420
`
`PROCEEDINGS OF THE EEE. VOL. 70, NO. 5, MAY 1982
`Silicon as a Mechanical Material
`KURT E. PETERSEN, MEMBER, IEEE
`
`I
`
`Abrtmct4h$eaysbl citioon is being inaerdaety employed m a
`be
`miniaturized mechanical devies and components must
`integrated or interfaced with electronics such as the exampleg
`w i e t y o f n e w a r m m e r c h l ~ ~ ~ n o t b e a u s e o f i t s w e l l - ~ d
`electronicpropgties,bptratherbeauseofitsexcdlentmeduuiai
`given above.
`of silicon micromechanical
`properties Jnrddition,rscenttreadsmtheengineerine~mdi-
`The continuing development
`cateagrowingmtesestmtheuseofdiowasamechdulmatedrl
`withthedtimategotlofdedophgabrodnqgeofinexpcdve,
`applications is only one aspect of the current technical drive
`batdl-fabriaw bigh-pdl3?mmce rea#rcl d tnnsducenl which m
`toward miniaturization which is being pursued over a wide
`easily rnterfrced witb tbe rapidly pdifeating mi-.
`m a
`front in many diverse engineering disciplines. Certainly silicon
`review d g a i the .dpmt.ges o f . a a p l o y i a g rilicon as a mech.nicrl
`microdectronics continues to be the most obvious success in
`materid,therdevaotmecbrdaldum&&tbofIliticon,mdthep
`the ongoing pursuit of miniaturization. Four factors have
`played crucial roles in this phenomenal success story: 1) the
`I U C t O I e 3 ,
`l g W h i C h U e ~ t O ~ S t
`~ t s c h n i q l
`
`Fmrly,thepotcathbofthisnewtechadogymahgtritedbynama
`o o s d e $ d e d e x a m p k f r o m t b e t i ~ ItisdazthatsPiEonwiU
`active material, silicon, is abundant, inexpensive, and can now
`mntinne,tobeagpwidyellploitedmawidevPietyofmcdudcd
`be produced and processed controIlably to unparalleled stan-
`dards of purity and perfection; 2) silicon processing itself is
`~ ~ c a n p k m m ~ t o i t n d i t i o n r l r d e a s m ~
`Furthemnore, thge muWWphmy wes of silicm wiU
`ma-
`based on very thin deposited films which are highly amenable
`s@ifianttyrltathewaywethinLrboPtrIltypgofmiaiatnremb
`~ d e p i c a m d c o m p o a e n t a
`of the
`to miniaturization; 3) defmition and reproduction
`device shapes and patterns are performed using photographic
`techniques which have also, historically, been capable of high
`I. INTRODUCTION
`N THE SAME WAY that silicon has already revolutionized
`to miniaturization; fmally, and most
`precision and amenable
`of
`the way we think about electronics, this versatile material is
`important of all
`from a commercial and practical point
`view, 4) silicon microelectronic circuits are batch-fabricated.
`now in the process of altering conventional perceptions of
`miniature mechanical devices and components [ 1 1. At least
`The unit of production for integrated circuits-the
`wafer-is
`not one individual saleable item, but contains hundreds of
`eight fums now manufacture and/or market silicon-based pres-
`identical chips. If this were not the case, we could certainly
`sure transducers [ 21 (first manufactured commercially over 10
`never afford to install microprocessors in watches or micro-
`years ago), some with active devices or entire circuits integrated
`wave ovens.
` silicon chip and some rated up to 10 000 psi
`on the b
`e
`Texas Instruments has been marketing a thermal point head
`It is becoming clear that these same four factors which have
`[ 31 in several computer terminal and plotter products in which
`been responsible for the rise of the silicon microelectronics
`industry can be exploited in the design and manufacture of a
`the active printing element abrasively contacting the paper is a
`silicon integrated circuit chip. The crucial detector component
`wide spectrum of miniature mechanical devices and compo-
`nents. The high purity and crystalline perfection of available
`of a high-bandwidth frequency synthesizer sold by Hewlett-
`silicon is expected to optimize the mechanical properties of
`Packard is a silicon chip [4] from which cantilever beams have
`the same way that electronic
`devices made from silicon in
`been etched to provide thermally isolated regions for the diode
`properties have been optimized to increase the performance,
`detectors. High-precision alignment and coupling assemblies
`systems are produced by
`for fiber-optic communications
`reliability, and reproducibility of device characteristics. Thin-
`film and photolithographic fabrication procedures
`silicon chips
`make it
`Western Electric from anisotropically etched
`simply because this is the only technique capable of the high
`possible to realize a great variety of extremely small, high-
`accuracies required. Within IBM, ink jet nozzle arrays and
`precision mechanical structures using the same processes that
`have been developed for electronic circuits. High-volume
`charge plate assemblies etched into silicon wafers [ 51 have
`batch-fabrication techniques can be utilized in the manufac-
`been demonstrated, again because of the high precision capa-
`ture of complex, miniaturized mechanical components which
`bilities of silicon IC technology. These examples of silicon
`may not be possible by any other methods. And, finally, new
`micromechanics are not laboratory curiosities. Most are well-
`concepts in hybrid device design and broad new areas of appli-
`established, commercial developments conceived within about
`cation, such as integrated sensors [6], [7] and silicon heads
`the last 10 years.
`(for printing and data storage), are now feasible as a result of
`The basis of micromechanics is that silicon, in conjunction
`with its conventional role as an electronic material, and taking
`the unique and intimate integration
`of mechanical and elec-
`tronic devices which is readily accomplished with the fabrica-
`advantage of an already advanced microfabrication technology,
`can also be exploited as a high-precision highstrength high-
`tion methods we will be discussing here.
`While the applications are diverse, with significant potential
`reliability mechanical material, especially applicable wherever
`impact in several areas, the broad multidisciplinary aspects of
`silicon micromechanics also cause problems. On the one hand,
`the materials, processes, and fabrication technologies are all
`taken from the semiconductor industry. On the other hand,
`the applications are primarily in the areas of mechanical en-
`
`Manuscript received Deeember 2,198l;revised March 11,1982. The
`submission of thie paper was encouraged after the review of an advance
`ProPosaL
`The author was with IBM Research Labaatory, S m Jaw. CA 95193.
`He is now with Tnnaensory Devices, Fremont, CA 94539.
`0018-9219/82/0500-0420$00.75 @ 1982 IEEE
`
`1
`
`Petitioner Samsung - SAM1010
`
`

`
`PETERSEN: SILICON AS A MECHANICAL MATERIAL
`
`42 1
`
`TABLE I
`
`.-
`
`Yield
`Strength
`(10’0dync/m2)
`
`Knoop
`Hardness
`(kg/mm2)
`
`Young’s
`Density
`Modplvs
`(1012 dyac/m2) ( g r / c m 3 )
`
`nerrml Thed
`Erpoarion
`Conductivity
`(W/cm”C)
`(l@/OC)
`
`53
`
`
`
`*Diamond
`*Sic
`‘TIC
`
`
`
`10.35
`7.0
`21
`20
`
`15.4 5.3
`3.85
`14
`12.6
`8.4
`7.0
`0.97
`4.2
`4.0
`2.1
`2.1
`0.17
`
`
`
`4.1
`
`
`
`
`
`
`
`5.4
`
`
`7000
`2480
`2470
`2100
`
`3486
`400
`
`
`
`
`
`
`
`10.3
`
`1500 7.9
`
`485
`660
`275
`130
`
`4.97
`0.5
`
`1.96
`
`1.9
`2.1
`
`2.0 0.329
`
`3.43
`
`0.70
`
`
`
`
`
`3.5
`3.2
`4.9
`4.0
`3.1
`7.8
`2.5
`2.3
`
`7.9
`
`2.7
`
`20
`3.5
`3.3
`
`0.19 0.8
`0.803
`0.014
`1.57
`
`1.78
`
`1.38
`2.36
`
`1 .o
`3.3
`6.4
`
`12
`0.55
`2.33
`12
`4.5
`17.3
`5.0
`25
`
`*Si3N4
`‘Iron
`SiO, (fibers)
`820
`0.73
`
`8
`50
`
`*Si
`Steel (mu. strength)
`19.3
`
`W
`Stainless Stal
`M O
`Al
`‘Single crystal. See Refs. 8. 9, 10, 1 1 , 141, 163, 166.
`
`gineering and design. Although these two technical fields are
`now widely divergent with limited opportunities for communi-
`cation and technical interaction, widespread, practical exploi-
`tation of the new micromechanics technology in the coming
`years will necessitate an intimate collaboration between work-
`ers in both mechanical and integrated circuit engineering dis-
`ciplines. The purpose of this paper, then, is to expand the
`lines of communication by reviewing the area of silicon micro-
`mechanics and exposing a large spectrum of the electrical
`engineering community to itscapabilities.
`In the
`following section, some of
`the relevant mechanical
`aspects of silicon will be discussed and
`compared to other
`more typical mechanical engineering materials.
`Section I11
`describes the major “micromachining” techniques which have
`been developed to form the silicon “chips” into a wide variety
`of mechanical structures with IC-compatible processes ame-
`nable to conventional batch-fabrication. The next four sections
`comprise an extensive list of both commercial and experimen-
`on the ability to construct
`tal devices which rely crucially
`miniature, high-precision, high-reliability, mechanical struc-
`tures on silicon. This list was compiled with the primary pur-
`pose of illustrating the wide range of demonstrated applica-
`tions. Finally, a discussion of present and future trends will
`wrap things up in Section VIII. The underlying message is that
`silicon micromechanics is not a diverging, unrelated, or inde-
`pendent extension of silicon microelectronics, but rather
`a
`natural, inevitable continuation of the trend
`toward more
`complex, varied, and useful integration of devices on silicon.
`
`11. MECHANICAL CHARACTERISTICS OF SILICON
`Any consideration of mechanical devices made from silicon
`must certainly take into account the mechanical behavior and
`properties of single-crystal silicon (SCS).
`Table I presents a
`comparative list of its mechanical characteristics. Although
`SCS is a brittle material, yielding catastrophically (not unlike
`most oxide-based glasses) rather than deforming plastically
`(like most metals), it certainly is not as fragile as is often
`believed. The Young’s modulus of silicon (1.9 X 10” dyne/
`cm2 or 27 X lo6 psi) 181, for example, has a value approach-
`ing that of stainless steel, nickel, and well above that of quartz
`and most other borosilicate, soda-lime, and lead-alkali silicate
`glasses [ 91. The Knoop hardness of silicon (850) is close to
`quartz, just below chromium (939, and almost twice as high
`as nickel (557), iron, and most common glasses (530) [ 101.
`Silicon single crystals have a tensile yield strength (6.9 X 10”
`
`in silicon single crystals
`Fig. 1. Stresses encountered commonly
`are
`the growth of large boules. Seed crystals, typically
`very high during
`0.20 cm in diameter and supporting 40-kg boules, experience stresses
`over 1.25 X lo* Pa or about 18 000 psi in tension.
`dyne/cm2 or lo6 psi) which is at least 3 times higher than
`[ 81, [ 1 1 1.
`In practice, tensile stresses
`stainless-steel wire
`routinely encountered in seed crystals during the growth of
`large SCS boules, for example, can be over 18 000 psi (40-kg
`as illus-
`boule hanging from a 2-mm-diameter seed crystal,
`trated in Fig. €). The primary difference is that silicon wiU
`yield by fracturing (at room temperature) while metals usually
`yield by deforming inelastically.
`trouble
`Despite this quantitative evidence, we might have
`intuitively justifying the conclusion that silicon is a strong
`mechanical material when compared with everyday laboratory
`and manufacturing experience. Wafers do break-sometimes
`without apparent
`provocation; silicon wafers
`and parts of
`also easily chip. These
`occurrences are due to
`wafers may
`several factors which have contributed to the misconception
`that silicon is mechanically fragile. First, single-crystal silicon
`is normally obtained in large (5-13-cm-diameter) wafers, typi-
`cally only 10-20 mils (250 to 500 pm) thick. Even stainless
`
`2
`
`

`
`422
`
`PROCEEDINGS OF THE IEEE, VOL. 70, NO. 5, MAY 1982
`
`steel of these dimensions is very easy to deform inelastically.
`Silicon chips with dimensions on the order of 0.6 cm X 0.6
`cm, on the other hand, are
`relatively rugged under normal
`handling conditions unless scribed. Second, as a singlecrystal
`material, silicon has a tendency to cleave along crystallographic
`planes, especially if edge, surface, or bulk imperfections cause
`stresses to concentrate and orient along cleavage planes. Slip
`lines and other flaws at the edges of wafers, in fact, are usually
`responsible for wafer breakage. In recent years, however, the
`semiconductor industry has attacked this yield problem by
`contouring the edges of wafers and by regularly using wafer
`edge inspection instruments,
`specifically designed to detect
`mechanical damage on wafer edges and also to assure that
`edges are properly contoured
`to avoid the effects of stress
`concentration. As a result of these quality control
`improve
`ments, wafer breakage has been greatly reduced and the intrin-
`sic strength of silicon is closer to being realized in practice
`is also a potential
`during wafer handling Third, chipping
`problem with briffle materials such as SCS. On whole wafers,
`chipping occurs for the same qualitative reasons as breaking
`and the solutions are identical. Individual die, however, are
`subject to chipping as a result of saw- or scribinduced edge
`damage and defects. In extreme cases, or during rough han-
`dling, such damage mn also cause breakage of or cracks in in-
`dividual die. Finally, the high-temperature processing and
`multiple thin-film depositions commonly encountered in
`the
`fabrication of IC devices unavoidably result in internal stresses
`which, when coupled with edge, surface, or bulk imperfections,
`can cause concentrated stresses and eventual fracture along
`cleavage planes.
`These factors make it clear that although high-quality SCS
`is intrinsically strong, the apparent strength of a particular
`mechanical component or device will depend on its crystallo-
`graphic orientation and geometry, the number and
`size of
`surface, edge, and bulk imperfections, and the stresses induced
`and accumulated during
`growth, polishing, and subsequent
`processing. When these considerations have been properly
`accounted for, we can hope to obtain mechanical components
`with strengths exceeding that of the highest. strength alloy
`steels.
`General rules to be observed in this regard, which will be
`restated and emphasized in the following sections, can be for-
`mulated as follows:
`1) The silicon material should have the lowest possible bulk,
`surface, and edge crystallographic defect density to minimize
`potential regions of stress concentration.
`2) Components which might be subjected to severe friction,
`abrasion, or stress should be as small as possible to minimize
`the total number of crystallographic defects in the mechanical
`structure. Those devices which are never significantly stressed
`or worn could be quite large; even then, however, thin silicon
`wafers should be mechanically supported by some technique-
`such as anodic bonding to glass-to suppress the shock effects
`encountered in normal handling and transport.
`3) All mechanical processing such as sawing, grinding, scrib-
`ing, and polishing should be minimized or eliminated. These
`operations cause edge and surface imperfections which could
`result in the chipping of edges, and/or internal strains sub-
`quently leading to breakage. Many micromechanical compo-
`be separated from
`the wafer, for
`nents should preferably
`example, by etching rather than by cutting.
`4) If conventional sawing, grinding, or other mechanical
`operations are
`necessary, the affected surfaces and
`edges
`should be etched afterwards
`to remove the highly damaged
`regions.
`
`5 ) Since many of the structures presented below employ
`it often happens
`that sharp edges and
`anisotropic etching,
`comers are formed. These features can also cause accumula-
`tion and concentration of stress damage in certain geometries.
`The structure may require a subsequent isotropic etch or other
`smoothing methods to round such corners.
`6) Tough, hard, corrosion-resistant,
`thin-frlm coatings such
`as CVD Sic [ 121 or Si3N4 should be applied to prevent direct
`mechanical contact to the silicon itself, especially in applica-
`tions involving high stress and/or abrasion.
`as high-
`7) Low-temperature processing techniques such
`pressure and plasma-assisted oxide growth and CVD deposi-
`tions, while developed primarily for VLSI fabrication, will be
`just as important in applications of silicon micromechanics.
`High-temperature cycling invariably results in high stresses
`within the wafer due to the differing thermal coefficients of
`expansion of the various doped and deposited layers. Low-
`temperature processing will alleviate these thermal mismatch
`stresses which otherwise might lead to breakage or chipping
`under severe mechanical conditions.
`As suggested by 6) above, many of the structural or mechan-
`ical disadvantages of SCS can be alleviated by the deposition
`of passivating thin films. This aspect of micromechanics im-
`parts a great versatility to the technology. Sputtered quartz,
`for example, is utilized routinely by industry to passivate IC
`chips against airborne impurities and mild atmospheric corro-
`sion effects. Recent
`advances in the CVD deposition (high-
`temperature pyrolytic
`and low-temperature RF-enhanced)
`of Sic I121 have produced thin films of extreme hardness,
`essentially zero porosity, very high chemical corrosion resis-
`tance, and superior wear resistance. Similar films are already
`to protect pump and valve parts for han-
`used, for example,
`dling corrosive liquids. As seen in Table I, SisN4, an insulator
`IC structures, has a hardness
`which is routinely employed in
`second only to diamond and is sometimes even employed as a
`m s p e e d , rolling-contact bearing material [ 131, 1141. Thin
`films of silicon nitride will also fiid important uses in silicon
`micromechanical applications.
`On the other end of the thin-film passivation spectrum, the
`by Union Carbide for
`gas-condensation technique marketed
`depositing the polymer parylene has been shown to produce
`virtually pinhole-free, low-gorosity, passivating films in a high
`polymer form which has exceptional point, edge, and hole
`coverage capability [ 151. Parylene has been used, for example,
`to coat and passivate implantable biomedical sensors and
`electronic instrumentation. Other techniques have been devel-
`oped for the deposition of polyimide films which are already
`used routinely within
`the semiconductor industry [ 161 and
`which also exhibit superior passivating characteristics.
`One excellent example of the unique qualities of silicon in
`the realization of high-reliability mechanical components can
`the analysis of mechanical fatigue in SCS s t r u e
`be found in
`tures. Since the initiation of fatigue cracks occurs almost ex-
`clusively at the surfaces of stressed members, the rate of fatigue
`depends strongly on surface preparation, morphology, and
`In particular, structural components with
`defect density.
`highly polished surfaces have higher fatigue strengths than
`finishes as shown in Fig. 2 1171.
`those with rough surface
`Passivated surfaces of polycrystalline metal alloys (to prevent
`intergrain diffusion of Hz0) exhibit higher fatigue strengths
`than unpassivated surfaces, and, for the
`same reasons, high
`water vapor content in the atmosphere during fatigue testing
`will significantly decrease fatigue strength. The mechanism
`of fatigue, as these effects illustrate, are ultimately dependent
`on a surface-defect-initiation process. In polycrystalline ma-
`
`3
`
`

`
`PETERSEN: SILICON AS A MECHANICAL MATERIAL
`
`423
`
`120
`
`1 .o
`0.1
`0.01
`Surface Roughness (urn1
`Fig. 2. Generally, mechanical qualities such as fatigue and yield strength
`improve dramatically with surface roughness and defect density. In
`the case of silicon, it is well known that the electronic and mechanical
`perfection of SCS surfaces has been an indispensable part of inte-
`grated circuit technology. Adapted from Van Vlack [ 171.
`
`Fig. 3. A rotating MNOS disk storage device demonstrated by Iwamura
`et al. 121 1. The tungsten-carbide probe is in direct contact with the
`nitride-coated silicon wafer as the wafer rotates at 3600 rlmin. Sig-
`nals have been recorded and played back on such a system at video
`rates. Wear of the WC probe was a more serious problem than wear
`of the silicon disk.
`
`surface defects can be inclusions, grain bound-
`terials, these
`aries, or surface irregularities which concentrate local stresses.
`It is clear that the high crystalline perfection of SCS together
`with the extreme smoothness and surface perfection attainable
`by chemical etching of silicon should yield mechanical struc-
`tures with intrinsically high fatigue strengths [ 181. Even
`greater strengths of brittle materials can be expected with
`additional surface treatments [ 91. Since hydrostatic pressure
`has been shown to increase fatigue strengths [ 191, any film
`which places
`the silicon surface under compression should
`decrease the initiation probability of fatigue cracks. Si3N4
`films, for example, tend to be under tension [20] and there-
`fore impart a compressive stress on the underlying silicon sur-
`face. Such films may be
`employed t o increase the fatigue
`strength of SCS mechanical components.
`In addition, the
`smoothness, uniformity, and high yield
`strength of these
`thin-fim amorphous materials should enhance overall com-
`ponent reliability.
`A new rotating disk storage technology which has recently
`been demonstrated by Iwamura et al. [ 2 1 J not only illustrates
`some of the unique advantages derived from the use of silicon
`as a mechanical material but also indicates how well silicon,
`combined with wear-resistant Si3N4 films, can perform in
`demanding mechanical applications. As indicated in Fig. 3,
`
`an MNOS chargestorage
`data storage was accomplished by
`process in which a tungsten carbide probe is placed in direct
`contact with a 3-in-diameter silicon wafer, rotating at 3600
`r/min. The wafer is coated with 2-nm Si02 and 49-nm Si3N4,
`while the carbide probe serves as the top metal electrode.
`Positive voltage pulses applied to the metal probe as the silicon
`passes beneath will cause electrons to tunnel through the thin
`SiOz and become trapped in the Si3N4 layer. The trapped
`charge can be detected as a change in capacitance through the
`same metal
`probe, thereby
`allowing the signal to be read.
`Iwamura et al. wrote and read back video signals with this
`device over
`lo6 times with little signal degradation, at data
`densities as high as 2 X lo6 bitslcm' . The key problems
`encountered during this experiment
`were associated with
`wear of the tungsten carbide probe, not of the silicon substrate
`or the thin nitride layer itself. Sharply pointed probes, after
`scraping over the Si3N4 surface for a short time, were worn
`down to a 10+m by 10+m area, thereby increasing the active
`recording surface per bit and decreasing the achievable bit
`density. After extended operation, the probe continued
`to
`wear while a barely resolvable 1-nm roughness was generated
`in the hard silicon nitride film. Potential storage densities of
`lo9 bits/cm2 were projected if appropriate recording probes
`were available.
`Contrary to initial impressions, the rapidly
`rotating, harshly abraided silicon disk is not a major source of
`problems even in such
`a severely demanding mechanical
`application.
`
`111. MICROMECHANICAL PROCESSING TECHNIQUES
`Etching
`applications of old
`Even though new techniques-and novel
`techniques-are continually being developed for use in micro-
`mechanical structures, the most powerful and versatile process-
`ing tool continues to be etching. Chemical etchants for silicon
`are numerous. They can be isotropic or anisotropic, dopant
`dependent or not, and have varying degrees
`of selectivity to
`silicon, which determines the appropriate masking material(s).
`Table I1 gives a brief summary of the characteristics of a num-
`ber of common wet silicon etches. We will not discuss plasma,
`reactive-ion, or sputter etching here, although these techniques
`may also have a substantial impact on future silicon micro-
`mechanical devices.
`Three etchant systems are of particular interest due to their
`versatility: ethylene diamine, pyrocatechol, and water (EDP)
`[22] ; KOH and water [23 ] ; and HF, HNOJ, and acetic acid
`CH300H (HNA) [ 241, [ 25 1. EDP has three properties which
`make it indispensable for micromachining: 1) it is anisotropic,
`making it possible to realize unique geometries not otherwise
`feasible; 2 ) it is highly selective and can be masked by a variety
`of materials, e.g., SiOz, Si3N4, Cr, and Au; 3) it is dopant de-
`pendent, exhibiting near zero etch rates on silicon which has
`been highly doped with boron [26],[27].
`KOH and water is also orientation dependent and, in fact,
`exhibits much higher (1 10)-to-(1 11) etch rate
`ratios than
`EDP. For this reason, it is especially useful for groove etching
`on (1 10) wafers since the large differential etch ratio permits
`deep, high aspect ratio grooves with minimal undercutting of
`the masks. A disadvantage of KOH is that SiOz is etched at a
`rate which precludes its use as a mask in many applications.
`In structures requiring long etching times, Si3N4 is the pre-
`ferred masking material for KOH.
`HNA is a very complex etch system with highly variable etch
`rates and etching characteristics dependent on the
`silicon
`dopant concentration
`[28], the-mix ratios of the three etch
`
`4
`
`

`
`424
`
`PROCEEDINGS OF THE IEEE, VOL. 70, NO. 5, MAY 1982
`
`Etchant
`(Diluent)
`
`Typical
`Compo- Temp Rate Etch Rate
`
`
`
`siticm
`‘C
`
`Etch
`(pmlmin)
`
`TABLE I1
`Anisotropic
`(100)/(111)
`Ratio
`
`Doplnt
`Dependence
`
`Masking Filrm
`(etch ntc of nu&) Rcferel~cs
`
`HF
`HNo3
`(water,
`CHqCOOHl
`
`10 ml
`3Oml
`60 ml
`
`
`
`
`
` 22 0.7-3.0 1:l
`
`i 10”cm-3 n or p
`rcdncuetch nte
`by about I S 0
`
`S i
`
` (3OOA/min)
`
`24.25.28.30
`
`22
`
`25 ml
`50ml
`25 ml
`9 J
`75 J 22
` 7.0
`3od
`
`40
`
`1 : 1
`
`110 dependence
`
`ad44
`
`1:l
`
`--
`
`si* (700Undn)
`
`components, and even the degree of etchant agitation, as
`shown in Fig. 4 and Table 11. Unfortunately, these mixtures
`can be difficult to mask, since SiOz is etched somewhat for all
`mix ratios. Although Si02 can be used for relatively short
`etching times and Si3N4 or Au can be used for longer times,
`the masking characteristics are not as desirable as EDP in
`micromechanical s t r u c ~ where very deep patterns (and
`therefore highly resistant masks) are required.
`As descriied m detail by scvefal authors, SCS etching takes
`place in four basic steps [30], [3 1 I : 1) injection of hales into
`the semiconductor to raise the silicon to a higher oxidation
`state Si*, 2) the attachment of hydroxyl groups OH- to the
`positively charged Si, 3) the reaction of the hydrated silicon
`with the complexing agent in the solution, and 4) the dissolu-
`tion of the reacted products into the etchant solution. This
`process implies that any etching solution
`must provide a
`source of holes as well as hydroxyl groups, and must also con-
`tain a complexing agent whose reacted species is soluble in the
`etchant solution. In the HNA system, both the holes and the
`hydroxyl groups are effectively supplied by the strong oxidiz-
`ing agent HN03, while the flourine from the HF forms the
`soluble species H2SiF6. The overall reaction is autocatalytic
`since the HNO3 plus trace impurities of HN02 combine to
`form additional HNOz molecules.
`HN02 + HN03 + H20 + 2HN02 + 20H- + 2h*.
`This reaction also generates holes needed to raise the oxida-
`tion state of the silicon as well as the additional OH- groups
`necessary to oxidize the silicon. In the EDP system, ethylene
`diamine and H20 combine to generate the holes and the hy-
`droxyl groups, while pyrocatechol forms the soluble species
`Si(C6H402 )3. Mixtures of ethylene diamine and pyrocatechol
`
`<111>
`
`(a)
`
`(b)
`
`<110> Surface Orientation
`
`f sioz M=k
`
`(dl
`Fig. 4. A summary of wet chemically etched hole geometries which are
`commonly used m micromechaniul devices. (a) Anisotropic etching
`on (100) wrfaca. @) Anisotropic etching on (110) surfaces. (c) bo-
`tropic etching with agitation. (d) Isotropic etching without agitation.
`Adapted from S. Terry [ 291.
`
`5
`
`

`
`PETERSEN: SILICON AS A MECHANICAL MATERIAL
`
`silicon
`
`without water will not etch silcon. Other common
`etchants can be analyzed in the same manner.
`Since the etching process is fundamentally a charge-transfer
`mechanism, it is not surprising that etch rates might be depen-
`dent on dopant type and concentration. In particular, highly
`doped material in general might be expected to exhibit higher
`lightly doped silicon simply because of the
`etch rates than
`greater availability of mobile carriers. Indeed, this has been
`shown to occur in the HNA system (1 : 3 : 8) [ 28 I , where typi-
`cal etch rates are 1-3 km/min at p or n concentrations >lo"
`cm-' and essentially zero at concentrations < 10" cm-' .
`such as EDP 1261, [27] and KOH
`Anisotropic etchants,
`[32], on the other hand, exhibit a different preferential etch-
`ing behavior which has not yet been adequately explained.
`Etching decreases effectively to zero in samples heavily doped
`(-lo2' cm-').
`with boron
`The atomic concentrations
`at
`these dopant
`levels correspond to an average separation be-
`tween boron atoms of 20-25 A, which is also near the solid
`X 1019 cm-')
`substifutiunully
`solubility limit (5
`for boron
`introduced into the silicon lattice. Silicon doped with boron is
`placed under tension as the smaller boron atom enters the
`lattice substitutionally, thereby creating
`a local tensile stress
`field. At high boron concentrations, the tensile forces became
`80 large that it is more energetically favorable for the excess
`interstitial sites. Pre
`boron (above 5 X 1019 cm-j) to enter
`sumably, the strong B-Si bond tends to bind the lattice more
`rigidly, increasing the energy required to remove a silicon -+m
`high enough to stop etching altogether. Alternatively,
`since
`this etchstop mechanism is not observed in the HNA system
`(in which the HF component can readily dissolve BZO3),
`perhaps the boron oxides and hydroxides initially generated
`on the silicon surface are not soluble in the KOH and EDP
`etchants. In this case, high enough surface concentrations of
`boron, converted to boron oxides and hydroxides in an inter-
`mediate chemical reaction, would passivate the surface and
`prevent further dissolution of the silicon. The fact that KOH
`is not stopped as effectively as EDP by p+ regions is a further
`indication that this may be the c ~ s e since EDP etches oxides at
`a much slower rate than KOH. Additional experimental work
`along these lines will be required to fully understand the etch-
`stopping behavior of boron-doped silicon.
`The precise mechanisms underlying the nature of chemical
`are not wen
`anisotropic (or orientation-dependent) etches
`of such etchiag
`understood either. The principal feature
`behavior in silicon is that (1 11) surfaces are attacked at a
`much slower rate than all other crystallographic planes (etch-
`rate ratios as high as 1000 have been reported). Since (1 11)
`silicon surfaces exhiiit the highest density of atoms per square
`centimeter, it has been inferred that this density variation is
`responsible for anisotropic etching behavior. In particular, the
`screening action of attached Hz0 molecules (which is more
`effective at high densities, i.e., on (1 1 1) surfaces) decreases the
`interaction of the surface with
`the active molecules. This
`screening effect has also been used to explain the slower oxi-
`dation rate of (1 1 1) silicon wafers over (100). Another factor
`involved in the etch-rate differential of anisotropic etches is
`the energy needed to remove an atom from the surface. Since
`have two danglins bonds, while
`(100) surface atoms each
`(1 1 1) surfaces have only one dangling bond, (1 1 1) surfaces are
`again expected to etch more slowly. On the other hand, the
`differences in bond densities and
`the energies required to
`remove surface atoms do not differ by much more than a fac-
`tor of two