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On the chemical and mechanical properties of sputtered silicon nitride films
Thin Solid Films, 219 (1992) 139-145
139
On the chemical and mechanical properties of sputtered silicon nitride films F. Jansen and A. Day BOC Group Technical Center, Murray Hill, NJ 07974 (USA)
L. Wamboldt Airco Coating Technology, Concord, CA 94524 (USA)
(Received February 18, 1992; accepted May 27, 1992)
Abstract Selected chemical and mechanical properties of sputtered silicon nitride films have been studied as a function of deposition conditions. The presence of water vapor in the deposition chamber affects the properties of silicon nitride. This dependence on the chamber conditions can be significantly reduced by ion bombardment of the film during growth. Ion bombardment is thought to preferentially remove residual hydrogen, thus densifying the material. The extreme sensitivity of the etch rate of silicon nitride to relatively small amounts of oxygen present during deposition is demonstrated. When the network connectivity is systematically changed, by the controlled incorporation of hydrogen into the material, the etch rate of the silicon nitride films is shown to correlate with its mechanical wear and scratch properties.
I. Introduction
TABLE 1. Deposition methods for silicon nitride films
Owing to its unique combination of properties, silicon nitride is the material choice for a wide variety of industrial applications. Even in the form of an amorphous thin film, it combines hardness with a high atomic density (about 0.15 g at. cm-3); important properties when the film is applied as a protective or a barrier layer [1]. Transparency and ease of uniform deposition over large areas allow silicon nitride to be used as a durable optical material. The band gap of the material can be systematically changed as a function of the film composition, allowing control over the electrical conductivity in the 109 to 101° ~ cm range, important fovelectro-photographic applications [2]. A m o r p h o u s silicon nitride films can be prepared in several different ways (Table 1). Films prepared by chemical vapor deposition (CVD) provide the benchm a r k in performance but are deposited at high temperatures (700-900 °C). When low temperature deposition is important because of substrate limitations, a variety of plasma deposition methods can be used. These include plasma enhanced chemical vapor deposition (PECVD) techniques, and reactive sputtering. Novel P E C V D techniques (e.g. electron cyclotron resonance [3] and "rem o t e " deposition [4]) can yield nitride films with similar properties as CVD material. These hydrogen free silicon nitride films are restricted to much smaller areas than sputtered and conventional P E C V D silicon nitride films.
Technique
Deposition temperaturea
Areab
BHF etch ratec
Reactive sputtering PECVD diode PECVD remote PECVD ECR CVD
low medium low low high
large large small small small
low medium low high low low low
0040-6090/92/$5.00
~Low temperature: good quality films can be obtained at or close to room temperature; high temperature is greater than 700 °C. bLarge area means uniform deposition over 1 ft2. CLow etch rates are less than 50/~ min ~; high etch rates are greater than 500 A min ~. The hydrogen concentration is conventionally deposited P E C V D films can be up to 30%-40% and is minimized by heating during deposition, e.g. films deposited at 600 °C contain about 10% of bonded H2. An important advantage of sputtering over other thin film deposition techniques is that silicon nitride films with no or little hydrogen can be deposited at r o o m temperature (Table 1). The objective of this study is to determine the deposition conditions under which low temperature deposited silicon nitride films would exhibit optimum mechanical and chemical properties. It is not a priori clear whether these properties can be optimized simultaneously in the same material and how these optimized properties compare with silicon nitride deposited by
© 1992 - - Elsevier Sequoia. All rights reserved
140
F. Jansen et al. / S p u t t e r e d silicon nitride j i l m s
other methods. In this paper it will be shown that there are significant differences between sputtered and PECVD silicon nitride films with the same hydrogen concentration. It will also be shown that the mechanical and chemical properties of sputtered silicon nitride correlate when the network structure is changed by the systematic addition of hydrogen. The "quality" of silicon nitride films is often measured as its etch rate at room temperature in BHF (BHF, 9 volumes of 40% aqueous ammonium fluoride to 1 volume of 48"/0 aqueous hydrofluoric acid, other formulations are sometimes used). In the terminology used in this paper, " g o o d " silicon nitride is equated with slow etch rates. Kanicky and Voke [5], in the course of an extensive study of PECVD silicon nitride films, have determined which film properties are measured by the BHF test. PECVD films had been prepared in eight different commercial reactors under very different conditions of temperature and bombardment and with different gas mixtures. Of all the measured film properties (hydrogen concentration, Si/N ratio, density, stress, refractive index, band gap, BHF etch rate) the strongest correlation was observed [5] between the hydrogen concentration of a film and its BHF etch rate. Because hydrogen affects many other physical properties of the silicon nitride film, the BHF rate also correlated in a statistically significant manner with film properties such as density and refractive index. However these are weaker correlations, presumably because these properties are not determined by hydrogen alone but, be it to a lesser extent, by the Si/N ratio. One has to be careful in the interpretation of the BHF etch test results. A low etch rate of silicon nitride does not necessarily indicate a high quality nitride film with a low hydrogen content; it may also be indicative of a nitrogen deficient film. Both sputtered amorphous silicon and silicon nitride with very little hydrogen (e.g. CVD material) etch very slowly in BHF with rates of 2 and 10 A min J respectively. One needs additional tests to indicate where the material is on the Si/N compositional scale. As electron spectroscopies are notoriously difficult for silicon nitride films [6], ellipsometric measurements of the refractive index were used to complement the etch measurements, relying on the fact that silicon and silicon nitride have very different refractive indices, 4.5 and 2.0 respectively at a wavelength of 6328 A. The ability to produce dense and relatively hydrogen free silicon nitride films yields the opportunity to controllably add hydrogen to silicon nitride and systematically study its effects on the film properties. This is interesting because hydrogen is monovalent and its effect on the bonding properties of silicon nitride can be understood. The effect of hydrogen incorporation in amorphous silicon nitride is to reduce the degree of
substrate holder
(((el
N2/A r
I I
1
I
matching network
Fig. 1. Schematic representation of the experimental coater. crosslinking of the silicon nitrogen network. The weaker network manifests itself in inferior mechanical properties such as abrasive wear or scratch resistance [7]. It is also known that hydrogen incorporation has a chemical effect in that it increases the etch rate [5]. Here, an interpretation in terms of network connectivity is more difficult because one would have to consider in detail the chemical reaction rates at a multitude of different bonding sites as well as take surface effects into account. In this work it is investigated how these mechanical and chemical effects are correlated for a series of silicon nitride films in which the connectivity has been systematically changed by the controlled addition of hydrogen.
2. Experimental details Experiments were carried out in a small experimental batch sputter coater (Fig. 1), described elsewhere [8]. Ultimate background pressures are in the 10 6 Torr range. Films were r.f. self-biased during growth by capacitively coupling r.f. power (13.56MHz) to the substrate. Bias potentials ranged from 0 to - 3 5 0 V where bias potential refers to the r.f. amplitude of the voltage to the substrate holder. At bias voltages greater than 50 V, the substrates are bathed in a clearly visible plasma which exists independent of the sputter plasma. Substrates were lightly doped (IR transparent) silicon wafers which were not purposely heated. The sputter gas mixture consisted of 50% Ar and 50% N2, flowed at a total rate of 20 centimeters per minute (sccm). The sputter pressure was maintained at 5 mTorr. Deposition rates of the reactively sputtered nitride films under these conditions were about 200 A s ] and all the measurements were done on 1500 A thick films. BHF etch rates were determined by measuring the time that it took to completely remove the silicon nitride film from samples with a known thickness. All etch measurements were performed at room temperature and under slight mechanical agitation. The moment of complete removal was determined visually. By partially shielding selected specimens with polyimide tape and periodically interrupting the etch process, it
F. Jansen et al. / Sputtered silicon nitride films
was determined that the etch rate was constant throughout the thickness of the film within the 5% accuracy of the measurements. Coater qualification tests after cleaning of the chamber revealed a poor reproducibility for the BHF etch rate of the deposited nitride films. Etch rates decreased from several hundred to several tens of ~ngstroms per minute as the coater temperature slightly increased at the unloading/loading cycle due to usage. Controlled heating of the coater walls to 75 °C stabilized the etch rate of the silicon nitride films at 40/~ min -I. The application of a bias voltage of - 150 V improved the etch rate further to 25 + 10/k min- ~. Standard deposition conditions were adopted as follows: 50% Ar/50% N2 at a total flow rate of 20 sccm, 5 mTorr, coater wall temperature 75 °C, bias voltage - 1 5 0 V, substrates not intentionally heated and presumed close to 75 °C during deposition. Control runs to check the etch rate reproducibility of 25 + 10 ~ min -~ under standard deposition conditions were frequently performed during the course of the experiments. The mechanical properties were evaluated by measuring abrasive wear and scratch resistance. The abrasive wear apparatus was of the "falling bead" type. Chromium carbide beads of 320 mesh were dropped from about 10 cm high onto the films in a steady trickle for a fixed period of time (5 min). The advantage of using this type of test is that its interpretation is rather straightforward; the less damage to the sample by the abrasive beads after some predetermined time interval, the better the coating will stand up to the action of abrasive media. The disadvantage of this test is that the amount of damage to the coating is hard to quantify. For the present measurements, films were microscopically inspected and qualitatively ranked for damage. A scratch test was used to check the ranking quantitatively for hydrogen doped films. In this test, a sapphire stylus is traversed over the film surface under a continuously increasing mechanical load. During this traverse, the average horizontal pulling force is measured. The results of this test, although it yields a quantitative output, are much harder to interpret. At the load forces between 200 and 300 g that were used, the stylus scratches into but not through the coating. The horizontal pulling force can be expected to increase with the amount of material that is displaced, i.e. the groove depth and width. The deeper the stylus sinks into the film, the more bonds have to be severed when the stylus is pulled at a predetermined speed. Even ignoring the practical complications of what happens when the stylus approaches the film-substrate interface, the negative correlation between the horizontal pulling force and hardness can, obviously not be expected to hold true in e x t r e m a . One expects the horizontal pulling force to be near zero for extremely hard
141
as well as very soft samples. It is not known where the limits of applicability of the scratch test are and this test has therefore only been used as a supplementary measurement to the abrasive wear test. The film structure was determined for selected films to be amorphous as measured by X-ray diffraction. Optical micrography and scanning electron microscopy (SEM) measurements show featureless surfaces and no signs of a pronounced columnar bulk structure. The film composition and bonding characteristics were measured by Fourier transform infrared spectroscopy (FTIR) in the 4 0 0 - 4 5 0 0 c m -1 region. The hydrogen concentration of the silicon nitride films was calculated from the area under the S i - H and N - H peaks and using known calibration factors [9]. 3. Results The BHF etch rate of sputtered silicon nitride films can be reduced to bombarding the film during growth, in agreement with earlier observations [10]. The degree of improvement depends on the starting point; poor nitride can be improved a lot, good nitride can be improved somewhat. Figure 2 illustrates this point, summarizing the results of two experiments. One Set of films was deposited in a coater with walls at r o o m temperature, another in the same coater with heated walls (75 °C), under otherwise identical conditions. The bombardment energy was systematically changed by increasing the r.f. amplitude of the capacitively coupled r.f. bias. At bias voltages less than - 1 5 0 V the etch rates for nitride films made in an upheated reactor are much higher than for a heated reactor. At bias voltages greater than - 150 V the etch rates do no longer depend on whether the reactor walls are heated or not. At
A II
cold wall coater
•
hot wall coater
.=_ J¢
!
11 o
-1~
.2~o
-~o
.4oo
bias v o b g e (V) Fig. 2. The BHF etch rates of sputtered silicon nitride films as a function of the r.f. bias voltage, which is a measure for the ion bombardment energy o f the film during deposition. The upper curve (squares) is for an unheated chamber, the lower curve (black diamonds) for a chamber heated to 75 °C.
142
F. Jansen et al. / Sputtered silicon nitride films
.2.06
120 etch rate
2.04
100 80
2.02 "~
6O
z.oo .-~
40
1.98 &
20
1.96
.m
t_
1.94
0
100
0
200
300
400
bias v o l t a g e (V)
Fig. 3. The BHF etch rate and ellipsometric index of refraction (6328 A) of sputtered silicon nitride films deposited in a heated sputter chamber. voltages between - 1 0 0 and - 1 5 0 V the etch rate is minimized for heated and cold chambers. For the silicon nitride films deposited in heated chambers, in addition to the etch rates, the refractive indices were measured as a function of the bias voltage and the data are shown in Fig. 3. The refractive index is maximum for the slowest etching film. Starting from the conditions that produce relatively slow etching silicon nitride (heated ~valls, - 150 V bias), increasing amounts of hydrogen gas were admixed with the nitrogen/argon sputter gas mixture. The BHF rate of silicon nitride systematically increases with the percentage of hydrogen admixture as shown in Fig. 4. The smallest amount of oxygen that could be controllably admixed in our experiments with the nitrogen/argon sputter atmosphere was 2%. The BHF etch rate of the film deposited from this mixture was 1100/k min-1 and the material had a refractive index of 1.5, indicating that it is substantially silicon oxide. It is known that BHF attacks silicon oxide at high rates (about 2000/~min-~). To investigate the effects of what presumably are only trace amounts of oxygen,
Q
silicon nitride films were also deposited after exposing the deposition chamber to dry oxygen gas and to an oxygen plasma. The exposure of the coater interior to a pressure of 100 mTorr of oxygen gas for 5 min. (which was subsequently pumped out to pressures less than 10 _5 Tort for 5 min.) just prior to the standard silicon nitride deposition, increased the etch rates to 30 ~, min- ~. The exposure of the chamber to an oxygen plasma under the same conditions of time and pressure increased the etch rate to 70 ~ min -~. The oxygen contamination in these films was observable in the FTIR spectra at 1100 cm -1. Aluminum, silver and zirconium, were added to silicon nitride deposited at otherwise standard conditions by cosputtering from a second source. The results were always the same in that the etch rates were observed to increase, typically by an order of magnitude and more for additions at the percent level. Silicon nitride films deposited under standard conditions showed metallic impurities at trace levels of less than 0.01% combined and any possible effects of these (uncontrolled) impurities are ignored. To determine the possible effects of mechanical stress on the etch rate, compressive and tensile stresses were induced in long beam specimens by external bending forces. From the specimen geometry, it was estimated that the induced stresses were at least one order of magnitude higher than the compressive intrinsic stress typically observed. The etch rates of the compressive and tensile specimens were measured to be higher than for the control specimens but the increase was less than a factor of two. Therefore, any effects of (uncontrolled) intrinsic stress on the observations are ignored. Silicon nitride films with systematically changing etch rates, either due to hydrogen incorporation or changing bias conditions, were subjected to mechanical testing. The results of the dropping bead test are shown in Fig. 5. Within the limits of interpretation posed by a subjective measurement, it can be seen that minimum damage is sustained by the - 150 V bias sample and that damage increases with increasing hydrogen content. The results of the scratch test are shown in Fig. 6 for the hydrogen series. These measurements confirm the trends observed in the abrasive wear test and suggest, not surprisingly, that the ability of a material to withstand scratching correlates with its abrasive wear resistance.
m
4. Discussion
1
1'0
2'0
3'0
4'0
5'0
hydrogen (% in gas) Fig. 4. The BHF rate of sputtered silicon nitride as a function ~f the
percentage of hydrogenadmixtureto the 50/50 nitrogen-argonsputter gas. The total gas flow and sputter pressure are kept constant.
With the incorporation of hydrogen in sputtered silicon nitride films, the etch rate systematically increases. The same effect is observed [5] in PECVD SiN, :H. However, measurements of the hydrogen concentration in the films by F T I R show a quantitative
F. Jansen et al. / Sputtered silicon nitride films
143
5 0% H2
-0 V
.G ~
-50 V
3% H 2
3
o 0
•
0
6% H 2
-250 V
8% H2
-350 V
10% H2
12
.
i
N 4
0
2
4
6
8
•
,
•
i
4;
Fig. 7. Comparison of the BHF etch rates of sputtered (black squares) and PECVD silicon nitride films (open squares) as a function of hydrogen in the film. The PECVD data is taken from Kanicky, reference 5.
-150 V
Fig. 5. Optical micrographs of the surface appearance of sputtered silicon nitride films after a'5 minute bead drop (see text for details) as a function of bias voltage (unheated reactor) and hydrogen concentration in the sputtering atmosphere (heated reactor walls).
I ,9
i
10 20 30 hydrogen (% in film)
10
hydrogen concentration In sputter gas (%) Fig. 6. The horizontal pulling force of a sapphire stylus, averaged over the vertical load regime between 200 and 300 gr, as a function of the hydrogen admixture to the sputtergas (black circles). For comparison, a CVD sample is also shown (open circle).
difference between the behavior of sputtered and PECVD silicon nitride films (Fig. 7). For the same amount of hydrogen, the etch rates of sputtered films are about two orders of magnitude higher than for PECVD films. Various film characteristics that might account for this difference in an obvious way were examined and eliminated. Elements other than silicon, nitrogen and hydrogen could not be detected analytically other than
in trace amounts. The amount of oxygen that would cause a two orders of magnitude difference in etch rate would be easily detectable by FTIR as well as by a decrease in the refractive index. Although mechanical stress does increase the etch rates, the effect of a conceivable level of intrinsic stress would be within a factor of two, rather than two orders of magnitude. The main difference between the sputtered silicon nitride films of this study and PECVD films with a comparable amount of hydrogen is in the substrate temperature during deposition: room temperature vs. 500-600 °C. To rationalize the observed difference in etch rate between sputtered and PECVD films one might speculate that this difference is related to the deposition temperature of the film. The temperature at which the film material condenses most likely affects the degree of bond saturation ("dangling bonds") through the mobility of the species arriving at the film surface. Extensive spin density data for silicon nitride as a function of deposition temperature are not available. For a comparable material, a-Si, it has been shown [11] that the spin density systematically decreases by an order of magnitude when the deposition temperature is increased from room temperature (spin density 1020 cm -3) to 500 °C. Both the incorporation of hydrogen and the deposition of silicon nitride at low temperatures have a negative effect on the network connectivity. If indeed the BHF etch rate measures the cohesion of the network and does not discriminate between hydrogenated and unhydrogenated dangling bonds, one would conclude from the data that low temperature unhydrogenated silicon nitride has a dangling bond density equivalent to about 10% of hydrogen, the approximate shift between the etch rate lines (Fig. 7). One could easily imagine that one out of ten bonds would be unterminated for a severely bond constrained material such as amorphous silicon nitride, particularly when arriving atoms have a limited mobility. At an atomic density of 6 × 1022cm -3, these considerations would suggest a
144
F. Jansen et al. / Sputtered silicon nitride fihns
density of 6 x 10 2] c m - 3 of dangling bonds. However, the measured spin density of room temperature deposited silicon nitride is about an order of magnitude lower [12], between 1020 and 1021cm-3. This discrepancy might be related to the fact that only (an unknown) part of the total number of dangling bonds are visible in the spin density measurements. The spin signal is only due to the singly occupied silicon dangling bonds and nitrogen is invisible. It is thus plausible that the higher etch rates of low temperature deposited sputtered silicon nitride films, relative to high temperature deposited PECVD films with the same hydrogen concentration, is due to the higher unsaturated (broken) bond density of the sputtered material, compounding the effects of hydrogen common to both types of material. Oxygen incorporation increases the etch rate of silicon nitride films [13, 14]. It takes little oxygen in the sputter gas, on the order of one percent, to deposit a material that is more like silicon oxide than silicon nitride. This sensitivity can be explained when it is realized that nitrogen only reacts with silicon at appreciably high rates in its dissociated form as atomic nitrogen. Oxygen reacts as a molecule as one might conclude from the common observation that a bare silicon surface readily oxidizes. Equilibrium thermodynamic calculations also indicate that the silicon oxide formation rate is energetically favored over the reaction with nitrogen by several orders of magnitude. It can be reasonably estimated [15] that the atomic species are about two orders of magnitude less abundant than their parent molecules. Therefore, oxygen introduced at the percent level presents a comparable amount of reactive species as the nitrogen plasma and poses a severe contamination problem. The problem is experimentally illustrated by the results of the oxygen exposure experiments where it was observed that a brief exposure of the coater interior to oxygen gas about doubled the etch rate. This sensitivity to an ubiquitous impurity is exacerbated when one attempts to deposit multilayered stacks incorporating both oxide and nitride films. As the BHF etch rate of silicon nitride is so sensitive to incorporated hydrogen and oxygen, it has to be expected that also small traces of water vapor in the deposition chamber increases the etch rate. The differences observed in the etch rates of silicon nitride deposited in a cold chamber and a heated coater at zero bias voltage indicate that indeed residual water vapor increases the etch rate (Fig. 2). The application of a bias voltage of - 150 V negates this dependency on the chamber condition and a minimum etch rate is obtained right about where the refractive index is maximum (Fig. 3). The most likely mechanism for the bombardment to decrease the etch rate, is through the
removal of water vapor fragments, particularly hydrogen, from the film. The removal of hydrogen densities the film and increases the index of refraction. The increase in the refractive index with bias voltage quantitatively supports the interpretation that the applied bias lowers the etch rate by removing traces of hydrogen from the film. The refractive index systematically decreases from 2.09 to 1.76 when from 0% to 10% of hydrogen is admixed with the N 2/Ar feed gas. The etch rate then increases from 30 to 3 5 0 A m i n t. This is consistent with an increase of 0.1 in the refractive index of the bias series (Fig. 3) where the etch rate drops from 35 to 20/~min ~ when the bias voltage is increased from 0 to - 1 5 0 V. The beneficial effect on the etch rate of hydrogen leaving the film, does not continue with increasing bias. Rather, an increase in the etch rate is observed when bias voltages exceed - 150 V. One could surmise that at these bombardment energies, one observes the detrimental effects of damage to the material. It is generally observed that ion energies greater than 100V are needed to obtain appreciable sputter yields [16]. The refractive index data in Fig. 3 supports this interpretation as it decreases at high bombardment energies, presumably because the material becomes less dense due to sputter damage. The decrease in the network integrity by the addition of hydrogen shows the anticipated correlation between the mechanical and chemical properties. The quantitative mechanical test where a stylus is traversed over the surface of the film correlates with the etch rate of the silicon nitride as shown in Fig. 3. The bead drop test shows the same result in a qualitative manner (Fig. 5). The observed correlation between the mechanical and chemical properties suggests that these tests indeed measure the same or closely related properties of the silicon nitride film.
5. Conclusions
The etch rate of silicon nitride films systematically increases with its hydrogen concentration. For the same amount of hydrogen, the etch rates of sputtered films are about two orders of magnitude higher than for PECVD films. This difference is attributed to the higher degree of bond imperfection of sputtered silicon nitride films deposited at low temperature. The exposure of the deposition chamber to oxygen gas or an o x y g e n plasma prior to the deposition of silicon nitride films, increases the etch rate of the silicon nitride. Oxygen contamination at the percent level presents a comparable amount of reactive species as the nitrogen plasma and poses a severe contamination problem for sputter deposited silicon nitride.
F. Jansen et td. / Sputtered silicon nitride films
Water vapor in the deposition chamber increases the etch rate. The application of a bias voltage high enough to resputter the silicon nitride film negates any dependency on the chamber condition to a point where the materials have the same (minimum) etch rate, right about where the refractive index is maximum. This beneficial effect of bias does not continue as an increase in the etch rate is observed when bias voltages exceed - 150 V, likely a result of damage to the material. The decrease in the network integrity by the addition of hydrogen shows a correlation between the mechanical and chemical properties. This correlation suggests that these tests measure the same or closely related properties of the silicon nitride film. The most fundamental property that is Changed with the introduction of hydrogen in a covalently bonded material is the connectivity of the network, and it is this property that is measured by the mechanical and chemical tests.
Acknowledgments
We would like t o acknowledge the help of Randy Kurie with the development of the bead drop test. Dr. Jagdeesh Bandekhar did the FTIR measurements and Steve Nadel of Airco Coating Technology helped with the mechanical scratch test measurements. Finally we would like to thank Drs. Valery Koss, Abe Belkind, John Vossen and Ted Gens for valuable discussions.
145
References
1 J. R. Troxell, M. I. Harrington, J. C. Erskine, W. H. Dumbaugh, F. P. Fehlner and R. A. Miller, IEEE Electron Device Lett., EDL7 (1986) 597. 2 F. Jansen, J. Mort, S. Grammatica, M. Morgan and I. Chen, J. Non-Cryst. Sol., 66 (1984) 357. 3 S. Matsuo and M. Kiuchi, Jpn. J. Appl. Phys., 22 (1983) L210. 4 S. V. Hattangady, G. G. Fountain, R. A. Rudder and R. J. Markunas, J. Vac. Sci. Technol., A7 (1989) 570. 5 J. Kanicki and N. Voke, Mat. Res. Soc. Syrup. Proc., 68(1986) 167. 6 H. E. Maes, J. Remmerie, M. Hinoul, R. Vlaeminck and R. Vanden Berghe, Proc. Electrochem. Soc., 83-8 (1983)415. 7 F. Jansen and M. A. Machonkin, Thin Solid Films, 144 (1986) 227. 8 R. C. Ross, R. Sherman, R. A. Bunger and S. J. Nadel, SPIE Optical Mater. Technol. Energy Efficiency Solar Energy Conversion, 823 (1987) 47. J. L. Vossen, S. Krommenhoek and V. A. Koss, J. Vac. Sci. Technol., A9 (1991) 600. 9 W. A. Lanford and M. J. Rand, J. Appl. Phys., 49 (1978) 2473. 10 A. W. Stephens, J. L. Vossen and W. Kern, J. Electrochem. Soc., 123 (1976) 303. 11 P. A. Thomas, M. H. Brodsky, D. Kaplan and D. Lepine, Phys. Rev. B., 18 (1978) 3059. 12 T. Shimizu, S. Oozora, A. Morimoto, M. Kumeda and N. Ishii, Sol. Energy Mater., 8 (1982) 311. 13 W. R. Knolle, J. W. Osenbach and A. Elia, in V. J. Kapoor and K. T. Hankins (eds.), Proc. Symp. Silicon Nitride and Silicon Oxide Films, Vol. 10, The Electrochemical Society, 1987, 237. 14 L. M. Loewenstein and C. M. Tipton, J. Electrochem. Soc., 138 (1991) 1389. 15 F. Jansen, in J. Mort and F. Jansen (eds.), Plasma Deposited Thin Films, CRC Press, Boca Raton, FL, 1986, Chapter 1. 16 C. M. Melliar Smith and C. J. Mogab, in J. L. Vossen and W. Kern (eds.), Thin Film Processes, Academic Press, New York, 1978, Chapter V-2.
139
On the chemical and mechanical properties of sputtered silicon nitride films F. Jansen and A. Day BOC Group Technical Center, Murray Hill, NJ 07974 (USA)
L. Wamboldt Airco Coating Technology, Concord, CA 94524 (USA)
(Received February 18, 1992; accepted May 27, 1992)
Abstract Selected chemical and mechanical properties of sputtered silicon nitride films have been studied as a function of deposition conditions. The presence of water vapor in the deposition chamber affects the properties of silicon nitride. This dependence on the chamber conditions can be significantly reduced by ion bombardment of the film during growth. Ion bombardment is thought to preferentially remove residual hydrogen, thus densifying the material. The extreme sensitivity of the etch rate of silicon nitride to relatively small amounts of oxygen present during deposition is demonstrated. When the network connectivity is systematically changed, by the controlled incorporation of hydrogen into the material, the etch rate of the silicon nitride films is shown to correlate with its mechanical wear and scratch properties.
I. Introduction
TABLE 1. Deposition methods for silicon nitride films
Owing to its unique combination of properties, silicon nitride is the material choice for a wide variety of industrial applications. Even in the form of an amorphous thin film, it combines hardness with a high atomic density (about 0.15 g at. cm-3); important properties when the film is applied as a protective or a barrier layer [1]. Transparency and ease of uniform deposition over large areas allow silicon nitride to be used as a durable optical material. The band gap of the material can be systematically changed as a function of the film composition, allowing control over the electrical conductivity in the 109 to 101° ~ cm range, important fovelectro-photographic applications [2]. A m o r p h o u s silicon nitride films can be prepared in several different ways (Table 1). Films prepared by chemical vapor deposition (CVD) provide the benchm a r k in performance but are deposited at high temperatures (700-900 °C). When low temperature deposition is important because of substrate limitations, a variety of plasma deposition methods can be used. These include plasma enhanced chemical vapor deposition (PECVD) techniques, and reactive sputtering. Novel P E C V D techniques (e.g. electron cyclotron resonance [3] and "rem o t e " deposition [4]) can yield nitride films with similar properties as CVD material. These hydrogen free silicon nitride films are restricted to much smaller areas than sputtered and conventional P E C V D silicon nitride films.
Technique
Deposition temperaturea
Areab
BHF etch ratec
Reactive sputtering PECVD diode PECVD remote PECVD ECR CVD
low medium low low high
large large small small small
low medium low high low low low
0040-6090/92/$5.00
~Low temperature: good quality films can be obtained at or close to room temperature; high temperature is greater than 700 °C. bLarge area means uniform deposition over 1 ft2. CLow etch rates are less than 50/~ min ~; high etch rates are greater than 500 A min ~. The hydrogen concentration is conventionally deposited P E C V D films can be up to 30%-40% and is minimized by heating during deposition, e.g. films deposited at 600 °C contain about 10% of bonded H2. An important advantage of sputtering over other thin film deposition techniques is that silicon nitride films with no or little hydrogen can be deposited at r o o m temperature (Table 1). The objective of this study is to determine the deposition conditions under which low temperature deposited silicon nitride films would exhibit optimum mechanical and chemical properties. It is not a priori clear whether these properties can be optimized simultaneously in the same material and how these optimized properties compare with silicon nitride deposited by
© 1992 - - Elsevier Sequoia. All rights reserved
140
F. Jansen et al. / S p u t t e r e d silicon nitride j i l m s
other methods. In this paper it will be shown that there are significant differences between sputtered and PECVD silicon nitride films with the same hydrogen concentration. It will also be shown that the mechanical and chemical properties of sputtered silicon nitride correlate when the network structure is changed by the systematic addition of hydrogen. The "quality" of silicon nitride films is often measured as its etch rate at room temperature in BHF (BHF, 9 volumes of 40% aqueous ammonium fluoride to 1 volume of 48"/0 aqueous hydrofluoric acid, other formulations are sometimes used). In the terminology used in this paper, " g o o d " silicon nitride is equated with slow etch rates. Kanicky and Voke [5], in the course of an extensive study of PECVD silicon nitride films, have determined which film properties are measured by the BHF test. PECVD films had been prepared in eight different commercial reactors under very different conditions of temperature and bombardment and with different gas mixtures. Of all the measured film properties (hydrogen concentration, Si/N ratio, density, stress, refractive index, band gap, BHF etch rate) the strongest correlation was observed [5] between the hydrogen concentration of a film and its BHF etch rate. Because hydrogen affects many other physical properties of the silicon nitride film, the BHF rate also correlated in a statistically significant manner with film properties such as density and refractive index. However these are weaker correlations, presumably because these properties are not determined by hydrogen alone but, be it to a lesser extent, by the Si/N ratio. One has to be careful in the interpretation of the BHF etch test results. A low etch rate of silicon nitride does not necessarily indicate a high quality nitride film with a low hydrogen content; it may also be indicative of a nitrogen deficient film. Both sputtered amorphous silicon and silicon nitride with very little hydrogen (e.g. CVD material) etch very slowly in BHF with rates of 2 and 10 A min J respectively. One needs additional tests to indicate where the material is on the Si/N compositional scale. As electron spectroscopies are notoriously difficult for silicon nitride films [6], ellipsometric measurements of the refractive index were used to complement the etch measurements, relying on the fact that silicon and silicon nitride have very different refractive indices, 4.5 and 2.0 respectively at a wavelength of 6328 A. The ability to produce dense and relatively hydrogen free silicon nitride films yields the opportunity to controllably add hydrogen to silicon nitride and systematically study its effects on the film properties. This is interesting because hydrogen is monovalent and its effect on the bonding properties of silicon nitride can be understood. The effect of hydrogen incorporation in amorphous silicon nitride is to reduce the degree of
substrate holder
(((el
N2/A r
I I
1
I
matching network
Fig. 1. Schematic representation of the experimental coater. crosslinking of the silicon nitrogen network. The weaker network manifests itself in inferior mechanical properties such as abrasive wear or scratch resistance [7]. It is also known that hydrogen incorporation has a chemical effect in that it increases the etch rate [5]. Here, an interpretation in terms of network connectivity is more difficult because one would have to consider in detail the chemical reaction rates at a multitude of different bonding sites as well as take surface effects into account. In this work it is investigated how these mechanical and chemical effects are correlated for a series of silicon nitride films in which the connectivity has been systematically changed by the controlled addition of hydrogen.
2. Experimental details Experiments were carried out in a small experimental batch sputter coater (Fig. 1), described elsewhere [8]. Ultimate background pressures are in the 10 6 Torr range. Films were r.f. self-biased during growth by capacitively coupling r.f. power (13.56MHz) to the substrate. Bias potentials ranged from 0 to - 3 5 0 V where bias potential refers to the r.f. amplitude of the voltage to the substrate holder. At bias voltages greater than 50 V, the substrates are bathed in a clearly visible plasma which exists independent of the sputter plasma. Substrates were lightly doped (IR transparent) silicon wafers which were not purposely heated. The sputter gas mixture consisted of 50% Ar and 50% N2, flowed at a total rate of 20 centimeters per minute (sccm). The sputter pressure was maintained at 5 mTorr. Deposition rates of the reactively sputtered nitride films under these conditions were about 200 A s ] and all the measurements were done on 1500 A thick films. BHF etch rates were determined by measuring the time that it took to completely remove the silicon nitride film from samples with a known thickness. All etch measurements were performed at room temperature and under slight mechanical agitation. The moment of complete removal was determined visually. By partially shielding selected specimens with polyimide tape and periodically interrupting the etch process, it
F. Jansen et al. / Sputtered silicon nitride films
was determined that the etch rate was constant throughout the thickness of the film within the 5% accuracy of the measurements. Coater qualification tests after cleaning of the chamber revealed a poor reproducibility for the BHF etch rate of the deposited nitride films. Etch rates decreased from several hundred to several tens of ~ngstroms per minute as the coater temperature slightly increased at the unloading/loading cycle due to usage. Controlled heating of the coater walls to 75 °C stabilized the etch rate of the silicon nitride films at 40/~ min -I. The application of a bias voltage of - 150 V improved the etch rate further to 25 + 10/k min- ~. Standard deposition conditions were adopted as follows: 50% Ar/50% N2 at a total flow rate of 20 sccm, 5 mTorr, coater wall temperature 75 °C, bias voltage - 1 5 0 V, substrates not intentionally heated and presumed close to 75 °C during deposition. Control runs to check the etch rate reproducibility of 25 + 10 ~ min -~ under standard deposition conditions were frequently performed during the course of the experiments. The mechanical properties were evaluated by measuring abrasive wear and scratch resistance. The abrasive wear apparatus was of the "falling bead" type. Chromium carbide beads of 320 mesh were dropped from about 10 cm high onto the films in a steady trickle for a fixed period of time (5 min). The advantage of using this type of test is that its interpretation is rather straightforward; the less damage to the sample by the abrasive beads after some predetermined time interval, the better the coating will stand up to the action of abrasive media. The disadvantage of this test is that the amount of damage to the coating is hard to quantify. For the present measurements, films were microscopically inspected and qualitatively ranked for damage. A scratch test was used to check the ranking quantitatively for hydrogen doped films. In this test, a sapphire stylus is traversed over the film surface under a continuously increasing mechanical load. During this traverse, the average horizontal pulling force is measured. The results of this test, although it yields a quantitative output, are much harder to interpret. At the load forces between 200 and 300 g that were used, the stylus scratches into but not through the coating. The horizontal pulling force can be expected to increase with the amount of material that is displaced, i.e. the groove depth and width. The deeper the stylus sinks into the film, the more bonds have to be severed when the stylus is pulled at a predetermined speed. Even ignoring the practical complications of what happens when the stylus approaches the film-substrate interface, the negative correlation between the horizontal pulling force and hardness can, obviously not be expected to hold true in e x t r e m a . One expects the horizontal pulling force to be near zero for extremely hard
141
as well as very soft samples. It is not known where the limits of applicability of the scratch test are and this test has therefore only been used as a supplementary measurement to the abrasive wear test. The film structure was determined for selected films to be amorphous as measured by X-ray diffraction. Optical micrography and scanning electron microscopy (SEM) measurements show featureless surfaces and no signs of a pronounced columnar bulk structure. The film composition and bonding characteristics were measured by Fourier transform infrared spectroscopy (FTIR) in the 4 0 0 - 4 5 0 0 c m -1 region. The hydrogen concentration of the silicon nitride films was calculated from the area under the S i - H and N - H peaks and using known calibration factors [9]. 3. Results The BHF etch rate of sputtered silicon nitride films can be reduced to bombarding the film during growth, in agreement with earlier observations [10]. The degree of improvement depends on the starting point; poor nitride can be improved a lot, good nitride can be improved somewhat. Figure 2 illustrates this point, summarizing the results of two experiments. One Set of films was deposited in a coater with walls at r o o m temperature, another in the same coater with heated walls (75 °C), under otherwise identical conditions. The bombardment energy was systematically changed by increasing the r.f. amplitude of the capacitively coupled r.f. bias. At bias voltages less than - 1 5 0 V the etch rates for nitride films made in an upheated reactor are much higher than for a heated reactor. At bias voltages greater than - 150 V the etch rates do no longer depend on whether the reactor walls are heated or not. At
A II
cold wall coater
•
hot wall coater
.=_ J¢
!
11 o
-1~
.2~o
-~o
.4oo
bias v o b g e (V) Fig. 2. The BHF etch rates of sputtered silicon nitride films as a function of the r.f. bias voltage, which is a measure for the ion bombardment energy o f the film during deposition. The upper curve (squares) is for an unheated chamber, the lower curve (black diamonds) for a chamber heated to 75 °C.
142
F. Jansen et al. / Sputtered silicon nitride films
.2.06
120 etch rate
2.04
100 80
2.02 "~
6O
z.oo .-~
40
1.98 &
20
1.96
.m
t_
1.94
0
100
0
200
300
400
bias v o l t a g e (V)
Fig. 3. The BHF etch rate and ellipsometric index of refraction (6328 A) of sputtered silicon nitride films deposited in a heated sputter chamber. voltages between - 1 0 0 and - 1 5 0 V the etch rate is minimized for heated and cold chambers. For the silicon nitride films deposited in heated chambers, in addition to the etch rates, the refractive indices were measured as a function of the bias voltage and the data are shown in Fig. 3. The refractive index is maximum for the slowest etching film. Starting from the conditions that produce relatively slow etching silicon nitride (heated ~valls, - 150 V bias), increasing amounts of hydrogen gas were admixed with the nitrogen/argon sputter gas mixture. The BHF rate of silicon nitride systematically increases with the percentage of hydrogen admixture as shown in Fig. 4. The smallest amount of oxygen that could be controllably admixed in our experiments with the nitrogen/argon sputter atmosphere was 2%. The BHF etch rate of the film deposited from this mixture was 1100/k min-1 and the material had a refractive index of 1.5, indicating that it is substantially silicon oxide. It is known that BHF attacks silicon oxide at high rates (about 2000/~min-~). To investigate the effects of what presumably are only trace amounts of oxygen,
Q
silicon nitride films were also deposited after exposing the deposition chamber to dry oxygen gas and to an oxygen plasma. The exposure of the coater interior to a pressure of 100 mTorr of oxygen gas for 5 min. (which was subsequently pumped out to pressures less than 10 _5 Tort for 5 min.) just prior to the standard silicon nitride deposition, increased the etch rates to 30 ~, min- ~. The exposure of the chamber to an oxygen plasma under the same conditions of time and pressure increased the etch rate to 70 ~ min -~. The oxygen contamination in these films was observable in the FTIR spectra at 1100 cm -1. Aluminum, silver and zirconium, were added to silicon nitride deposited at otherwise standard conditions by cosputtering from a second source. The results were always the same in that the etch rates were observed to increase, typically by an order of magnitude and more for additions at the percent level. Silicon nitride films deposited under standard conditions showed metallic impurities at trace levels of less than 0.01% combined and any possible effects of these (uncontrolled) impurities are ignored. To determine the possible effects of mechanical stress on the etch rate, compressive and tensile stresses were induced in long beam specimens by external bending forces. From the specimen geometry, it was estimated that the induced stresses were at least one order of magnitude higher than the compressive intrinsic stress typically observed. The etch rates of the compressive and tensile specimens were measured to be higher than for the control specimens but the increase was less than a factor of two. Therefore, any effects of (uncontrolled) intrinsic stress on the observations are ignored. Silicon nitride films with systematically changing etch rates, either due to hydrogen incorporation or changing bias conditions, were subjected to mechanical testing. The results of the dropping bead test are shown in Fig. 5. Within the limits of interpretation posed by a subjective measurement, it can be seen that minimum damage is sustained by the - 150 V bias sample and that damage increases with increasing hydrogen content. The results of the scratch test are shown in Fig. 6 for the hydrogen series. These measurements confirm the trends observed in the abrasive wear test and suggest, not surprisingly, that the ability of a material to withstand scratching correlates with its abrasive wear resistance.
m
4. Discussion
1
1'0
2'0
3'0
4'0
5'0
hydrogen (% in gas) Fig. 4. The BHF rate of sputtered silicon nitride as a function ~f the
percentage of hydrogenadmixtureto the 50/50 nitrogen-argonsputter gas. The total gas flow and sputter pressure are kept constant.
With the incorporation of hydrogen in sputtered silicon nitride films, the etch rate systematically increases. The same effect is observed [5] in PECVD SiN, :H. However, measurements of the hydrogen concentration in the films by F T I R show a quantitative
F. Jansen et al. / Sputtered silicon nitride films
143
5 0% H2
-0 V
.G ~
-50 V
3% H 2
3
o 0
•
0
6% H 2
-250 V
8% H2
-350 V
10% H2
12
.
i
N 4
0
2
4
6
8
•
,
•
i
4;
Fig. 7. Comparison of the BHF etch rates of sputtered (black squares) and PECVD silicon nitride films (open squares) as a function of hydrogen in the film. The PECVD data is taken from Kanicky, reference 5.
-150 V
Fig. 5. Optical micrographs of the surface appearance of sputtered silicon nitride films after a'5 minute bead drop (see text for details) as a function of bias voltage (unheated reactor) and hydrogen concentration in the sputtering atmosphere (heated reactor walls).
I ,9
i
10 20 30 hydrogen (% in film)
10
hydrogen concentration In sputter gas (%) Fig. 6. The horizontal pulling force of a sapphire stylus, averaged over the vertical load regime between 200 and 300 gr, as a function of the hydrogen admixture to the sputtergas (black circles). For comparison, a CVD sample is also shown (open circle).
difference between the behavior of sputtered and PECVD silicon nitride films (Fig. 7). For the same amount of hydrogen, the etch rates of sputtered films are about two orders of magnitude higher than for PECVD films. Various film characteristics that might account for this difference in an obvious way were examined and eliminated. Elements other than silicon, nitrogen and hydrogen could not be detected analytically other than
in trace amounts. The amount of oxygen that would cause a two orders of magnitude difference in etch rate would be easily detectable by FTIR as well as by a decrease in the refractive index. Although mechanical stress does increase the etch rates, the effect of a conceivable level of intrinsic stress would be within a factor of two, rather than two orders of magnitude. The main difference between the sputtered silicon nitride films of this study and PECVD films with a comparable amount of hydrogen is in the substrate temperature during deposition: room temperature vs. 500-600 °C. To rationalize the observed difference in etch rate between sputtered and PECVD films one might speculate that this difference is related to the deposition temperature of the film. The temperature at which the film material condenses most likely affects the degree of bond saturation ("dangling bonds") through the mobility of the species arriving at the film surface. Extensive spin density data for silicon nitride as a function of deposition temperature are not available. For a comparable material, a-Si, it has been shown [11] that the spin density systematically decreases by an order of magnitude when the deposition temperature is increased from room temperature (spin density 1020 cm -3) to 500 °C. Both the incorporation of hydrogen and the deposition of silicon nitride at low temperatures have a negative effect on the network connectivity. If indeed the BHF etch rate measures the cohesion of the network and does not discriminate between hydrogenated and unhydrogenated dangling bonds, one would conclude from the data that low temperature unhydrogenated silicon nitride has a dangling bond density equivalent to about 10% of hydrogen, the approximate shift between the etch rate lines (Fig. 7). One could easily imagine that one out of ten bonds would be unterminated for a severely bond constrained material such as amorphous silicon nitride, particularly when arriving atoms have a limited mobility. At an atomic density of 6 × 1022cm -3, these considerations would suggest a
144
F. Jansen et al. / Sputtered silicon nitride fihns
density of 6 x 10 2] c m - 3 of dangling bonds. However, the measured spin density of room temperature deposited silicon nitride is about an order of magnitude lower [12], between 1020 and 1021cm-3. This discrepancy might be related to the fact that only (an unknown) part of the total number of dangling bonds are visible in the spin density measurements. The spin signal is only due to the singly occupied silicon dangling bonds and nitrogen is invisible. It is thus plausible that the higher etch rates of low temperature deposited sputtered silicon nitride films, relative to high temperature deposited PECVD films with the same hydrogen concentration, is due to the higher unsaturated (broken) bond density of the sputtered material, compounding the effects of hydrogen common to both types of material. Oxygen incorporation increases the etch rate of silicon nitride films [13, 14]. It takes little oxygen in the sputter gas, on the order of one percent, to deposit a material that is more like silicon oxide than silicon nitride. This sensitivity can be explained when it is realized that nitrogen only reacts with silicon at appreciably high rates in its dissociated form as atomic nitrogen. Oxygen reacts as a molecule as one might conclude from the common observation that a bare silicon surface readily oxidizes. Equilibrium thermodynamic calculations also indicate that the silicon oxide formation rate is energetically favored over the reaction with nitrogen by several orders of magnitude. It can be reasonably estimated [15] that the atomic species are about two orders of magnitude less abundant than their parent molecules. Therefore, oxygen introduced at the percent level presents a comparable amount of reactive species as the nitrogen plasma and poses a severe contamination problem. The problem is experimentally illustrated by the results of the oxygen exposure experiments where it was observed that a brief exposure of the coater interior to oxygen gas about doubled the etch rate. This sensitivity to an ubiquitous impurity is exacerbated when one attempts to deposit multilayered stacks incorporating both oxide and nitride films. As the BHF etch rate of silicon nitride is so sensitive to incorporated hydrogen and oxygen, it has to be expected that also small traces of water vapor in the deposition chamber increases the etch rate. The differences observed in the etch rates of silicon nitride deposited in a cold chamber and a heated coater at zero bias voltage indicate that indeed residual water vapor increases the etch rate (Fig. 2). The application of a bias voltage of - 150 V negates this dependency on the chamber condition and a minimum etch rate is obtained right about where the refractive index is maximum (Fig. 3). The most likely mechanism for the bombardment to decrease the etch rate, is through the
removal of water vapor fragments, particularly hydrogen, from the film. The removal of hydrogen densities the film and increases the index of refraction. The increase in the refractive index with bias voltage quantitatively supports the interpretation that the applied bias lowers the etch rate by removing traces of hydrogen from the film. The refractive index systematically decreases from 2.09 to 1.76 when from 0% to 10% of hydrogen is admixed with the N 2/Ar feed gas. The etch rate then increases from 30 to 3 5 0 A m i n t. This is consistent with an increase of 0.1 in the refractive index of the bias series (Fig. 3) where the etch rate drops from 35 to 20/~min ~ when the bias voltage is increased from 0 to - 1 5 0 V. The beneficial effect on the etch rate of hydrogen leaving the film, does not continue with increasing bias. Rather, an increase in the etch rate is observed when bias voltages exceed - 150 V. One could surmise that at these bombardment energies, one observes the detrimental effects of damage to the material. It is generally observed that ion energies greater than 100V are needed to obtain appreciable sputter yields [16]. The refractive index data in Fig. 3 supports this interpretation as it decreases at high bombardment energies, presumably because the material becomes less dense due to sputter damage. The decrease in the network integrity by the addition of hydrogen shows the anticipated correlation between the mechanical and chemical properties. The quantitative mechanical test where a stylus is traversed over the surface of the film correlates with the etch rate of the silicon nitride as shown in Fig. 3. The bead drop test shows the same result in a qualitative manner (Fig. 5). The observed correlation between the mechanical and chemical properties suggests that these tests indeed measure the same or closely related properties of the silicon nitride film.
5. Conclusions
The etch rate of silicon nitride films systematically increases with its hydrogen concentration. For the same amount of hydrogen, the etch rates of sputtered films are about two orders of magnitude higher than for PECVD films. This difference is attributed to the higher degree of bond imperfection of sputtered silicon nitride films deposited at low temperature. The exposure of the deposition chamber to oxygen gas or an o x y g e n plasma prior to the deposition of silicon nitride films, increases the etch rate of the silicon nitride. Oxygen contamination at the percent level presents a comparable amount of reactive species as the nitrogen plasma and poses a severe contamination problem for sputter deposited silicon nitride.
F. Jansen et td. / Sputtered silicon nitride films
Water vapor in the deposition chamber increases the etch rate. The application of a bias voltage high enough to resputter the silicon nitride film negates any dependency on the chamber condition to a point where the materials have the same (minimum) etch rate, right about where the refractive index is maximum. This beneficial effect of bias does not continue as an increase in the etch rate is observed when bias voltages exceed - 150 V, likely a result of damage to the material. The decrease in the network integrity by the addition of hydrogen shows a correlation between the mechanical and chemical properties. This correlation suggests that these tests measure the same or closely related properties of the silicon nitride film. The most fundamental property that is Changed with the introduction of hydrogen in a covalently bonded material is the connectivity of the network, and it is this property that is measured by the mechanical and chemical tests.
Acknowledgments
We would like t o acknowledge the help of Randy Kurie with the development of the bead drop test. Dr. Jagdeesh Bandekhar did the FTIR measurements and Steve Nadel of Airco Coating Technology helped with the mechanical scratch test measurements. Finally we would like to thank Drs. Valery Koss, Abe Belkind, John Vossen and Ted Gens for valuable discussions.
145
References
1 J. R. Troxell, M. I. Harrington, J. C. Erskine, W. H. Dumbaugh, F. P. Fehlner and R. A. Miller, IEEE Electron Device Lett., EDL7 (1986) 597. 2 F. Jansen, J. Mort, S. Grammatica, M. Morgan and I. Chen, J. Non-Cryst. Sol., 66 (1984) 357. 3 S. Matsuo and M. Kiuchi, Jpn. J. Appl. Phys., 22 (1983) L210. 4 S. V. Hattangady, G. G. Fountain, R. A. Rudder and R. J. Markunas, J. Vac. Sci. Technol., A7 (1989) 570. 5 J. Kanicki and N. Voke, Mat. Res. Soc. Syrup. Proc., 68(1986) 167. 6 H. E. Maes, J. Remmerie, M. Hinoul, R. Vlaeminck and R. Vanden Berghe, Proc. Electrochem. Soc., 83-8 (1983)415. 7 F. Jansen and M. A. Machonkin, Thin Solid Films, 144 (1986) 227. 8 R. C. Ross, R. Sherman, R. A. Bunger and S. J. Nadel, SPIE Optical Mater. Technol. Energy Efficiency Solar Energy Conversion, 823 (1987) 47. J. L. Vossen, S. Krommenhoek and V. A. Koss, J. Vac. Sci. Technol., A9 (1991) 600. 9 W. A. Lanford and M. J. Rand, J. Appl. Phys., 49 (1978) 2473. 10 A. W. Stephens, J. L. Vossen and W. Kern, J. Electrochem. Soc., 123 (1976) 303. 11 P. A. Thomas, M. H. Brodsky, D. Kaplan and D. Lepine, Phys. Rev. B., 18 (1978) 3059. 12 T. Shimizu, S. Oozora, A. Morimoto, M. Kumeda and N. Ishii, Sol. Energy Mater., 8 (1982) 311. 13 W. R. Knolle, J. W. Osenbach and A. Elia, in V. J. Kapoor and K. T. Hankins (eds.), Proc. Symp. Silicon Nitride and Silicon Oxide Films, Vol. 10, The Electrochemical Society, 1987, 237. 14 L. M. Loewenstein and C. M. Tipton, J. Electrochem. Soc., 138 (1991) 1389. 15 F. Jansen, in J. Mort and F. Jansen (eds.), Plasma Deposited Thin Films, CRC Press, Boca Raton, FL, 1986, Chapter 1. 16 C. M. Melliar Smith and C. J. Mogab, in J. L. Vossen and W. Kern (eds.), Thin Film Processes, Academic Press, New York, 1978, Chapter V-2.