Silicon wafers (n-type, (1 1 l), one face polished) were purchased from Barrington Chemical Corp. (Mamaroneck, NY). Disk ultramicroelectrodes were made of.
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J. CHEM. SOC. FARADAY TRANS., 1995, 91(6), 1019-1024
Study of Silicon Etching in HBr Solutions using a Scanning Electrochemical Microscope Sheffer Meltzer and Daniel Mandler" Department of Inorganic and Analytical Chemistry, The Hebrew University of Jerusalem, Jerusalem 91904, Israel
The etching of silicon has been studied by the scanning electrochemical microscope (SECM) technique. Etching has been accomplished in acidic fluoride solutions by electrogenerating a strong oxidant, i.e. bromine in this case, at an ultramicroelectrode which was held closely above a silicon (1 11) wafer. The parameters that affect the process and control the efficiency of the silicon etching were examined. A detailed mechanism of the process, which was derived from the unique advantages of the SECM and is in agreement with previous reports, is proposed.
Silicon is the major substrate material used in the fabrication of solid-state electronic devices.' The monolithic chip, usually based on silicon, is batch-fabricated through a multi-step process which involves many operations such as masking, selective doping and etching. Etching of silicon has been the subject of numerous The earliest processes used were based on wet etching, in which silicon wafers were agitated in etchant baths. The wet etching of silicon involves its chemical oxidation, which is usually driven by strong oxidants such as nitric followed by oxide layer disso!ution with fluoride compounds.6-8 The demand for sub-pm technology, as well as environmental legislation and employee safety regulations, has pushed forward the development of dry processes' such as reactive ion-etching. The major advantages of dry processes over wet etching are the higher resolution and aspect ratio, owing to the anisotropy of the dry process, as well as better control of the etching rate. Nevertheless, efforts are still being made to develop better and more controllable approaches for wet etching of silicon. These are particularly interesting, as a result of the recent discovery that the wet etching of silicon can result in porous photo- and electro-luminescent silicon. Among the different microprobe techniques' which have followed the invention of the scanning tunnelling microscope, the scanning electrochemical microscope (SECM) has been very promising in modifying surfaces with pm resolution. The SECM is based12,13 on approaching and imaging a surface with an ultramicroelectrode while measuring the faradaic current that flows through it. The steady-state current that flows at the ultramicroelectrode, as a result of generating electroactive species, senses the surface; its magnitude depends on both the distance between the microelectrode and the surface and the electrochemical characteristics of the surface. The SECM, which has been developed mainly by Bard and his co-workers, was used to image a variety of surface~,'~.' to study electrochemical processes occurring on surfaces and in solution'6"7 and to modify surfaces with pm resolution.'8-2' The modifying potential of the SECM has been demonstrated by driving the deposition18" and etching of metals18' and of semiconductor^,^ in electropolymerization of conducting polymers2' and recently in the deposition of structures of a metal hydroxide.2' Although 11-VI and 111-V semiconductors, such as gallium arsenide and cadmium telluride, have been already etched using the SECM, the etching of silicon has never been undertaken. We report here on the application of the SECM for silicon etching. Our approach combines the major advantage of the SECM (i.e. driving electrochemical reactions on surfaces with high spatial
''
resolution), with the chemistry involved in the wet etching of silicon. Specifically, bromine electrogenerated at an ultramicroelectrode, which was brought close to a silicon wafer in acidic fluoride solutions, resulted in the formation of etching patterns. A mechanism is proposed based on the effect of the different parameters that control the efficiency of the process.
Experimental The SECM, which is based on a controlled micropositioning device, has been previously described.21 Micrographs of the etching patterns were taken by an optical microscope (Universal Microscope, Zeiss). Profiles of the etching pits were recorded using an Alpha Step 200 profilometer (Tencor Instruments). All chemicals were of analytical grade and used as received. Deionized water (Milli-Q, Millipore) was used for solution preparation. Silicon wafers (n-type, (1 1 l), one face polished) were purchased from Barrington Chemical Corp. (Mamaroneck, NY). Disk ultramicroelectrodes were made of 50, 25 or 10 pm diameter platinum wires (Goodfellow, Cambridge, England) following conventional methods.22 The microelectrodes were polished with diamond paste and electrocycled in 1 mol dm-3 H2S04before each experiment. Prior to mounting in the SECM cell, the Si wafers were immersed in a 10% hydrofluoric solution for 30 s followed by rinsing with clean water. The microelectrode was brought manually to the surface and then withdrawn to a known distance from the surface before the solution (typically 5 mmol dm-3 HBr, 1 mol dm-3 H F an 1 mol dm-3 H2S0,) was added to the SECM cell. Finally, the current, was recorded as a function of time or distance, while the microelectrode approached the surface. In all experiments the silicon wafer was not biased by any external voltage source. All potentials are quoted us. an Hg/Hg2S04 (K2S04 saturated) reference electrode. To examine different parameters the microelectrode was left at several places above the surface and the charge that passed through it was monitored. The volume of silicon that was etched was estimated from profiles of the cross-sections of the etching pits and their optical micrographs.
Results and Discussion Fig. 1 shows the normalized current, i (the actual current divided by the current measured far, i.e. several times the electrode diameter, from the surface i,) measured with a 50 pm diameter platinum electrode ( E = 0.66 V) as a function of the normalized distance, d (the distance divided by the electrode radius, r) while approaching an unbiased silicon surface in 5 mmol dm-3 HBr and 1 mol dm-3 H2S04 solution. This
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1.6 1.2
0.8
0-4
0
t 1
t
I
I
1
I
I
2
I
I
3
t I
!
I
4
dlr Fig. 1 Normalized feedback current us. the normalized distance of a 50 pm Pt microelectrode approaching an n-type Si( I 1 1) surface in 5 mmol dm-3 HBr and 1 rnol dm-3 H,SO,: 0 ,in the absence of H F and A,with 1 rnol dm-3 H F (E = 0.66 V)
0
E 0.4
so-called feedback current23is constant (equal to ),i as long as the microelectrode is held far above the surface. However, while approaching a surface the current senses the surface and either increases (positive feedback current) or decreases (negative feedback current). The fact that a negative feedback current is observed in Fig. 1 indicates that the electroactive species generated at the microelectrode, i.e. bromine, is not regenerated at the surface. In this case, the hindrance of the bromide flux to the microelectrode by the surface causes the steady-state current to decrease. Low surface conductivity, as well as sluggish heterogeneous electron transfer, are the two major reasons that can prevent efficient electrogeneration of the species on the surface. Note that, in spite of the fact that the wafers were treated with H F before each experiment, a negative feedback current was always detected in the absence of fluoride ions in the SECM cell. Fig. 1 also shows the feedback current obtained upon introducing 1 mol dm-3 HF. A positive feedback current is detected which confirms that the negative feedback current was due to the presence of an oxide layer that prevented the regeneration of bromide ions. The positive feedback current is driven by the difference in the brominelbromide concentration beneath the tip as compared to the solution.23Therefore, the Si surface need not be biased in order to drive the regeneration process locally on the surface. The feedback current attained in 1 mol dm-3 H F and 1 mol dm-3 H2S0, solutions was positive and constant, suggesting that the oxide layer was removed, thus allowing fast electron transfer between the surface and the electrogenerated bromine. The dissolution of silicon oxide by fluoride solutions has been the subject of numerous although the mechanism has never been fully resolved. At least three different mechanisms have been suggested for the attack of exposed SiO, sites by HF. Different fluoride species, e.g. H F and HF,-, are involved in this process and their concentrations depend on the total H F ~ o n c e n t r a t i o nand ~ ~ acidity. The dissolution products are mainly SiF6,-, although it has been claimed that SiF, is also formed as a result of the reaction of the uppermost layer of silicon atoms.25 Furthermore, the etching rate of SiO, depends not only on the fluoride concentration but also on the acidity,26as will be discussed later. E x situ IR absorption studies followed by in situ studies unambiguously proved that the silicon surface remains predominantly covered by hydrogen rather than by F- or 0-containing The H-terminated silicon surface is resistive to further dissolution by HF. Leaving the microelectrode biased above the unbiased silicon surface for a few minutes resulted in etching patterns. Fig. 2 shows the micrographs and the profiles of etching pits
s. --. A 0.8
0
500
1000 XlPm
1500
Fig. 2 Optical micrograph and profiles of etching pits formed as a result of leaving a 50 pm Pt microelectrode (E = 0.66 V) above an n-type Si( 111) surface for durations of (a) 10, (b) 10 and (c) 15 min in a solution consisting of 5 mmol dm-3 HBr, 1 mol dm-3 H,SO, and 1 mol dm-3 HF
formed with a 50 pm microelectrode, as a consequence of leaving the microelectrode for different durations above the silicon wafer. It can be seen that the diameters of the patterns are of the order of the microelectrode (including insulator) diameter (note that the length scales are different on the x and y axes of all profilometer scans). On the other hand, comparing the charge that passed through the microelectrode (that generates Br,) with the volume of silicon which was etched reveals that the current efficiency is relatively high (close to 100%) and depends significantly on the distance between the microelectrode and the surface. Replacing the Pt by an Au microelectrode resulted in gold deposits owing to its anodic dissolution in the presence of bromide. This fact has been used by to ‘microwrite’, i.e. deposit thin lines of gold on a conducting surface. To understand better the mechanism of the etching process by the SECM, we studied the different parameters that affect the process and control its efficiency.
Nature of Oxidant The first process in the electro- and photoefectro-chemical etching of semiconductors involves hole injection via the valence band.29 We showed3* that n-GaAs could be etched using the SECM by generating strong one-electron oxidants as well as bromine. We concluded that the first step that led to n-GaAs etching employing one-electron oxidants was also hole injection. Nevertheless, the chemical etching by bromine of 111-V semiconductors, such as GaAs, is believed to proceed through chemical decomposition rather than by hole injection.29aTherefore, we also attempted to etch silicon by generating strong one-electron oxidants such as Ru(bpy),, +, F e ( ~ h e n ) , ~ + (bpy = 2,2’-bipyridine, phen = 1,lO-phenanthroline) and IrC16,-. Specifically, the reduced state of one of those redox couples, e.g. IrC163-, was added into a solution consisting of 1 mol dm-3 H F and 1 mol dm-3 H,SO, and
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the tip was made to approach the unbiased Si surface while generating (at the tip) the oxidized state, e.g. 1rClG2-. Though the redox potentials of these couples are positive as is that of bromine, no etching pits were observed, even after prolonged generation of these oxidants close to the surface. Note that, in spite of the fact that silicon was not etched by employing one-electron redox couples, the feedback current was positive. The fact that a positive feedback current was observed indicates that electrons were transferred from the surface to the oxidized species. Moreover, this indicates that the holes that were injected into the valence band diffused away and were eliminated by electrons coming from the rest of the wafer. In other words, the silicon surface behaved as a conductor only, exactly in the same way as, for example, platinum would behave in the case of generating F e ( ~ h e n ) , ~ + above it. Note that the driving force for electron transfer from the surface (even though it was not biased) stems, as stated before, from the difference in the Ox/Red ratio beneath the tip and in the solution.23 On the other hand, our attempts to etch silicon by electrogenerating iodine failed. Iodine was adsorbed on the Si surface and resulted in erratic positive feedback currents. The positive feedback current was not followed by any observable structural changes on the surface. Type of Silicon
The effect of the dopant and the orientation of the silicon on the etching was also examined. No significant difference was observed in the etching efficiency between a p- and an n-type Si. In both cases one-electron oxidants did not result in observable etching structures, while local generation of bromine caused similar etching pits. In addition, similar etching rates were obtained with Si (100) which implies that our process is isotropic. We showed3' that the type of dopant of GaAs affects the etching by SECM. n-GaAs and undoped GaAs were etched using one-electron redox couples, while p-GaAs was completely resistive toward etching. The results were interpreted based on the energy-level diagram of GaAs and assumed hole injection as the initial step. In addition, bromine etched n-GaAs three to five times faster than p-GaAs. All these results prove that the chemical etching of silicon in our study proceeds through a different process than 'simple' hole injection.
Bromine Concentration The effect of bromine concentration on the overall rate of etching was investigated. Fig. 3 shows the profiles of etching structures formed by employing different bromide concentrations. The concentration of bromine was controlled by varying the concentration of bromide and applying sufficiently positive potentials to ensure a diffusion-limited process for bromine electrogeneration at the microelectrode. We find that the volume of silicon which was etched (calculated from the profiles) varied linearly with the number of mol of electrogenerated bromine. The latter was calculated by integrating the current over the length of time of the experiment. The current efficiency (moles of Si etched divided by moles of electrons that passed through the microelectrode) was 60 & 30%, assuming a 1 : 1 ratio between bromide and silicon.? Although silicon ends in tetravalent state, a bromine molecule attacks two adjacent silicon atoms, vide infra, whereas the subsequent oxidation is driven by HF. These results also
t The calculated efficiency represents the average of more than five experiments. It should be noted that the relatively low precision is mainly due to the measurement of the etched silicon that is based on the cross-section of the etching pits.
0
2 4
€ 6
-? A
8 10 12
500
1000
1500
xlpm
Fig. 3 Profiles of etching pits obtained as a result of leaving a 50 pm Pt microelectrode( E = 0.66 V) above an n-type Si( 111) wafer in a solution consisting of 1 mol dm-3 H,SO,, 1 rnol dm-3 HF and (a) 5 mmol dmW3HBr, (b) 25 mmol dm-3 HBr and (c) 50 mmol dm-3 HBr for 10 min
suggest that the reaction of bromine with silicon is the ratelimiting step of the whole process.
Fluoride Concentration Fig. 4 shows the profiles of etching pits which were formed using different concentrations of HF. It is apparent that similar etching pits were formed using 0.1 and 1 rnol dm-3 HF at constant pH (the pH was kept constant by adding 1 mol dm-3 H2S0,). Obrriously under these conditions, most H F is unprotonated owing to its low dissociation constant. Decreasing the concentration of hydrogen fluoride more, i.e. using a 0.01 mol dm-3 H F solution, resulted in a negative feedback current and no etching structures were detected. Thus, it is clear that fluoride concentration affects the etching rate, presumably by controlling the dissolution of the native oxide layer. As long as its concentration is kept higher than a certain level (ca. 0.01 mol dm-3) the total etching rate is fluoride-independent. Therefore, dissolution of the oxide layer is not the rate-limiting step in the process.
Acidity The last parameter which was examined was the pH. Fig. 5 shows profiles which were formed in 0.1 and 1 mol dm-3 0 0.2 E
0.4
-3 0.6 34 0.8 1.o
1000
1500 xhm
Fig. 4 Profiles of etching pits formed as a result of passing 1.26 x lo-, C through a 50 pm Pt microelectrode ( E = 0.66 V) which was left above an n-type Si( 11 1) wafer in a solution consisting of 5 mmol dmW3HBr, I mol dma3 H,SO, and (a) 0.1 rnol drn-3 HF, (b) 1.0 rnol dm-3 HF
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I
0
I
100
I
I
I
200
I
1
300
xlpm Fig. 5 Profiles of etching pits formed upon passing 1.2 x loW4C through a 50 pm Pt microelecteode ( E = 0.66 V) which was left above an n-type Si(ll1) wafer in a solution consisting of 5 mmol dm-3 HBr, 1 mol dm-3 HF and (a) 0.1 rnol dm-3 H,SO,; (b) 1.0 mol dm-3 H,SO,
H2S04 at a constant H F concentration (1 mol dm-3). Obviously, the concentration of protons does not affect the rate of etching. The experiments were carefully conducted such that equal charge passed through the microelectrode, which ensured that equal amounts of bromine were generated. However, no etching structures could be detected when the concentration of the acid was lower than 0.01 mol dm-3. The feedback current in this case turned almost instantaneously from positive to negative upon approaching the surface.
Mechanism A detailed mechanism of the overall process must take into account all these observations. Basically, the process involves the local attack of bromine on unbiased silicon in acidic fluoride solutions. Only a small number of reports discuss the chemical etching of silicon by molecular b r ~ m i n e . ~ Fuller and Allison3' mentioned that bromine in methanol exhibited superior etching properties toward GaAs, but etched silicon and germanium less efficiently. Bromine, and in particular, chlorine have been used in the gas phase for silicon e t ~ h i n g . ~Sm ~ 00th - ~ ~ surfaces with moderate etching rates (< 1 pm s-') are obtained at relatively low halogen concentrations and high temperatures. The fundamental steps in silicon etching by halogens (in the gas phase) have been studied and discussed.gbThe desorption products observed in the gas phase depend on the halogen. A general trend was detected, whereby the desorption products switch over from silicon halides to halogen species as the halogen changes from chlorine to iodine. To the best of our knowledge, the only detailed report dealing with the wet etching of silicon by bromine and other oxidants in the presence of fluoride, was published by Gerischer and Lubke.32 Although they proposed a mechanism
which involves hole injection as the first step, they found differences between the process driven by a one-electron oxidant, i.e. IrCl,, -, and multiple electron-transfer agents such as Br, and Mn04-. The different behaviour of bromine us. IrCl,*- was attributed to the adsorption of intermediates on the surface and was assigned as 'chemical oxidation' or 'chemical decomposition'. The chemical decomposition of semiconductors by strong bifunctional oxidants in solutions was also described earlier by Gerischer and Mindt.,'" Such bifunctional oxidants are mainly the halogens and H,O, . The initial step of this process must proceed uia formation of two new bonds with the semiconductor surface, as suggested by the authors. Thus, the attack on exposed Si by bromine (generated at the microelectrode) in our study must have led to the formation of Si-Br bonds (Fig. 6, step 1). Clearly, such attack can be successful only after the removal of the oxide layer by fluoride. The initial dissolution of the native oxide layer depends on the presence of fluoride ions as well as on the acidity. Thus, decreasing the concentration of H F to below a minimum level stopped the etching process, and made the feedback current negative as a result of formation of an oxide layer. The effect of pH on the etching rate of silicon dioxide is well documented.26 A number of studies showed that increasing the acidity had a significant impact on the rate of SiO, dissolution. Several mechanisms were proposed in which the protons were involved in the formation of silanol groups. Thus, we believe that the acidity controls the initial step of SiO, dissolution. If this step is sufficiently fast (requires relatively low pH), the overall rate is determined by the reaction of bromine with silicon. However, fluoride ions also play an important role, not only in the initial dissolution of silicon dioxide but also in the continuous etching process. The polarized Si-Br bond increases the chemical reactivity of the other Si back bonds which are, therefore, more easily attacked by H F (Fig. 6, step 11). We cannot completely exclude the possible cleavage of these bonds by water (Fig. 6, step IIa), although it is conceivable that hydrolysis (by water) occurs efficiently only in alkaline solutions. In this case, the silanol bonds will be subsequently cleaved by F- to form unstable Si-F bonds (Fig. 6, step 111). Finally, the activated Si-H bonds, which are also unstable in aqueous and fluoride media, will undergo nucleophilic attack evolving hydrogen (Fig. 6, step IV). Notice that the stoichiometry between electrogenerated bromine, Br2, and silicon is 1 : 2. Bromine is responsible for the oxidation of silicon only to its + 1 state. This is in accordance with the current efficiency that is obtained. If a higher ratio had been assumed, e.g. two molecules of bromine per one Si atom, a higher current efficiency than 100% would have been resulted. Moreover, the fact that a positive feedback current is observed while etching means that bromide is regenerated very rapidly on the surface. Since H
\ I./Br / '\
' \
\
/ \
Br,
/H HF Si -. F/ \H StepIV
SiF4
a +Br-+H2 SiF6*-
\T.P Br\
/H
Pi\H HO
Fig. 6 Scheme of the proposed mechanism for Si etching using the SECM
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fabricating patterns. Fig. 7 shows an optical micrograph picture of a pattern which was formed using a 10 pm platinum electrode. The microelectrode was scanned laterally at a rate of 0.1 pm s-'. Higher scanning rates resulted in profiles too shallow to be detected, although the lateral resolution might have been higher. Fig. 7 shows also the profile of two of the lines. Interestingly, it can be seen that the etching profile is not symmetrical. Examining the microelectrode surface revealed that the platinum wire was not located at the centre of the glass insulator. Therefore, the flux of electrogenerated bromine was not symmetrical, which caused this peculiar profile. This suggests that the shape and structure of the microelectrode can be used to control the structure of the pattern and its profile.
0.1
c
Conclusions
I
L
0.3k 100
I
I 200
I
xlw
I
I
J
300
Fig. 7 'HU' microwriting etching patterns obtained as a result of scanning (0.1 pm s - l ) a 10 pm Pt microelectrode across an n-type Si(ll1) wafer (other experimental conditions as in Fig. 2). The profile is of a cross-section of the ' U '.
the current efficiency is close to loo%, it strongly indicates that the Si-Br bonds are easily cleaved by fluoride to regenerate the bromide ions. Therefore, the final dissolution products are essentially SiF,2- and SiF, (Fig. 6). This mechanism is in full agreement with our results. The rate-limiting step is the chemical attack of silicon by bromine to form Si-Br bonds. This explains also why other one+ , not lead to Si etching. electron oxidants, e.g. F e ( ~ h e n ) , ~did It is crucial to cleave immediately Si-Si bonds to result finally in silicon etching. The formation of Si-Br bonds scavenges the holes on the Si surface. Note that this behaviour is unique to the SECM technique in which the Si surface is kept unbiased. The anodic dissolution of Si can be driven by one-electron oxidants such as as has been demonstrated by G e r i ~ c h e r .On ~ ~ the other hand, the local injection of holes (in an SECM experiment) into the valence band, i.e. by one-electron oxidants, is insufficient, since the holes are not trapped on the Si surface (beneath the tip) and can diffuse away. This model can also account for the difference in the etching of GaAs us. Si. While holes injected to n-GaAs remain at the electrode/solution interface and result in GaAs oxidation, holes injected to Si must be trapped via the formation of a chemical bond to result eventually in Si etching. The overall etching rate is determined by the initial step as long as the concentrations of protons and fluoride ions are sufficient to maintain an oxide-free Si surface as well as to dissolve effectively the etching products. Otherwise, these steps will govern the overall etching rate. Optimum conditions for silicon etching by the SECM require, therefore, a certain concentration of fluoride ions and protons as well as high bromide concentrations. Finally, we took advantage of the SECM capability to move the microelectrode laterally as a means of micro-
The formation of etching structures in silicon by the scanning electrochemical microscope technique has been demonstrated. The SECM made it possible to etch silicon with spatial resolution in one step without the requirement of lithographic steps, such as masking. Furthermore, the SECM enables the quantitative study of the etching process by following the feedback current and examining the etching structures. Once again, the SECM has proven its versatility as a modifying as well as an analytical tool. Natan Zeldes from Intel Electronics (Jerusalem, Israel) is acknowledged for the profilometer measurements. This research was supported by the Basic Research Foundation administrated by the Israel Academy of Sciences and Humanities.
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Paper 4/05445H;Received 6th September, 1994
