Actuators (Transducers 87), Tokyo, Japan, 1987, 827-829. 18. M. Mehregany, C.A. Zorman, N. Rajah, and C.H. Wu, Proc. IEEE, vo186 1998, 1594-1610. 19.
Materials Issues in MEMS
Materials issues in MEMS H. Kahn and A.H. Heuer, Department of Materials Science and Engineering, Case Western Reserve University, Cleveland, Ohio USA S.J. Jacobs, Texas Instruments, Dallas, Texas USA IIIII
The field of microelectromechanical systems (MEMS) involves the interaction of the physical environment (either passively using sensors, or actively using actuators, or both) with electrical signals, through the use of small ("micro") batch-fabricated devices. It takes advantage of many of the wafer-level processing technologies developed for integrated circuits (ICs), while providing new capabilities of sensing and actuating. (It has also established its own, often confusing, jargon; the terms "microstructure" and "mechanism"in the MEMS community both refer to devices.) MEMS is currently one of the fastest growing technologies in microelectronics, in terms of published research papers, scientific conferences, potential application areas, and research funding. Commercial M E M S p r o d u c t s are becoming commonplace, including pressure transducers (which have been available since the 1970s), accelerometers, inkjet print heads, chemical sensors, and projection displays. As the field has matured, the impetus for advancement has made some shift from device design to material design. The best devices are being recycled; for example, simple resonators, which have served well as accelerometers, are being used with only minor modifications as gyroscopes[I] and micromechanical filters[2]. Fabricating the standard polysilicon accelerometer from a different material, such as nickel, can be a great innovation. To optimize MEMS designs, the materials properties must be thoroughly characterized and controlled. Optimal performance often involves size reduction, usually to an unexplored size regime. Higher device density on a wafer results in lower cost per part; smaller gaps between capacitive plates
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provide higher electrostatic actuation forces and greater output signals; smaUer mirrors give better optical resolution. The tradeoff is concomitant tighter tolerances. The materials deposition, patterning, and etching must be uniform, repeatable, and precise. For reduced lateral dimensions, small deflections result in large strains. Device designers need to know the allowable strain limits, as well as other materials properties such as Young's modulus, fracture strength and fracture toughness, thermal conductivity, etc. While these values are generally known for bulk materials, it is not d e a r that they will be valid for the materials with unique and characteristic microstructures and morphologies which result from MEMS processing. Also, as devices shrink, some features of the device, such as a tether beam, could be so small as to contain only a few grains. Even if the device were comprised of a moderately isotropic material, such as polysilicon, the orientation of those grains would play an important role. On the other hand, some designs improve with increasing size. Larger plates provide greater capacitances; higher masses improve inertial measurements; larger motions create greater pumping volumes; larger diaphragms improve pressure sensing sensitivity. Increased size, however, also creates complications. Larger devices are harder to release from the substrate, more susceptible to processing defects, and tend to be more fragile. The structures must be very uniform and flat across large distances, w h i c h requires precise control of materials deposition and residual stresses. Greater surface areas exacerbate problems with stiction, the phenomenon of two surfaces adhering after coming into contact.
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Bulk Micromachining MEMS devices are fabricated by two generic technologies: bulk micromachining, in which material is removed from a substrate, and surface micromachining, in which material is added to a substrate, in order to create structures. More details can be found in [3]. Several on-line tutorials are also available, many of which are catalogued at http://mems.isi.edu. Bulk micromachining was the first of the two technologies to be explored, using anisotropic silicon etchants and single-crystal wafers. These etchants, such as KOH, TMAH (tetramethyl ammonium hydroxide), and EDP (ethylene diamine pyrocatechol), etch sil-
icon much more slowly in the (111) directions than in the other crystallographic directions, and also etch heavily boron-doped (p+) silicon, silicon dioxide, and silicon nitride m u c h more slowly than lightly doped silicon. With the appropriate masking, deep channels can be formed in silicon wafers, with or without overlying diaphragms, beams, and bridges. In this manner, the first MEMS pressure transducers were fabricated. For these devices, holes were etched through silicon wafers, leaving just thin membrane diaphragms which contained piezoresistive silicon resistors[4].
Materials W h e n exposed to an increased pressure through the hole, the diaphragm w o u l d deflect outwards, stretching the piezoresistors to generate the signal. A portion of a bulk-micromac h i n e d f l o w sensor is s h o w n in Figure 115]. The SixNy-Ni bridges, which use electrical signals to create and sense a temperature pulse in the flowing medium, are seen s u s p e n d e d over the etched channel. O n e materials problem w h i c h had to be solved for this application was t h e development of a low-stress silicon-rich SixNy film, since chemical-vapor-deposited (CVD) stoichiometric Si3N 4 has too high residual c o m p r e s s i v e stresses to be used[6]. The recent d e v e l o p m e n t o f deep-silic o n r e a c t i v e i o n e t c h i n g (DRIE), w h i c h can e t c h t h r o u g h an entire wafer w i t h nearly vertical sidewalls, has greatly e x p a n d e d the capabilities of bulk micromachining[7-9]. Since the e t c h e d areas are no longer bounded b y the (111) planes, any shapes can b e realized, i n c l u d i n g c u r v e d m i c r o t u r b i n e blades[10]. Very thick, high a s p e c t ratio structures can also be fabricated.These devices will have the advantages of increased mass and s i d e w a l l c a p a c i t a n c e w i t h o u t increased lateral dimensions. In addition, the DRIE p r o c e s s is m o r e compatible w i t h IC p r o c e s s i n g than the w e t etchants, w h i c h often involve p o t e n t i a l contaminants. Besides structural materials, MEMS also utilize active materials as sensors and actuators. In addition to the silicon piezoresistive strain sensor and the nickel temperature sensor previously mentioned, MEMS devices can be used to d e t e c t oxygen, hydrogen, humidity, magnetic and electric fields, and o t h e r environmental conditions. Actuator materials are those w h i c h c o n v e r t electrical (or other) signals into physical motion. Actuator schemes differ in such aspects as force generated, extent of motion, p o w e r consumption, etc., and various applications will call for different actuators. For example, MEMS microvalves have b e e n designed w i t h actuators utilizing a variety of behaviors, including electrostatic [ 11 ], piezoelectric[12], s h a p e m e m o r y [ 1 3 , 1 4 ] , m a g n e t i c [ 1 5 ] , bimetallic[16], and
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thermopneumatic[17] phenomena. Due to space limitations, sensor and a c t u a t o r materials will n o t be discussed further in this article.
Surface Micromachining Structural materials for surface-micromachined d e v i c e s are t y p i c a l l y d e p o s i t e d in o n e o f t w o ways: CVD and electroplating. By far, the most c o m m o n CVD m a t e r i a l u s e d for MEMS is polysilicon, t h o u g h o t h e r m a t e r i a l s have b e e n i n v e s t i g a t e d , including SiC[18], Ge[19], GaAs[20],
W[21], and diamond[22]. Some issues o f c o n c e r n for p o l y s i l i c o n MEMS devices include residual stress, stress gradients t h r o u g h the film thickness, surface roughness, and statistical variations of the effective Young's modulus due to variations in the local m i c r o s t r u c t u r e s [ 2 3 ] . T h e relationships b e t w e e n the d e p o s i t i o n parameters and s u b s e q u e n t heat treatments, and the resulting polysilicon m i c r o s t r u c t u r e s and stress states, have b e e n e x a m i n e d in s o m e detail[23]. Some s c h e m e s for achieving suitable films include l o w t e m p e r a t u r e deposition followed b y annealing[24], very
Materials
high t e m p e r a t u r e deposition[25,26], and d e p o s i t i o n of alternating multilayers of l o w t e m p e r a t u r e (tensile) and high t e m p e r a t u r e ( c o m p r e s s i v e ) polysilicon[27]. Etching thick polysilicon films can be achieved using the DRIE t e c h n i q u e s discussed above. Figure 2 is a surface-micromachined polysilic o n m i c r o m o t o r ; it s u p p o r t s a m i r r o r w h i c h c a n b e t u r n e d for o p t i c a l deflection[28]. Recently, an alternative to e t c h i n g w a s d e v e l o p e d w h e r e b y m o l d s are defined in the substrate, followed b y d e p o s i t i o n of the structural material in w h i c h the features are defined by l a p p i n g and p o l i s h i n g b a c k to t h e t o p of t h e molds.This is analogous to the "damascene" p r o c e s s o f f o r m i n g c o p p e r i n t e r c o n n e c t s for ICs[29], and has b e e n d e m o n s t r a t e d for b o t h polysilic o n ( k n o w n as HexSil)[23], and polycrystalline SiC[30]. The o t h e r class of surface-micromachining involves e l e c t r o p l a t i n g metal structures into pre-formed molds.This was first realized by t h e LIGA (lithographie galvanoformung a b f o r m u n g ) process, w h i c h utilizes x-ray lithograp h y w i t h PMMA as the p h o t o p o l y m e r [ 3 1 , 3 2 ] . In brief, a sacrificial release layer is d e p o s i t e d on t h e substrate, followed b y the electroplating seed layer and PMMA, w h i c h is t h e n p a t t e r n e d into a mold. The structural metal is e l e c t r o p l a t e d into the m o l d w h i c h is subsequently removed. In this manner, v e r y high a s p e c t ratio structures can be formed. The metals u s e d i n c l u d e nickel, c o p p e r , a n d p e r m a l l o y . N e w p h o t o r e s i s t s have r e c e n t l y b e e n d e v e l o p e d w h i c h can form v e r y thick coatings and still b e e x p o s e d b y UV light to create suitable m o l d s for e l e c t r o p l a t i n g [ 3 3 ] . This a l l o w s e l e c t r o p l a t e d MEMS to b e
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stiction by successive rinsing with a series of low surface tension liquids, terminating in a sublimation or supercritical drying step[41-43]. Dry methods that minimize the o c c u r r e n c e of release-related stiction i n c l u d e HF v a p o r phase etching and plasma ashing of photoresist[44-47].
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