Materials Science and Engineering B66 (1999) 79–82

Real-time strain monitoring in thin film growth: cubic boronnitride on Si (100)

Dmitri Litvinov a,*, Roy Clarke a, Charles A. Taylor II b, Darryl Barlett b

a Randall Laboratory of Physics, Uni6ersity of Michigan, Ann Arbor, MI 48109-1120, USAb k-Space Associates, Inc., 555 S. Forest A6e., Ann Arbor, MI 48104, USA

Abstract

We demonstrate the application of real-time film-stress monitoring and control using a multi-beam optical sensor. In situmeasurements on wide-bandgap boron nitride films grown by ECR-assisted sputtering reveal a critical stress beyond which defectsare injected into the silicon substrate. This is marked by a rapid onset of wafer curvature. The method should be particularlyuseful for monitoring stress build-up in other wide-bandgap nitride films where no appropriate lattice-matched substrates arepresently available. © 1999 Elsevier Science S.A. All rights reserved.

Keywords: Real-time strain monitor; In situ stress control; Wafer curvature; III-Nitrides; Cubic boron nitride; Yield strength

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1. Introduction

There is an increasing need for real-time, in situmonitors for the expert control of electronic materialsfabrication and processing. Here, we demonstrate anew application to determine strain accumulation dur-ing thin-film growth and wafer processing. While theapproach is very general and can be applied to a widevariety of materials and coatings, we illustrate ourstudies with results on wide-band gap thin films wherethe very strong inter-atomic bonding can lead to highvalues of residual stress.

The zinc-blende nitrides (c-BN, cubic GaN, ...) forman important emerging class of wide-bandgap semicon-ductors of interest for high-power optoelectronicdevices operating under rugged conditions. An ongoingchallenge towards fabrication of devices from suchmaterials is to find appropriate substrates and bufferlayers that will promote epitaxial growth while mini-mizing stress build-up. In this work we describe a novelapproach to this issue, recognizing the existence ofpolymorphic structures in the III-nitrides. The wurtziticand hexagonal (turbostratic) forms are particularly in-teresting as strain-relieving buffer-layers for the growth

of cubic phases of the same III-N compound. We termsuch buffer layers, ‘self-compliant’.

Drawing on the example of cubic boron nitride (c-BN), a III–V analog of diamond, our previous studies[1] demonstrate the use of hexagonal buffer layers ofBN to relieve stress build-up and to promote orientedgrowth of c-BN on Si (100) substrates. In this paper insitu measurements of film stress are presented utilizinga recently developed technique [2] in which an array oflaser beams reflected from the substrate surface is mon-itored in real time by a CCD area detector. In this waythe curvature of the substrate can be determined, andfrom this the residual stress in the BN film is calculated.We have found that a dramatic reduction in stress canbe achieved by optimizing the deposition parameterssuch as substrate temperature and incident ion (N+)energy, leading to improved adhesion, increased growthrates, and a well-oriented mosaic structure [1].

2. Experimental set up

We conducted the growth studies in a custom-de-signed UHV chamber with a base pressure of 1×10−10

Torr [3]. Hot pressed boron nitride of 4 N purity wasused as a sputtering target and nitrogen ions weresupplied from an electron cyclotron resonance (ECR)source. The silicon (100) substrate, which was heated to

* Corresponding author. Present address: Seagate Technology,1520 Penn Avenue, Pittsburgh, PA 15222, USA.

E-mail address: dmitri_litvinov@notes.seagate.com (D. Litvinov)

0921-5107/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.

PII: S0921 -5107 (99 )00128 -2

D. Lit6ino6 et al. / Materials Science and Engineering B66 (1999) 79–8280

over 1000°C by direct Joule heating, was biased with anegative dc voltage to control the energy of nitrogenions arriving at the film surface. At this temperature,the low base pressure of our vacuum chamber allowedthermal desorption of oxygen from the substrate sur-face, eliminating the need for any special surface treat-ment prior to deposition.

The stress measurements were performed using themulti-beam optical stress sensor (MOSS) developedjointly by Sandia National Laboratory and k-SpaceAssociates (Ann Arbor, MI) [2,4]. The principle of themethod is that a thin film under stress will induce acurvature k in the underlying substrate. The film stresscan be calculated from k by the following equation [5,6]

s=�Mst s

2

6tf

�(k−k0) (1)

where Ms is the substrate elastic modulus, tf and ts arethe film, substrate thicknesses, and k0 is the initialcurvature of the substrate prior to growth.

We can also calculate stress in the substrate s2, usingthe exact solution for the curvature in a bilayer system[7]:

k=6s2

M1t1

I0,x(a) (2)

where function I0,x(a) is defined by

I0,x(a)=& x= t2/t1

0

1+2x+ax2

1+ax(4+6x+4x2+ax3)dx (3)

with

a=M2

M1

(4)

Here M1,2 are the elastic moduli of the 1st and the2nd layers and t1,2 are the thicknesses of the 1st and the2nd layers in the bilayer system. For the 1st layer beingthe film and the 2nd layer being the substrate, t2/t1�1and I0,x(a):I0,�(a) Then we have for the stress in thesubstrate

ss=kMftf

6I0,�(a)(5)

where a=Ms/Mf. For the c-BN/Si system, I0,�(a):1.4.

A schematic of the set up to measure wafer curvatureis shown in Fig. 1. The curvature is determined by

DDD0

=2Lk

cos u(6)

where D is the spacing between adjacent beams, D=D0

at t=0, DD=D(t)−D0, L is the distance from thesample to the CCD camera, and u is the angle ofincidence on the sample.

Reflection high energy electron diffraction (RHEED)was used as an after-growth in situ technique to charac-terize the structure of the films. In addition we used exsitu Fourier transform infrared spectroscopy (FTIR) toprovide additional information about film stress [8,9]and c-BN content [10,11]. The vibrational frequenciesof the infrared-active phonon modes for the cubic andhexagonal phases of BN are well known and tabulated[12]. In particular, the h-BN phase has two modes at770 cm−1 (A2u mode) and 1383 cm−1 (E1u mode) andc-BN has a peak in its absorption spectrum at :1065cm−1 corresponding to the TO phonon mode [13]. Theposition of the c-BN peak depends strongly on thestress in the film [8,9] and provides an extra tool formeasuring residual stress in the films.

3. Results and discussion

The growth of cubic boron nitride on Si (100) pro-ceeds in four steps: first a thin (:50 A, ) amorphouslayer forms at the interface with the substrate; then abuffer layer of hexagonal (turbostratic) BN grows witha thickness of about 500 A, . The formation of the h-BNbuffer layer is followed by nucleation and coalescenceof c-BN. We find that a critical N+ ion energy (sub-strate bias :90 V) is required to initiate cubic phaseformation [14]. After this template layer of c-BN isformed, the substrate bias can be reduced to a lowervalue (:60 V) below which no cubic phase formationis observed.

Fig. 2 shows the stress-induced curvature in BN/Si(100) as a function of film thickness/growth time. Thefilm was grown at a constant substrate bias of −89 V,which optimizes c-BN nucleation as determined byFTIR. The film was grown for a sufficiently long time

Fig. 1. Experimental setup for wafer curvature measurements. Anetalon placed at an angle to the laser beam generates a linear array ofparallel beams. These beams reflect off the sample surface and aredirectly imaged by a CCD detector. Note that by inserting a secondetalon a two-dimensional array of beams can easily be generated,producing an areal map of the wafer curvature [4].

D. Lit6ino6 et al. / Materials Science and Engineering B66 (1999) 79–82 81

Fig. 2. Stress-induced curvature of a 0.45-mm-thick Si (100) wafer asa function of BN film thickness and growth time. The dashed lineindicates missing data where the specular reflectivity is stronglysuppressed during nucleation. The arrow marks the rapid onset ofsubstrate bending accompanied by dislocation injection into thesubstrate. The film stress at this point is 6 GPa.

periods of time [9,15] (see Fig. 3). Since for coalescedfilms (time ]180 min) the TO phonon frequency isdirectly related to the amount of residual stress in thefilms, we conclude that the amount of stress in the filmsstays constant when the thickness reaches some criticalvalue. This proves that, at late times, the bending of thesubstrate observed in Fig. 2 is dominated by defectsinjected into the silicon substrate. Using cross-sectionTEM, we have confirmed the presence of dislocationsin the Si substrate when the critical stress has beenexceeded.

It is possible to estimate the yield strength of siliconat a particular temperature from the thickness of BNfilm at which the rapid onset of defects injected into thesubstrate takes place. Using Eq. (5) and 395 and 184GPa for the elastic moduli of c-BN and silicon respec-tively, we get :0.005 GPa for the yield strength ofsilicon at 1050°C which is the temperature at which thisparticular c-BN film was grown. This result is in goodagreement with previously published data [16].

Obviously, it is desirable to prevent such damage tothe substrate from happening. One way of achievingthis is to decrease the growth temperature thus increas-ing the yield strength of the substrate. Although thiscertainly is possible; it should be noted that the reduc-tion of growth temperature increases the residual stressin the c-BN film and deteriorates adhesion of the filmto the substrate not allowing us to grow thick films inexcess of :1000 A, .

Another way to prevent substrate warping whilepreserving good adhesion is to reduce stress in the filmby implementing reduced bias growth [17]. This tech-nique does not allow us to eliminate the stress com-pletely but it helps to reduce it, thus postponing themoment when the plasticity limit of the substrate isreached. Indeed, we found that reducing the substratebias by 25 V leads to a less strained c-BN film. The filmthickness at which the yield strength of the siliconsubstrate is achieved, is moved from :1000 to :1500A, .

It is interesting to consider the stress build-up in BNfilms in the context of recent suggestions [18] that thecubic phase (c-BN) is stabilized by film stress. The levelof intrinsic stress observed in Fig. 2 prior to nucleationof c-BN (:3 GPa at a film thickness of :500 A, ) isindeed consistent with published phase equilibrium datafor bulk c-BN at T:1000°C [19,20]. However, bychanging the bias conditions such that growth of purephase h-BN is maintained (bias :40 V), we observe(see Fig. 4) initially even larger values of intrinsic stress(\5 GPa at film thickness 5150 A, ). These stressvalues are easily in excess of what is required to stabi-lize the cubic phase in the bulk at the growth tempera-tures we are working with. This suggests that while theresidual stress may help to stabilize the cubic phase,other mechanisms must also be important. In particu-

Fig. 3. Center TO phonon frequency as a function of BN film growthtime. The steep initial drop in TO frequency is related to depolariza-tion effects in isolated BN grains prior to coalescence [9].

so that all the phases of growth described above areprobed. It should be noted that during the nucleationof c-BN on the h-BN buffer layer, a significant surfaceroughening takes place which greatly reduces the specu-lar reflectivity of the film surface. The effect is sopronounced that the reflected beams disappear for ashort period of time, which explains the absence of datafor a growth time interval around 180 min.

When the thickness of the c-BN film reaches somecritical value, the stress it exerts on the substrate ex-ceeds the plasticity limit (yield strength) of silicon at agiven growth temperature. At that point dislocationsstart to form in the substrate leading to a sharp increasein surface curvature. For a 1 mm thick c-BN film on a0.45-mm-thick Si substrate, the radius of curvature canbe as small as 20 cm.

To test whether the sharp increase in the curvature ofthe film surface is related to the induced damage of thesubstrate, we conducted studies of the TO phononfrequency for a series of films grown for different

D. Lit6ino6 et al. / Materials Science and Engineering B66 (1999) 79–8282

Fig. 4. Stress as a function of film thickness and growth time for apure phase h-BN film. The plateau at :50 A, coincides with thenucleation of h-BN on an amorphous interface layer.

91-J-1398 and N0001494-J-0763, and by k-Space Associ-ates, Inc.

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lar, epitaxial matching at the film/substrate interface islikely to play a role in stabilizing the film structure. It isalso interesting to note that the film stress shows atendency to be relieved during the initial (hexagonal)phase of the growth, as evidenced by the decreasingcurvature prior to the nucleation of the cubic phase at:500 A, (see Fig. 2). This raises the possibility that therelatively weak interlayer bonding of the hexagonalphase could be useful as a compliant buffer for subse-quent growth of the cubic phase. We are currentlyinvestigating the relationship between stress in theseinterface structures and the energetics of biased filmgrowth in order to shed more light on this issue.

4. Conclusions

We have demonstrated how the MOSS technique canbe applied to real-time monitoring of strain and itscontrol in thin films using cubic boron nitride as anexample. We have shown that the build-up of stress inhigh modulus films such as BN can lead to significantdistortion of the substrate. At some critical film thicknessthe residual stress is large enough to distort the siliconsubstrate beyond its elastic limit. Our measurementssuggest a possible route to mediate such effects byutilizing the compliant nature of the hexagonal (h-BN)buffer layers. The ability to perform in situ, real-timewafer curvature measurements using the MOSS tech-nique will greatly aid this task, as well as being generallyuseful for monitoring stress in the growth of otherstrained-layer systems.

Acknowledgements

This work is supported by ONR Grant Nos. N00014-

.