Deep Reactive Ion Etching for Bulk Micromachining of ...

21Deep Reactive Ion

Etching for BulkMicromachining

of Silicon Carbide

21.1 Introduction21.2 Fundamentals of High-Density Plasma Etching21.3 Fundamentals of SiC Etching Using

Fluorine Plasmas21.4 Applications of SiC DRIE: Review21.5 Applications of SiC DRIE: Experimental Results21.6 Applications of SiC DRIE: Fabrication

of a Bulk Micromachined SiC Pressure Sensor21.7 Summary

21.1 Introduction

It is often desired to insert microsensors and other microelectromechanical systems (MEMS) into harsh(e.g., hot or corrosive) environments. Silicon carbide (SiC) offers considerable promise for such appli-cations, because SiC can be used to fabricate both high-temperature electronics and extremely durablemicrostructures. One of the attractive characteristics of SiC is the compatibility of its process technologieswith those of silicon, which allows for the co-fabrication of SiC and silicon MEMS. However, a veryimportant difference in the processing of these semiconductors arises from the chemical inertness of SiC,a characteristic that makes it attractive for use in corrosive environments but also makes it very difficultto micromachine.

Realization of the full potential of SiC MEMS will require the development of a set of micromachiningtools for SiC comparable to the tool set available for silicon. Micromachining methods are generallyclassified as bulk, in which the wafer is etched, or surface, in which deposited surface layers are patterned.Surface micromachining methods for deposited SiC layers have been developed to a high level [Mehreganyet al., 1998]. Silicon carbide can be readily etched to the required depths of just several microns usingreactive ion etching (RIE) processes [Yih et al., 1997]. Further work remains to be done, however, indeveloping RIE processes with greater selectivity for SiC. Current RIE processes lack the selectivityneeded to etch a SiC layer entirely through while minimally modifying an underlying silicon or silicondioxide layer. This limitation has motivated the development of a micromolding method in which SiCis deposited into molds formed by RIE of silicon or silicon dioxide [Yasseen et al., 1999].

The emphasis here is bulk micromachining of SiC, for the fabrication of SiC microstructures withvertical dimensions from approximately 10 µm to several hundred microns. Three methods for bulk

Glenn M. BeheimNASA Glenn Research Center

© 2002 by CRC Press LLC

micromachining of SiC have been developed: electrochemical etching, micromolding and deep reactiveion etching (DRIE). Each of these has an important role to play in SiC MEMS fabrication.

Electrochemical etching of SiC can provide the high rates (>1 µm/min) needed for economical deepetching [Shor and Kurtz, 1994]. In addition, electrochemical etching provides a high degree of selectivityto other materials, and it allows for the use of p–n junction etch-stops for precise depth control. Etchdirectionality, however, is poor, resulting in significant undercutting of the mask. Highly directional (oranisotropic) etch processes, which provide vertical etch sidewalls, are needed to accurately transfer themask pattern deeply into the wafer.

Bulk micromolding methods have been developed to circumvent the limitations of available deep-etchprocesses for SiC [Lohner et al., 1999]. Here, silicon molds are fabricated using highly developed siliconDRIE processes. The molds are filled with polycrystalline SiC using chemical vapor deposition (CVD)and then removed by wet etching. Thick, finely featured SiC microcomponents have been fabricatedusing this approach. The molded polycrystalline SiC, however, does not have the excellent mechanicaland electronic properties of single-crystal SiC. In particular, deep etching of single-crystal SiC wafers isrequired if high-quality SiC electronics are to be integrated with thick SiC microstructures.

Conventional parallel-plate RIE is not well suited for deep etching (i.e., etch depths greater than about10 µm) of SiC because it provides low etch rates, high rates of mask erosion and relatively poor direc-tionality (although superior to that which can be achieved using electrochemical etching). The use ofadvanced high-density-plasma (HDP) reactors can alleviate all these difficulties. High-density plasmahas enabled the DRIE process for silicon, and its usefulness for deep etching of SiC is becoming wellestablished.

21.2 Fundamentals of High-Density Plasma Etching

The advantages of high-density plasma for DRIE are primarily the results of a more highly reactiveplasma, which provides for increased etch rates, and a lower operating pressure, which improves direc-tionality by minimizing the scattering of ions. There are several types of HDP systems, the most commonbeing electron cyclotron resonance (ECR) and inductively coupled plasma (ICP). Inductively coupledplasma has come to dominate the HDP market [Bhardwaj and Ashraf, 1995], in large part due to lowercomplexity and cost (ICP uses RF frequencies, while ECR uses microwaves to generate the plasma). Thefocus here will be on etching with ICP, although ECR etching shares many of the same characteristics.

Inductively coupled plasma has several important advantages relative to conventional RIE. First, ICPcan have a plasma density (ions per unit volume) which is two orders of magnitude higher than that ofconventional RIE. This produces a much greater flux of ions and reactive species to the substrate surface.Also, ICP etching is performed at lower pressures, which helps minimize bowing of the etch sidewallscaused by scattering of ions. Lower pressures also facilitate the transport of etchants and etch productsinto and out of narrow trenches. In addition, the low operating pressure helps to provide smoothersurfaces, as sputtered mask materials are less likely to be scattered back onto the etched surface, whichcan cause micromasking and the formation of “grass” or other etch residues. Another important advan-tage of ICP is the independent control it provides for the plasma density and the energy with which ionsbombard the substrate. This allows for the adjustment of the chemical and mechanical components ofthe etch process to give a satisfactory trade-off between etch rate and the erosion rate of the mask. Theratio of the substrate and mask etch rates, the mask selectivity, is an important parameter for deep etchingas it determines the maximum depth that can be etched with a given mask thickness.

A schematic of an inductively coupled plasma etching system is shown in Figure 21.1. Typically, afluorinated gas, such as SF6, is supplied to the reactor at a controlled rate. A high throughput pumpingsystem maintains the system at a low pressure and enables a high gas flow to rapidly replenish depletedetchants. A plasma is generated by supplying RF power to a coil wrapped around the ceramic walls ofthe chamber. Power supplied to the coil produces a time-varying axial magnetic field inside the chamber.This induces an azimuthal electric field, which accelerates the electrons to high energies. The circumfer-ential electric field helps to confine the electrons to the plasma, which results in the much higher plasma


densities characteristic of ICP. Because confinement of the electrons increases the probability that anelectron will undergo an ionizing collision with a gas molecule before it leaves the plasma, the ionizationrate is increased. The plasma can therefore be sustained at lower pressures (as low as 1 mTorr), whichincreases the mean path length the electrons travel between collisions. This enables the acceleration ofthe electrons to higher energies, which increases the likelihood that a collision with a gas molecule willbreak the molecule apart. An ICP reactor, therefore, is very effective at dissociating a relatively inertfluorinated feed gas, such as SF6, to produce high concentrations of atomic fluorine and other highlyreactive radicals.

At the same time the substrate is subjected to a high flux of reactive radicals, it can be bombardedwith energetic ions. A second generator supplies RF power to an electrode, onto which the wafer is clamped.Upon application of the AC voltage, electrons and ions are alternately attracted. The less massive electronsare considerably more mobile so the electrode acquires a negative charge, which gives it a time-averagednegative potential with respect to the plasma. This negative bias potential acts to repel electrons, pre-venting further net accumulation of charge. The potential gradient between the plasma and substrateelectrode causes ions drifting out of the plasma to be accelerated across the dark space (or sheath) betweenthe plasma and substrate. Typically, the ions are accelerated to energies from several tens of electronvoltsto several hundred electronvolts. The damage which these ions cause to the substrate can dramaticallyincrease the etch rate—for example, by creating highly reactive dangling bonds. The ions strike the substrateat normal incidence, provided they do not undergo scattering collisions with gas molecules in the sheath.Inductively coupled plasma enables highly anisotropic etching, as the low operating pressures of ICPensure that ions strike only the horizontal wafer surfaces and are not scattered to the etch sidewalls.

21.3 Fundamentals of SiC Etching UsingFluorine Plasmas

The use of fluorine-based plasma etch chemistries is most easily implemented, as, unlike chemistriesbased on other halogens such as chlorine, it allows the use of nontoxic feed gases such as SF6. The plasmadissociates the relatively inert fluorinated gas (e.g., SF6) to produce highly reactive radicals such as atomicfluorine. For DRIE of silicon, a fluorine etch chemistry is almost always used, in part because of conve-nience, but primarily because fluorine provides the high rates needed for economical deep etching.Fluorine plasmas are widely used in SiC etching, also. Limited experimentation with chlorine plasmaetching of SiC has yielded relatively low etch rates [Wang et al., 1998].

The reactivity of SiC in a fluorine plasma contrasts strongly with that of silicon. Exposure to atomicfluorine produces rapid isotropic etching of silicon, as shown in Figure 21.2. Silicon reacts spontaneouslywith atomic fluorine, producing volatile etch products (e.g., SiF4 and SiF2) which rapidly desorb from

FIGURE 21.1 Schematic of an inductively coupled plasma (ICP) etching system. The RF generators are labeledRF1 and RF2.

RF1

RF2

gas inlet aluminachamber

coil

pump port


the surface. Etching proceeds laterally as well as vertically, which results in an isotropic profile. In contrast,the etch rate of SiC when exposed to atomic fluorine is extremely low, unless energy is supplied to thesurface. In reactive ion etching, this energy is provided by ion bombardment, which results in ananisotropic etch profile, as shown in Figure 21.3. Energetic ions damage the SiC lattice (e.g., breakingSi–C bonds) which promotes reactions with atomic fluorine to produce volatile SiFx and CFx species. Ifthe ions are well collimated, only the horizontal surfaces are etched, producing the desired anisotropy.

In fluorine-based DRIE of silicon, the required anisotropic profile is produced by modifying theprocess to cause the formation of a passivating layer on the etch sidewall. Most commonly, the reactoris programmed to switch back and forth between etching of silicon and deposition of an etch-resistantfluorocarbon polymer. Low energy ion bombardment is used during the etch step to remove the polymerfrom the horizontal surfaces, while leaving it intact on the etch sidewalls. Proper balancing of the etchand passivation steps produces a vertical sidewall, which at high magnification in a scanning electronmicroscope has a scalloped appearance.

In DRIE of SiC, the low reactivity of the substrate makes the etch profile inherently anisotropic.However, the low reactivity of SiC creates a number of other problems, including low etch rate, lowselectivity to the mask and a tendency for the formation of residues on the etched surfaces. The net resultis that DRIE of SiC is decidedly more difficult than DRIE of silicon. It is the high reactivity of siliconwith fluorine that enables the high rate, photoresist etch masks and tunable sidewall slope of the DRIEprocess for silicon. Because the silicon DRIE process uses low ion energies, it provides a high selectivitywith respect to photoresist, the most economical etch mask. Also, the silicon DRIE process providesexcellent control of the etch profile, as the sidewall can be adjusted from outward to inward sloping bysimply increasing the duration of the etch step relative to that of the passivation step. With inherentlyanisotropic etch processes, such as DRIE of SiC, it may prove more difficult to obtain the same highaspect ratios (etch depth divided by minimum feature size) that can be realized in silicon.

The high ion energies needed to etch SiC at acceptable rates make it impractical to use photoresist asan etch mask, as the etch rate of photoresist under these conditions is not much different from that ofSiC. Nickel and indium–tin–oxide (ITO) are widely used because these materials do not form a volatilereaction product with fluorine and so are etched only by sputtering.

The formation of residues on the etched surface is a significant problem in SiC etching. The highlyenergetic ions cause significant erosion of the etch mask, and sputtered mask materials that deposit ontothe SiC surface act as micromasks. Because the etch process is highly anisotropic, such micromasks arenot undercut and pillar-like features and other residues can result. High pressures increase the likelihood

FIGURE 21.2 Etching of silicon in a fluorine plasma, using a photoresist etch mask.

FIGURE 21.3 Etching of SiC in a fluorine plasma, using a nickel etch mask.

photoresist

Si

F SiFx

nickel

SiC

F ions SiFx + CFx


of residue formation, as sputtered mask materials are more likely to be scattered back onto the SiCsurface. Also, the use of aluminum masks, rather than nickel or ITO, has been found to promote theformation of etch residues.

21.4 Applications of SiC DRIE: Review

Throughput is a key economic factor, and much work has focused on the development of high-rate etchprocesses for SiC. Various types of HDP reactors have been used to demonstrate SiC etch rates greaterthan 1 µm/min, which compares fairly well with the 2-µm/min rates typical of silicon DRIE. In SiC,however, the attainment of such a high etch rate necessitates highly energetic ion bombardment, whichdramatically increases the sputter erosion of the etch mask. This can severely limit the etch depths thatcan be obtained using practically feasible mask thicknesses. In addition, high-rate etching often does notprovide the smooth etched surfaces needed, due to the increased tendency for micromasking.

Several groups have demonstrated deep reactive ion etching of SiC. In one case, a parallel-plate RIEwas used to etch a via through an 80-µm-thick substrate at a rate of 0.23 µm/min [Sheridan et al., 1999].The feed gas was NF3, and the chamber pressure was relatively high, 225 mTorr, which provided for ahigh concentration of reactive species. The high operating pressure also resulted in relatively low ionenergies, which minimized sputtering of the mask. The selectivity to the mask was only 25, and thickelectroplated nickel–alloy masks were used for deep etching of SiC. Anisotropy was quite good for theroughly 100-µm-diameter via, but scattering of ions will limit the aspect ratios that can be achieved atsuch a high pressure.

Etching of 4H-SiC was reported using a helicon HDP reactor [Chabert et al., 2000]. The feed gas wasSF6 and 25% O2. It was stated that the addition of O2 did not affect the etch rate but prevented thedeposition of sulfur compounds on the chamber walls. A 50-µm-thick nickel sheet was used as a shadowmask to etch a via through a 330-µm-thick 4H-SiC substrate. The wafer was etched through in approx-imately 6 hr, giving an etch rate of 0.9 µm/min. The selectivity with respect to the nickel mask was 40:1.Etch rates as high as 1.35 µm/min were obtained, but rough surfaces were produced at these high rates.

Using an ICP reactor with SF6 and 33% O2, holes were etched to a depth of 97 µm in 4H-SiC, at arate of 0.32 µm/min [Cho et al., 2000]. A cross-sectional scanning electron micrograph showed that thebottom of the etched hole was relatively clean. The low operating pressure of 2 mTorr may have beenthe key to providing clean etched surfaces despite the use of an aluminum mask, which often results inmicromasking. Selectivity with respect to the mask was reported to be greater than 50.

Using an ICP reactor with SF6, very smooth etched surfaces were obtained for an etch depth of 45 µm,and a via was etched through a 100-µm-thick 6H-SiC wafer [Beheim and Salupo, 2000]. At a rate of0.3 µm/min, the selectivity with respect to a nickel mask was 80. Since the presentation of these results,this laboratory has made substantial progress in SiC DRIE, as will be described in the next section.

21.5 Applications of SiC DRIE: Experimental Results

This section will present experimental results for DRIE of SiC using an inductively coupled plasmareactor (STS Multiplex ICP). The reactor has an automated load-lock and a 1000-L/sec turbopump witha dry backing pump. Helium is supplied to the backside of the electrostatically clamped wafer to provideeffective heat transfer to a water-cooled chuck. The SiC substrates, which range in size from 10 mm

2 to

50-mm diameter, are attached to 100-mm silicon carrier wafers using a thin layer of photoresist. The useof a sacrificial carrier wafer (which etches at approximately 2 µm/min) helps to minimize roughnesscaused by the sputtering of etch-resistant materials onto the SiC surface.

Liftoff of electron-beam evaporated films is a convenient means of fabricating both nickel and ITOetch masks. Liftoff of ITO masks as thick as 3.5 µm was readily accomplished, while film stresses limitedthe evaporated nickel masks to thicknesses no greater than 2500 Å. For extremely deep etching, nickelmasks with thicknesses up to 15 µm were fabricated by selective electroplating.


A series of experiments was performed to determine the effects of various process parameters on theSiC etch rate and selectivity to the mask. During these experiments, the coil power was 800 W and the feedgas was 100% SF6. The gas flow was maintained as high as practical, given the constraints of the pumpingsystem and the mass flow controller. The 10-mm

2 n-type 4H-SiC substrates were masked using evaporated

nickel.Figure 21.4 shows the etch rate of 4H-SiC as a function of pressure for a fixed platen power of 75 W.

The highest etch rate of 0.30 µm/min was obtained at a pressure of 5 mTorr. The etch rates for 4H-SiCare not significantly different from those reported for the 6H polytype [Beheim and Salupo, 2000]. Thedopant type also has been found to have little effect on the etch rate.

Figure 21.5 shows the SiC etch rate and the selectivity to the nickel mask (ratio of the SiC and Ni etchrates) as functions of the power supplied to the substrate electrode or platen. The platen power determinesthe kinetic energy of the ions which bombard the substrate. Selectivity was found to decrease withincreasing ion energies, due to increased sputtering of the mask. For a platen power of 75 W, the etchrate was 0.30 µm/min and the selectivity to the nickel mask was 125.

An investigation was performed to determine the effects of the various parameters on the surface mor-phology of deeply etched features. It was found that 5-mTorr pressure and 75-W platen power, in conjunctionwith 800-W coil power and 55-sccm SF6, provide a deep etch that is satisfactory for many applications.

FIGURE 21.4 Etch rate of n-type 4H-SiC as a function of pressure for 800-W coil power, 75-W platen power and100% SF6.

FIGURE 21.5 Etch rate of n-type 4H-SiC and selectivity to a nickel mask as functions of platen power for 800-Wcoil power, 5-mTorr pressure, and 100% SF6.

SiC

Etc

h R

ate

(/m

in)

3500

3000

2500

2000

1500

1000

500

00 5 10 15 20 25 30 35 40 45

Pressure (mTorr)

6000

5000

4000

3000

2000

1000

00 50 100 150 200 250

0

20

40

60

80

100

120

140

160

Rate (Å/min)Selectivity

Platen Power (Watts)

SiC

Etc

h R

ate

(Å/m

in)

Sel

ectiv

ity to

Mas

k


Relative to these baseline process parameters, higher platen powers give lower selectivity to the mask andhigher pressures promote residue formation, while lower pressures cause increased trenching at the baseof the sidewall.

The morphology of the etched surface was found to be strongly influenced by the cleanliness of thesurface at the start of the etch process. A number of cleaning procedures (solvents, hot sulfuric acid etch,oxidation followed by etch in hydrofluoric acid) were tried, but none was found satisfactory. Figure 21.6shows results typical of those obtained by cleaning a 6H-SiC substrate using standard methods prior toloading it in the reactor. After etching to a depth of 45 µm, the etched surface is covered with dimplesand spike-like residues.

An in situ plasma cleaning procedure was found to be effective at providing smooth surfaces for etchdepths up to roughly 75 µm. Figure 21.7 shows the result when the SiC surface was sputter cleaned inargon immediately prior to a 45-µm-deep etch using the baseline process. A disadvantage of the argonsputter clean is the significant mask erosion that it causes. The 10-min sputter etch in argon was foundto remove 1 µm of ITO, or 2500 Å of nickel, while removing only 800 Å of SiC. An oxygen plasma cleaningprocess, with moderately energetic ion bombardment (75W platen power), was found as effective as theargon sputter cleaning, while causing much less mask erosion. In the case of ITO masks, however, the oxygenplasma pretreatment was found to leave etch-resistant residues on the SiC surface, unless the openingsin the mask were quite wide (>100 µm).

When the SiC surface is plasma cleaned prior to deep etching, the etched surface is initially smooth,but with increasing etch depth, first dimples and then rough residues gradually appear. For etch depthsless than roughly 75 µm the surfaces are generally smooth, but for greater depths the etched surfacesbecome increasingly rough. Figure 21.8 shows a 160-µm-deep etch of a p-type 6H-SiC substrate, whichwas oxygen-plasma cleaned prior to the start of the baseline etch process. Auger analysis of the roughresidues showed small quantities of aluminum. The residues are apparently the result of micromaskingcaused by aluminum sputtered from the vacuum chamber. The rough residues are found only if dimplesare also present. The surface of a dimple may react more readily with aluminum than the untexturedSiC surface, thereby trapping aluminum to produce a micromask. If no dimples are present, aluminummay remain mobile on the surface until it is sputtered off, thereby causing no roughening of the surface.Experimentally, residual hydrocarbons and water were found to greatly increase the density of dimples,

FIGURE 21.6 6H-SiC (n-type) etched to a depth of 45 µm using the baseline 100% SF6 process. The SiC substratewas cleaned in hot sulfuric acid prior to etching. The electroplated nickel etch mask has not been stripped.


in the absence of in situ plasma cleaning prior to etching. As determined by X-ray photon spectroscopy(XPS), the etched SiC surface was found to be covered with a thin (<10 Å) fluorocarbon layer, whichmediates the etch reaction. Hydrogen is known to reduce the thickness and passivating properties ofsuch a fluorocarbon layer on an etched silicon surface, so it is feasible that hydrogen-bearing contaminantsmay cause the local enhancement of etch rate which results in the dimples observed here. The texture-inducing contaminants are apparently found in the background vacuum of the chamber, as the dimplesgradually appear with continued etching, despite an initial in situ cleaning of the surface.

Smooth surfaces, even for extremely deep etching, were obtained by diluting the SF6 etchant gas withargon to cause continuous sputter cleaning of the surface throughout the etch process. In a series of

FIGURE 21.7 A 1-mm-diameter well etched to a depth of 45 µm in n-type 6H-SiC using the baseline 100% SF6

etch process, after an in situ sputter cleaning in argon. The ITO etch mask has been stripped.

FIGURE 21.8 A 1-mm-diameter well etched to a depth of 160 µm in p-type 6H-SiC using the baseline 100% SF6

etch process, after an in situ oxygen plasma clean using energetic ion bombardment. The ITO etch mask has beenstripped.


experiments, the argon concentration was gradually increased until smooth etched surfaces were obtainedat 85% argon and 15% SF6. Other parameters were unchanged from the baseline process (5-mTorrpressure, 800-W coil and 75-W platen powers). Dilution with 85% argon reduces the SiC etch rate from0.30 to 0.22 µm/min. The selectivity to nickel is reduced from 125 to 55. The selectivity to ITO is only20, insufficient for deep etching. For DRIE using the Ar-diluted SF6 process, thick nickel masks werefabricated by selective electroplating. Figure 21.9 shows the smooth surface that resulted after etchingp-type 6H-SiC to a depth of 245 µm, using SF6 and 85% argon. The mask in this case was 6-µm-thickelectroplated nickel.

A key attribute of the silicon DRIE process is its ability to accurately transfer finely detailed featuresdeep into the wafer, as quantified by the aspect ratio, or etch depth divided by lateral feature size. Typically,DRIE of silicon can provide aspect ratios of 30 or greater. Deep RIE processes suitable for the fabricationof high-aspect-ratio SiC microstructures are still in the very early stages of development. Figure 21.10shows trenches with moderately high aspect ratios which were fabricated by DRIE of n-type 6H-SiC.The etch depth was 60 µm, and the trenches which form the letters shown were 10 µm wide, for anaspect ratio of 6. The electroplated nickel mask (initially 6 µm thick) was stripped prior to SEM. Thewafer was etched using the standard 100% SF6 etch for 200 min, after a 10-min sputter clean in argon.In developing a high-aspect-ratio DRIE process for SiC, the primary objective will be to control thedeposition of passivating polymer on the etch sidewall in order to minimize the sidewall roughnessapparent in Figure 21.10. This can be accomplished by using a switched process, as for DRIE of silicon,or by modifying the SiC DRIE process to provide for simultaneous etching and polymer deposition.

21.6 Applications of SiC DRIE: Fabrication of a BulkMicromachined SiC Pressure Sensor

This section will describe a practical application for SiC DRIE—specifically, a pieozoresistive SiC pressuresensor, which can be used at temperatures as high as 500°C. Silicon piezoresistive pressure transducerswere some of the first MEMS sensors and now represent a large fraction of the MEMS devices manu-factured. Silicon pressure sensors with precisely controlled dimensions can be inexpensively fabricatedusing highly anisotropic wet etch processes. At moderate temperatures, silicon is a nearly ideal elasticmaterial; however, at about 450°C, silicon begins to deform plastically. Silicon carbide is a superiormaterial for high-temperature pressure sensors, because SiC maintains its excellent mechanical properties

FIGURE 21.9 A 1-mm-diameter well etched to a depth of 245 µm in p-type 6H-SiC using the SF6 + 85% Ar etchprocess, after an in situ sputter cleaning using argon. The electroplated nickel mask has not been stripped.


to very high temperatures (>1000°C). A production-worthy DRIE process for SiC is a key to the successfulcommercialization of high-temperature SiC pressure sensors, as DRIE is the only bulk-micromachiningmethod for SiC that provides the required dimensional control.

The packaged SiC pressure sensor chip is shown in Figure 21.11 [Ned et al., 2001]. A SiC diaphragmis micromachined by using DRIE to etch a 1-mm-diameter well to a depth of about 60 µm in thebackside of a 120-µm-thick 6H-SiC wafer. On the front side of the diaphragm, four SiC strain gaugesare fabricated in an n-type epitaxial layer. An underlying p-type epitaxial layer provides electricalisolation between the strain gauges. The strain gauges are defined using a photoelectrochemical etchprocess which automatically stops when the p-type layer is reached. The strain gauges are configuredin a Wheatstone bridge, with two gauges positioned over the edge of the diaphragm while the othertwo gauges are positioned in the center of the diaphragm. The gauges on the edge of the diaphragmhave the greatest sensitivity to pressure, while the sensitivity of the gauges in the center of the diaphragmis smaller and opposite in sign. The output of the Wheatstone bridge responds to the difference in theresistances of the edge-mounted and center-mounted gauges. The temperature-induced resistancechanges, therefore, are compensated, as they are largely the same for all four gauges. A three-layermetallization system, consisting of layers of titanium, tantalum silicide and platinum, is used toelectrically contact the SiC piezoresistors.

The highly anisotropic DRIE process for SiC produces diaphragms that are uniform in thickness andhave precisely positioned edges. This maximizes the sensitivity and provides a high uniformity of responsefrom one sensor to the next, as the sensitivity is strongly influenced by the positions of the strain gaugesin relation to the edge of the diaphragm. For a 60-µm etch depth, the 1-mm-diameter diaphragm wasdetermined to have a thickness that was uniform to better than ±1 µm.

The SiC pressure sensor of Figure 21.11 was tested in the compressor discharge of a gas turbine engine.The sensor performed as expected during this hour-long test, in which it was exposed to gas temperaturesas high as 520°C. Current efforts are focused on reducing the cross-sensitivity to temperature: first byrefining the design and fabrication processes to more accurately balance the bridge, and, second, byincorporating an on-chip temperature sensor to enable the compensation of temperature-inducedchanges in gauge factor.

FIGURE 21.10 6H-SiC (n-type) etched to a depth of 60 µm using the 100% SF6 baseline process, after an in situsputter cleaning using argon. The electroplated nickel mask has been stripped. Visible on the tops of the sidewall arefragments of the polymer layer which had built up on the edge of the mask.


21.7 Summary

Deep RIE of SiC is a key to enabling the development of a wide range of SiC MEMS for use in harshenvironments. High-density plasma makes DRIE of SiC possible, as it provides the required high chemicalreactivity, low operating pressure and independent control of ion energy. Deep RIE of silicon carbide isquite different from DRIE of silicon, due to the chemical inertness of SiC. The need for energetic ionbombardment to produce an appreciable etch rate causes RIE of SiC to be inherently anisotropic. Energeticion bombardment necessitates the use of a nonreactive etch mask such as nickel or ITO. The attainableetch depth is typically limited by sputter erosion of the etch mask. Often, higher ion energies can be usedto increase the SiC etch rate, but this can produce an unacceptable reduction in the selectivity with respectto the mask. The etch depths for which smooth residue-free surfaces are obtained can be limited bybackground contaminants in the reaction chamber. Argon can be introduced to the chamber to providecontinuous sputter cleaning of the surface during the etch process, but this has detrimental effects onetch rate and mask selectivity. At present, SiC DRIE processes have been developed which are well suitedfor the fabrication of low-aspect-ratio microstructures such as pressure sensors. However, high-aspect-ratio DRIE processes are still in the early stages of development.

Defining Terms

Anisotropy: Degree of directionality of an etch process. In reactive ion etching, an ideal anisotropicprocess provides negligible etching in the lateral direction.

Aspect ratio: Etch depth divided by lateral feature size. Highly anisotropic etch processes are needed tofabricate microstructures with high aspect ratios.

Deep reactive ion etching (DRIE): Highly anisotropic plasma etching to depths greater than about 10 µm.

FIGURE 21.11 (Color figure follows p. 12-26.) Packaged SiC pressure-sensor chip. Through the semitransparentSiC can be seen the edge of the well that has been etched in the backside of the wafer to form the circular diaphragm.The metal-covered, n-type SiC that connects the strain gauges is highly visible, while the n-type SiC strain gaugesare more faintly visible. There is a U-shaped strain gauge over the edge of the diaphragm at top and bottom, andthere are two vertically oriented linear gauges in the center of the diaphragm.


High-density plasma (HDP): A reactor that employs some means of electron confinement to producea plasma density (ions per unit volume) which is typically two orders of magnitude greater thanthat of a conventional parallel plate RIE.

Inductively coupled plasma (ICP): A type of high-density plasma reactor in which power is coupled tothe plasma using a coil. The time-varying magnetic field induced by the coil helps to confineelectrons to the plasma, which increases the plasma density.

Mask selectivity: The substrate etch rate divided by the etch rate of the mask.Micromasking: The formation of residues due to the masking effect of materials sputtered from the etch

mask onto the etched surface. In some reactors, sputtering of the substrate electrode can also causemicromasking.

References

Beheim, G., and Salupo, C.S. (2000) “Deep RIE Process for Silicon Carbide Power Electronics and MEMS,”in Proc. of the MRS 2000 Spring Meeting, May 24–28, San Francisco, CA, MRS Proc. 622, paper T8.9.

Bhardwaj, J.K., and Ashraf, H. (1995) “Advanced Silicon Etching Using High Density Plasmas,” in Micro-machining and Microfabrication Process Technology, ed. K.W. Markus, Proc. SPIE 2639, pp. 224–233.

Chabert, P., Proust, N., Perrin, J., and Boswell, R.W. (2000) “High Rate Etching of 4H-SiC Using a SF6/O2

Helicon Plasma,” Appl. Phys. Lett. 76, pp. 2310–2312.Cho, H., Leerungnawarat, P., Hays, D.C., Pearton, S.J., Chu, S.N.G., Strong, R.M., Zetterling, C.-M.,

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