Control of strain gradient in doped polycrystalline silicon carbide films through tailored doping

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2006 J. Micromech. Microeng. 16 L1

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INSTITUTE OF PHYSICS PUBLISHING JOURNAL OF MICROMECHANICS AND MICROENGINEERING

J. Micromech. Microeng. 16 (2006) L1–L5 doi:10.1088/0960-1317/16/10/L01

BRIEF COMMUNICATION

Control of strain gradient in dopedpolycrystalline silicon carbide filmsthrough tailored dopingJingchun Zhang1, Roger T Howe2 and Roya Maboudian1

1 Department of Chemical Engineering, University of California, Berkeley, CA 94720, USA2 Department of Electrical Engineering, Stanford University, Stanford CA 94305, USA

E-mail: maboudia@berkeley.edu

Received 27 May 2006, in final form 11 July 2006Published 29 August 2006Online at stacks.iop.org/JMM/16/L1

AbstractPolycrystalline 3C-SiC (poly-SiC) films are deposited by low-pressurechemical vapor deposition on Si substrates using 1,3-disilabutane and arein situ doped by NH3. Microstrain gauges and cantilever beam arrays arefabricated to study the doping effect on average residual strain and straingradient. A bi-layer deposition scheme consisting of films with differentresidual strains due to differing doping content is developed to minimize thestrain gradient without compromising the electrical resistivity of thedeposited film. In this way, a 3 µm thick poly-SiC film with a straingradient of 5 × 10−5 µm−1 and a resistivity of 0.024 cm is achieved.

1. Introduction

As a wide band-gap semiconductor with many extraordinaryproperties, silicon carbide (SiC) is attracting renewed attentionfor microelectromechanical systems (MEMS) application inharsh environments [1]. Compared with single crystallineSiC, polycrystalline 3C-SiC (poly-SiC) films, benefiting fromthe relatively low growth temperatures and the capability ofgrowth on various substrates, are preferred as a structural layerin MEMS as they offer maximum flexibility in developingSiC micromachining processes [2]. Residual stress (strain)including average residual stress and stress gradient throughthe thickness of the film are among the most importantfilm properties, especially for surface micromachined MEMSdevices, which are typically constructed by constrained andcantilevered structures. A moderate average tensile stress isdesired because compressive stress leads to the buckling inconstrained structures such as double clamped beams; stressgradients should be minimized to avoid out-of-plane bendingof cantilevered structures. Through a specific device design(e.g., folded-suspension), excess average residual stress canbe relieved in some situations [3]. However, the depressionof stress gradient in films remains challenging. Resistivityis another important film property that needs to be controlled.

Film conductivity is required for electrical actuation of MEMSdevices. Generally, a low resistivity is preferred to reduce theenergy dissipation, and enhance the system energy transferperformance, which is critical to MEMS transducers [4].

Recently, several MEMS-targeted poly-SiC thinfilm deposition processes have been developed [5–7].Film resistivity and stress are found to depend on filmmicrostructure, which is in turn determined by the details of thedeposition process [8, 9]. Although film resistivity and stresscan be controlled separately, the approach to optimize themsimultaneously is still a challenge because both of them aresensitive functions of deposition and post-deposition processconditions. Tailored multi-layer is one approach to controlfilm stress. In the case of polycrystalline Si (poly-Si), theprevailing material for a MEMS structural layer, the averagestress varies from tensile to compressive with the increase indeposition temperature. Poly-Si films with overall near-zerostresses and near-zero stress gradients have been achieved bythe so-called Multipoly process, which consists of alternatelydeposited compressive and tensile poly-Si layers, obtainedby varying deposition temperatures [10]. However, theMultipoly film is composed of distinct partially amorphousand fully crystalline layers, which compromise other filmproperties, such as resistivity, and may have stability problems

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Brief Communication

with time and temperature due to recrystallization. We recentlyhave reported residual strain characterization of undoped poly-SiC films and electrical characterization of NH3 doped poly-SiC films [8, 11]. In this study, the effect of NH3 doping onthe residual strain is investigated. A bi-layer scheme, whichis capable of controlling strain gradient while preserving thefilm resistivity, is proposed and demonstrated.

2. Experimental details

Poly-SiC films are deposited on 40 × 80 mm Si(1 0 0)substrates in a conventional horizontally oriented hot-walltubular LPCVD reactor at 800 C using 1,3-disilabutane(DSB), SiH3−CH2−SiH2−CH3, as the single precursor. Thefilms are in situ n-type doped by ammonia with a varyingNH3 to DSB flow rate ratio of 0 to 5:100 (referred to as 0 to5% doped films in this study). The details of the depositionprocess can be found elsewhere [12]. In the bi-layer depositionruns, NH3 flow rate is adjusted during deposition. The poly-SiC thickness is maintained at around 1 µm unless specifiedotherwise. The Si substrate is immersed in HF aqueoussolution before being loaded into the reactor to remove thenative oxide on the surface. To monitor the film resistivity,another Si substrate with a thermally grown SiO2 top layeris also included in each deposition. The film thicknessis controlled by deposition time and measured by opticalreflectometry using a NanoSpec Model 3000 interferometer.In the bi-layer deposition runs, the thickness of each layer isestimated by deposition time and the growth rate information.Film resistivity is calculated using the film thickness and thesheet resistance value, obtained with a Signatone S-301 four-point probe via in-line configuration. Average residual strainand strain gradient are evaluated by microstrain gauges andcantilever beam array (CBA) which are fabricated by surfacemicromachining techniques. After each deposition, poly-SiC micromechanical structures are dry etched using a HBr-based reactive ion etching with SiO2 as etch mask [13]. Thefreestanding SiC structures are released by time etching of theSi substrate using XeF2. The details of fabrication processare described elsewhere [11]. Scanning electron microscopy(SEM) is used to examine the microstrain gauge and CBAafter release. The beam profile is also measured by a WykoNT3300 interferometer profiling system. Assuming that strainchanges linearly through the film thickness, the beam profileis used to quantitatively determine the residual strain gradient(γ ) of the films using:

γ = 2z/L2 (1)

where z is the cantilever beam tip deflection and L is thelength of the cantilever beam.

3. Results and discussion

Figure 1 shows SEM images of the microstrain gauge fora representative 3% doped poly-SiC film. The strain gaugeconsists of two test beams (250 µm in length), one indicatorbeam (500 µm in length), and two vernier gauges. Residualtensile strain in the film causes test beams to contract, whichrotates the indicator beam. The small change in the lengthof test beams is therefore amplified and is proportional to the

(a)

(b)

Figure 1. SEM images of a poly-SiC (3% doped) microstraingauge: (a) the top view, (b) close-up side view.

10-2

10-1

100

101

102

0 0.02 0.04 0

0.1

0.2

0.3

Res

istiv

ity (

Ω c

m)

Ave

rage

str

ain

(%)

NH3/DSB ratio

ResistivityAverage strain

Figure 2. Effect of NH3/DSB flow rate ratios on resistivity andaverage strain of poly-SiC films.

displacement at the vernier gauges. Figure 2 shows the effectof NH3 doping on resistivity and average strain of poly-SiCfilms. With the increase in doping content, the resistivityinitially exhibits a fast drop and then stabilizes around 0.02–0.03 cm, while the tensile strain increases from 0.10% to0.21%. The strain gradient of all films is negative with valuesranging from −2 × 10−4 to −5 × 10−4 µm−1. This makes thepoly-SiC cantilever beams bend down towards the substrate,as shown in figures 3(a) and (b). The strain gradient is solarge in magnitude that the long beams eventually touch thesubstrate and are deflected upwards.

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(a)

(a)(b)

(c)

(d )40 µm

40 µm

40 µm

200 µm

Figure 3. Cantilever beam array of ((a), (b)) 1 µm thick single layerpoly-SiC (3% doped film), (c) 1 µm thick bi-layer poly-SiC (0.5 µm5% doped top layer; 0.5 µm 3% doped bottom layer), (d) 3 µm thickbi-layer poly-SiC (0.43 µm 5% doped top layer; 2.57 µm 3% dopedbottom layer). The beams are 100 to 1500 µm long and 10 µm wide.

The residual strain of polycrystalline thin films consistsof thermal and intrinsic components [14]. Thermal strain isgenerated during the cooling process after deposition, dueto the differences in thermal expansion coefficients (TEC)between the thin film and the substrate. The intrinsic strainmostly depends on the film microstructure, which is developedduring deposition process. Taking the TEC of SiC to be4 × 10−6 K−1 [15] and that of Si to be 2.5 × 10−6 K−1, SiC

-4x10-4

-2x10-4

0 x100

2x10-4

4x10-4

6x10-4

8x10-4

0 20 40 60 80 100

Str

ain

grad

ient

(µm

-1)

Top layer to total thickness ratio (%)

Figure 4. Simulation of the strain gradient for 1 µm thick bi-layerpoly-SiC films consisting of 5% doped top layer and 3% dopedbottom layer as a function of the ratio of the top layer thickness tothe total thickness.

film deposited at 800 C is expected to have a tensile strainof 0.12%, which is in the range of our results. Since all filmsare deposited at same temperature in this study, the effect ofNH3 doping on the poly-SiC strain can only be explained bythe intrinsic strain. Our previous studies on NH3 doping ofpoly-SiC showed that N atoms occupy the C sites in the SiClattice, which causes the crystalline lattice to contract from4.360 to 4.345 A [8]. The reduction in the SiC lattice increasesthe mismatch between SiC and Si (5.43 A) lattice constantswith doping, resulting in an increase in the tensile strain. Forpoly-SiC films with tensile strain, negative strain gradientis usually observed [9, 11]. The development of the straingradient is related to the variation in microstructure through thefilm thickness, as discussed in a recent transmission electronmicroscopy (TEM) study on poly-SiC films [16].

The strain gradient control approach developed in thisstudy is by depositing two layers of poly-SiC with differentstrains obtained by changing the doping content. Specifically,a layer with higher doping content (higher tensile strain) isdeposited on the top of a less doped layer (lower tensile strain)to compensate for the negative strain gradient. To preserve thelow resistivity, 3% and 5% doped layers are chosen to formthe bi-layer films. Figure 3(c) shows the CBA of a 1 µm thickbi-layer film consisting of 0.5 µm 3% doped layer (bottom)and 0.5 µm 5% doped layer (top). Although single layers of3% and 5% doped films both have negative strain gradients,by applying a top layer with higher average tensile strain inthe bi-layer film, the strain gradient is reversed. The cantileverbeams bend up, indicating a positive strain gradient. The straingradient of this bi-layer film is 5×10−4 µm−1, with an averagetensile strain of 0.18%, and the film resistivity of 0.022 cm.

The strain gradient (γ ) can be calculated from the strainprofile along the film thickness ε(z), using [4]

γ = 12

t3

∫ t/2

−t/2ε(z)z dz (2)

where t is the film thickness. Assuming that the average strainin each layer is identical to those of 1 µm thick single-layerfilms with the same doping content, and assuming a linearstrain gradient, equation 2 may be used to estimate the straingradient for the bi-layer films. The results for a 1 µm thick bi-layer film with 3% doped layer on the bottom and 5% dopedlayer on the top as a function of the ratio of the top layerthickness to the total thickness are shown in figure 4. With

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4.0x10-5

6.0x10-5

8.0x10-5

1.0x10-4

1.2x10-4

1.4x10-4

14 16 18 20 22

Str

ain

grad

ient

(µm

-1)

Top layer to total thickness ratio (%)

Figure 5. Strain gradient of 3 µm thick bi-layer poly-SiC filmsconsisting of 5% doped top layer and 3% doped bottom layer as afunction of the ratio of the top layer thickness to the total thickness.

the increase in the relative thickness of the top layer, the straingradient changes from negative through a positive maximumto negative, providing two zero strain gradient compositions.When the top layer is 0.5 µm thick, this analysis gives astrain gradient of 6.7 × 10−4 µm−1, which is quite close tothe experimental value of 5 × 10−4 µm−1. It should be notedthat this analysis can only be used to predict trends due tothe deviation of actual strain profile from the assumed linearprofile.

Based on this bi-layer scheme, several 3 µm thick bi-layerfilms are deposited in order to minimize the strain gradientwhile preserving the low film resistivity. The CBA of a 3 µmthick bi-layer film with the lowest strain gradient reported inthis study is shown in figure 3(d). The cantilever beams havea slight out-of-plane bending, with a 200 µm long beam onlybending up by 1 µm. The resistivity of this film is 0.024 cm.Figure 5 shows the strain gradient of 3 µm thick bi-layer poly-SiC films as a function of the top layer to total thickness ratio.With the reduction in the thickness of the high tensile-strain toplayer, the strain gradient decreases. The lowest strain gradientachieved is 5 × 10−5 µm−1. Lower strain gradient is possiblewith further adjustment in the relative thickness of each layer.

Although only one bi-layer scheme consisting of 3% and5% doped layers is present in this study, other doping contents(between 3 and 5% in order to maintain low resistivity) forthe two composite layers are clearly applicable. By a properchoice of doping content (and hence, strain) of the two layers,a large process window can be achieved for obtaining lowstrain gradient. The strain gradient control by the bi-layerscheme is achieved by a mechanical method; therefore it doesnot inhibit the optimization of other film properties throughdeposition conditions. In addition, the strain of each layer inthis study is controlled by doping content, which has severaladvantages to the use of deposition temperature to controlstrain, as done in the Multipoly process. First, gas flow canbe more quickly adjusted and stabilized than temperature.Second, the constant deposition temperature decreases thepossible properties variation of the preceding layer(s) duringthe deposition of the subsequent layers in the case that the laterdeposition is carried out at higher temperature. This is a majoradvantage for a material like poly-SiGe, whose strain can beeasily shifted, even reversed, by post-deposition annealingat the temperature only slightly higher than the depositiontemperature [17]. Finally, although the Multipoly poly-Si filmis found to retain its overall low strain gradient even after

annealing at elevated temperatures, the thermal expansioncoefficient of each layer is measured to be very different[18, 19]. On the other hand, the SiC bi-layer films presentedin this study are continuous without large variations in filmproperties (e.g., microstructure) for various layers; as aconsequence, the low strain gradient achieved through thisscheme is expected to have good time and temperature stability.Work is underway to investigate the effect of temperature onthese bi-layer films.

4. Conclusion

In conclusion, NH3 doping is found to increase the tensilestrain of poly-SiC films deposited using DSB-based low-pressure chemical vapor deposition. A single layer of dopedpoly-SiC has a negative strain gradient with the magnitudeon the order of −10−4 µm−1. A bi-layer deposition schemeconsisting of films with different doping contents (and henceresidual strain) is developed to control the strain gradient ofpoly-SiC films without compromising the electrical resistivityof the film. A 3 µm thick poly-SiC film with a strain gradientof 5 × 10−5 µm−1 and a low resistivity of 0.024 cm isachieved, while further lower strain gradient is possible.

Acknowledgments

The authors gratefully acknowledge the financial support ofDARPA MTO program (contract number NBCH1050002) andDr Clark Nguyen, the program manager.

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