds. The 40% solid formulation gives a 5 µm thick layer. To form a thinner layer, one can increase the spin speed or dilute with mesitylene. Dilution has a larger effect. After spin coating, we bake the layer at 100 ◦ C on a hot plate for 10–15 seconds. Without baking, the layer is tacky and will stick to the mask when the mask and substrate are put into contact. On the other hand, too much baking will cause cross-linking and make developing impossible. The photo-BCB is exposed using a positive mask, which causes the polymer to further cross-link, so where it is exposed it will remain after developing. Because the polymer layer may flow during exposure, which takes 30 seconds at 5 mW cm−2 at a wavelength of 365 nm, the features can sometimes be up to 10 µm larger on every side than the mask. The mask should be designed with this in mind. The final structure should have enough clearance between it and the edge of the adhesion layer that it can be lifted off. The edges of the BCB features are bell shaped. The BCB is best developed on a spinner. The wafer is placed on the spinner, and a pipette used to transfer a puddle of developer onto the wafer; the photo-BCB is allowed to develop for approximately 1 minute. Then the wafer is spun, and while spinning more developer is dribbled onto it. This washes the wafer clean. It is then spun until dry and inspected. If there is still BCB in the unexposed areas, the procedure is repeated. Usually it is necessary to do this twice. BCB that is developed in a beaker re-deposits onto the wafer, and this method is thus not recommended. After developing, there is normally a curing step. This should be done in a nitrogen or argon atmosphere, not in air, for best results. (Curing can also be done in air, but the layers the two layers comprising the beam [161]. For Au, E = 80 GPa [162], and for PPy(DBS) we have measured it to be ∼3 GPa in the dry state, which is typical for plastics [162] and other polypyrroles [163].

11

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may develop compressive stress.) Curing is done at ∼200 ◦ C with a slow ramp up and ramp down; the temperature and time determine the degree of cross-linking. (Details of the curing procedure are available from Dow.) More highly cross-linked films are stiffer, but they also have more internal stress. In order to obtain flat paddles, we found it best to avoid curing, so we omitted this step. Although BCB layers stand up well to all water-based solutions, contact with organic solvents should be minimized because the BCB is swollen by them. Thus, RIE patterning of PPy (below) is preferable. The final step in patterning this layer is the RIE ‘via de-scum’. This step removes the final very thin residue of BCB that is still present over the entire surface even after developing. If the Au is not completely cleaned of this insulator, then the PPy will not grow on it. The RIE plasma consists of 5 sccm 5% CF4 in O2 and 45 sccm O2 , and the pressure should be 40 mTorr, the power 300 W. The etch time is one minute. Regular BCB was also used instead of photo-BCB. It was cured after spin-coating, then patterned with photoresist as a mask using the same RIE parameters. The etch rate for BCB is lower than for photo-BCB; it took approximately 30 minutes for a 1 µm thick film of BCB. If only simple bilayers are desired, with no rigid parts, this third step is simply omitted. The rest of the fabrication sequence is unaffected. 4.4. PPy deposition and patterning, final Au etch In the fourth step (figure 11(d)) the PPy is deposited and patterned. We use either PPy growth in resist openings or RIE etching. In general, both work equally well, but one or the other may be more advantageous in a given process sequence. As mentioned above, a photoresist pattern can be used to define areas for electrodeposition of PPy (figure 9(c)). The Au film is coated with resist, which is patterned to leave exposed metal areas for PPy growth. The PPy thickness is limited to some extent by the thickness of the resist layer: above that the polymer has the opportunity to grow laterally. However, PPy(DBS) does not have a fast lateral growth rate over photoresist, and we have produced structures with good definition that are 0.5 µm higher than the resist surface. Surprisingly, this method results in films with much more uniform thickness, both within each resist well and among wells, than using patterned electrodes. However, the growth rate is faster in small wells† due to the greater availability of reactants and reduced diffusion of oligomers away from the surface. The resist is removed with ethanol. To finish patterning the electrodes (figure 11(e)), a new layer of resist is applied to protect the PPy and the desired metal regions. This is exposed using a negative mask (or alternatively, negative resist). (This step requires that the polymer does not dissolve when the resist is applied.) The metal can then be etched and the resist removed. The second alternative for patterning PPy is to use RIE (figure 9(d)). PPy cannot be wet-chemically etched, but an † In wells with the dimensions of our typical hinges, of the order of 30 × 60 µm2 , the growth rate is twice as fast as on an electrode with an area of some cm2 .

12

Figure 12. RIE etching procedure for patterning PPy and underlying gold. (a) The sample and a resist-covered dummy wafer are put into the chamber together. (b) The PPy is etched until the gold is exposed. (c) The gold is wet chemically etched. (d) The sample and the dummy are etched until the dummy is clean of resist.

oxygen plasma will dry etch it. (The plasma does not damage the polymer, but only etches it. Thus, if it is exposed to the plasma and thinned, the remaining material is unimpaired.) A protective mask is necessary, and one must determine the best etching conditions. We use the following parameters for the RIE etching process: 30 sccm oxygen flow, 40 mTorr pressure, and 350 W RF-power. The simplest mask is photoresist, but PPy and photoresist etch at approximately the same rate under these conditions, ∼3000 Å min−1 . Thus, the maximum final PPy thickness is less than or equal to the thickness of the resist. (The Top-CARL process would circumvent this problem [147].) After the exposed PPy is removed, the gold layer can be wet etched directly (figure 11(e)). This is best done if some resist still protects the PPy, so that PPy alone is not the sole mask material for the gold. We usually use a 1 µm thickness of PPy, and a 1.4 µm thickness of resist. Therefore, if the etching is halted immediately after the PPy is removed, 4000 Å of photoresist remains. This prevents the gold etchant from permeating through the porous PPy layer to etch the gold underneath. After the gold is patterned, the wafer can be returned to the RIE for another minute to remove the remaining resist. Acetone will not adequately remove RIE-hardened resist. A dummy wafer consisting only of a resist-covered substrate that is processed in the RIE at the same time can be used to determine when all the resist has been removed. This sequence is illustrated in figure 12. Patterning with RIE produces the most uniform films, because the PPy is grown over the entire wafer, but it has one significant disadvantage. As the PPy becomes thicker, it becomes increasingly opaque; it is almost impossible to perform an alignment on films thicker than 1 µm, especially when they are covered by a further 1.4 µm of resist, because you cannot see the underlying features. This can be circumvented by ensuring that there are alignment marks uncovered by PPy. Another disadvantage is that the final PPy thickness is difficult to control precisely.

Microfabrication with conjugated polymers

Note that this fabrication sequence is quite simple and only requires three masks (four if PPy is grown in patterned resist). Under ideal conditions, such devices can be produced in two days. When the film is electrochemically oxidized and reduced (see below), the bilayer lifts from the surface, bending and straightening depending on the applied potential. The plates are carried by the hinges. (The BCB and PPy are connected through the Au structural layer, which underlies them both.) PPy/Au hinges of just 30 × 30 µm2 have pulled free and rotated rigid areas of 900 × 900 µm2 using this technique [28]. The device illustrated in figure 11 can rotate from 0 to 180◦ ; further movement is prevented by the plate coming into contact with the substrate. 5. Operation of PPy microactuators

We actuate the devices in monomer-free 0.1 M aqueous NaDBS. Because the DBS is immobile in PPy [51, 54, 100, 165, 166], it does not move during redox, and the volume change is therefore determined by cation transport. (It is thought that the cation is hydrated, and water has been found to play an important role in the movement [167, 168].) Before electrochemical actuation, the PPy(DBS) film is in the oxidized (doped) state. During the first reduction scan, cations (+ water) are inserted, and one would therefore expect the PPy(DBS) to expand and the bilayers to bend backwards, towards the wafer. Instead, during the first reduction scan a change takes place in the PPy film, and there is almost no movement [100] (except in films with a thickness of several microns, which may bend backwards during the first few cycles). Then during the first re-oxidation, the PPy(DBS) contracts, and the bilayers bend away from the wafer. Thus, the completed structures lift themselves off the surface; this happens almost immediately if the fabrication has been done perfectly, otherwise it may take a few minutes. This anomalous first reduction peak has been extensively investigated, but the reason for this behavior is still not understood; it has been suggested that uncycled PPy has a special structure that is irreversibly changed during reduction [169, 170]. The phenomenon is sensitive to the electrolyte. Thereafter, the bilayers bend upon oxidation (Na+ is expelled) and straighten upon reduction (Na+ is incorporated). In the reduced state, the bilayers lie flat against the wafer because the PPy volume is nearly the same as it was originally, when freshly deposited and oxidized. The bilayer motion is smooth and uni-directional, i.e. it does not change direction midway during the application of a voltage or current, but expands or contracts continuously until reaching the corresponding redox state [51]. If the potential is removed, the PPy returns to its stable oxidized state. A reducing potential must therefore be applied to hold the bilayers straight. If the PPy is grown and cycled with mobile anions such as ClO4 , then the bilayers may bend during reduction (anions expelled) and straighten during oxidation (anions incorporated), the opposite of what occurs in PPy(DBS) [166]. The potential can be varied smoothly by cyclic voltammetry or abruptly by stepping the potential. The

potential limits should be chosen by observing a cyclic voltammogram. To see the devices move, we use limits between −1 and 0 V against Ag/AgCl, somewhat above the oxidation peak and below the reduction peak. Although more extreme limits make the bilayers move faster, a too high positive voltage will overoxidize the PPy and result in degradation, and too negative a voltage will result in hydrogen evolution. Another way to control the movement is through the current [171], rather than voltage, stopping when the desired charge has been consumed. (Warning: additional current beyond that required for redox can only arise from other, unwanted reactions, including pyrrole decomposition and gas evolution.) PPy layers 1 µm thick take ∼2 seconds to undergo their complete volume change from minimum to maximum or vice versa. The speed is determined by the transport of Na+ (and perhaps its hydration shell) in and out of the film, so thinner films react more quickly. The fastest devices we have made were able to go from completely bent to completely straight and back again at 2.5 Hz. We have observed that oxidation and reduction usually take the same amount of time, but sometimes oxidation (Na+ leaving the film) is faster. It has likewise been observed by other workers that ion expulsion can be faster [172]. The voltage limits can also affect the relative speed of the two processes. It should be emphasized that the application of a voltage that lies between the fully oxidized and reduced states will result in a partially oxidized polymer, and consequently an intermediate position. This position can be held fixed, although over long times the polymer chains will relax and the position will drift somewhat [51]. The redox level can of course also be changed from one intermediate level to another, without returning to either fully oxidized or reduced states; for instance by changing the voltage from −0.5 V to −0.7 V and back to −0.5 V the bilayer will go from somewhat bent to less bent and back again. Up to now, the devices have only been operated in liquid, which is an advantage in biological systems, but we are developing the methods necessary to make allpolymer devices that work in air. This is a critical step for many applications. Such dry systems have already been demonstrated on the macroscopic scale. Instead of a liquid, a polymer layer is used as the ion reservoir, which can be either a gel [47] or a solid polymer electrolyte (SPE) [46]. Gel systems are swollen by a solvent, and ion transport takes place in the solvent phase. This has the advantage of high ionic conductivity, and therefore speed, but the disadvantageous possibility of drying out. A further important advantage is that methods have been developed to micro-pattern gels [173]. Solid polymer electrolytes are less attractive for microfabrication because they have a relatively low ionic conductivity, typically only 10−5 or 10−6 S cm−1 at room temperature (ion transport takes place in the polymer and is therefore hindered by chain segment motions). Patterning is also problematic because the materials are sticky, and crosslinking would further hinder ion motion. In addition to finding a suitable polymer electrolyte layer, the three electrodes must all be fabricated on chip. We have recently accomplished this, and the on-chip electrodes have been used to oxidize and reduce a PPy layer [174]. To form the reference electrodes, silver was electroplated over the Au 13

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structural layer in regions defined by openings in photoresist. Part of this layer was converted to AgCl electrochemically. The counter-electrodes were defined photolithographically on the Au structural layer (on adherent regions), and the PPy bilayers were fabricated as usual [175]. Following these first demonstrations of the micromuscles, new applications are now being developed and their performance improved. More detailed studies on device behavior are also being performed to gain a better understanding of the mechanisms involved in the volume change. 6. Fabrication of polyaniline microactuators

Macroscopic polyaniline actuators have been demonstrated and characterized previously [85, 176, 177]. Although we have less experience with polyaniline, we have also successfully microfabricated simple bilayer actuators with this polymer, again using the differential adhesion method. For these devices, the Cr/Au adhesion layer had a Cr thickness of 50 Å and a gold thickness of 10 Å; the structural Au layer was 200 Å thick. For useful information on forming and patterning PANI films, see [58]. We used PANI (obtained from Neste Oy) in the undoped, emeraldine base form, which is a blue powder. (For a description of a method to synthesize PANI, see [178].) We added 0.5 g of the PANI to 10 g of the solvent N-methyl pyrrolidon (NMP) and stirred the mixture to dissolve the powder using a stirring bar and a magnetic stirrer. This took from some minutes to an hour, mainly because the PANI was several years old. The solution was then centrifuged for 1 hour at 3800 rpm to remove the undissolved particles. We obtained a dark solid at the bottom of the tube and a dark blue liquid (supernatant) on top. The supernatant could be spin-coated and dried to obtain a particle-free dark purple film. At 6000 rpm, a film thickness of 700 Å was obtained, and at 2000 rpm the thickness was 1250 Å. With more freshly synthesized PANI, we have found that dissolution is facile, and centrifugation unnecessary. If the PANI/NMP solution was allowed to stand, it thickened considerably. This is due to gelling [179]. To obtain reproducible results, it is therefore best to prepare the mixture freshly before each use. Because NMP has a high boiling point, it evaporates slowly. Thick films took some hours to dry. Gentle heating (45 ◦ C) in an oven accelerated the process and gave good films. Blow drying, however, resulted in streaks and small holes, and excessive heat, especially from a hot plate, resulted in dewetting, the formation of puddles and non-uniform thicknesses; these measures to speed the drying process are therefore not recommended. After the PANI film had dried, it was possible to spincoat resist over it. However, even a PANI film as thin as 1250 Å was very dark and opaque, and thus difficult to align. The substrate should be prepared with alignment marks that will be clearly visible after the PANI is coated onto it. After exposure, the resist was developed. The 1250 Å thick PANI film was dry etched in an oxygen plasma using the same parameters as for PPy, above, which took one minute. The Au was then wet chemically etched, and the remaining resist removed in the O2 plasma. 14

It is also possible to obtain PANI doped with DBS acid. We tried to make bilayers using this, also. We added 1 g PANI(DBS) to 2.5 g xylene, stirred, and centrifuged. At 1000 rpm we obtained a uniform, smooth, green, transparent film with a thickness of 15 000 Å. Unfortunately, when spin coating resist over it, the PANI(DBS) film ripped and tore off in several places. Furthermore, it flaked and completely peeled off the gold in the developer. The lack of mechanical integrity and adhesion, as well as its susceptibility to decomposition in bases, thus prove problematic for this form of PANI. The PANI/gold bilayers were actuated by electrochemical cycling in acid. We used 1 M HCl, and applied voltages between −0.1 V and +0.5 V against Ag/AgCl. The bilayers curled upwards at −0.1 V (leucoemeraldine state, yellow color) and straightened for +0.5 V (protonated emeraldine, transparent green), the opposite of the PPy(DBS). In this reaction, Cl− anions compensate the emeraldine state, and these are expelled during reduction. For a review of the electrochemistry of PANI, see [43]. 7. Conclusions

A number of laboratories have been developing the techniques necessary for microfabricating conducting polymer devices. This has led to a wide variety of methods available for depositing these materials, as well as numerous ways to pattern them. In this paper, specific procedures have been described in detail, and references to other work provided, with the hope that this will encourage more micromachinists to begin working with this interesting new class of semiconductors. Their potential applications as micro-light emitters, sensors, actuators, batteries etc make them quite attractive, and we are only limited by our imaginations in what we might invent. Acknowledgments

I am grateful to Carsten Worsoe Moeller for performing the HPLC on the DBS and to Ib Johannsen for discussions about this. I would also like to thank the Danish Research Councils, the Swedish Research Council for Engineering Sciences and the Volvo Research Foundation, Volvo Educational Foundation and Dr Pehr Gyllenhammar Research Foundation for their financial support. References [1] Gardner J W and Bartlett P N 1991 Potential applications of electropolymerized thin organic films in nanotechnology Nanotechnology 2 19 [2] Gardner J W and Bartlett P N 1995 Application of conducting polymer technology in microsystems Sensors Actuators A 51 57 [3] Lidzey D G, Pate M A, Weaver M S, Fisher T A and Bradley D D C 1996 Photoprocessed and micropatterned conjugated polymer LEDs Synth. Met. 82 141 [4] Renak M L, Bazan G C and Roitman D 1997 Microlithographic process for patterning conjugated emissive polymers Adv. Mater. 9 392

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