- Email: [email protected]
Effect of substrate bias on properties of silicon nitride films prepared by activated reactive evaporation
Effect of substrate bias on properties of silicon nitride films prepared by activated reactive evaporation Jong S. Yoon Materid?
Chandra Department
Design
Lahorutor):
Korea
V. Deshpandey qf Materials
Instiiute
of‘ Science and Technulog~~, P.O.
and Rointan
Science and Engineering,
Box
I3 I
Cheongrymg,
Seoul (Korerr)
F. Bunshah
School of Engineering
and Applied
Science.
Unicersify
oj’ Cuiifhrnirr.
Los Angeles,
CA, (USA)
Abstract Substrate biasing was explored to improve the quality of activated reactive evaporation processed (ARE) silicon nitride films. The samples were prepared at various r.f. (radio frequency) substrate bias power for a given electron beam current, pressure, and r.f. discharge power. The quality of the ARE silicon nitride deposited at over 150 W of substrate bias was comparable with high temperature processed CVD films. These films showed a refractive index of 2.00, an absorption edge of 4.9 eV, a dielectric constant of 7.2-7.7 and hydrogen and oxygen concentrations far less than 1 at.%.
1. Introduction There has been a long history of interest in the use of thin films of silicon nitride for various applications, especially in electronic devices. One of the most widely recognized advantages of the silicon nitride film in the applications for electron devices is its low permeability to the alkali ions, such as Na+, K+, etc. [l-3] and moisture [4-61. It has been known that silicon dioxide has an inherent structural porosity which leads to high permeability toward moisture and other impurities [4]. Furthermore, silicon nitride films are chemically inert and mechanically strong [6, 71. Therefore, they provide good physical protection against scratching during packaging, mounting and bonding operations and also help to protect the metallization from corrosion in electronic devices. Silicon nitride films have been prepared mainly by chemical vapor deposition (CVD) [4, 8-101 and plasma assisted CVD (PACVD) [3, 6, 1l- 161. It is well known that PACVD silicon nitride films possess a considerable amount of hydrogen in the films, lo-30 at.% [6, 11, 12, 17- 191. Even though high temperature CVD silicon nitride films have less hydrogen than PACVD silicon nitride films, they contain 6-8 at.% of hydrogen [ 17, 201. Hydrogen in silicon nitride films can play a positive or negative role depending on the application of the films. When silicon nitride films are used as a passivation layer, some amount of hydrogen in the film is desirable for electrical reasons. Amorphous silicon nitrides contain a significant number of dangling bonds and other types of native defects due
0040-6090/92/$5.00
to its high coordination number [20, 211. Hydrogen incorporation can passivate these defects and reduces the midgap density of states [22-241. On the other hand, potential uses of Si,N, films are prevented by the residual H atoms in the silicon nitride due to the following problems: (1) Because of its excellent barrier effect for alkali ions and its good electrical insulation, amorphous Si,N, can be an ideal candidate for gate insulators. However, silicon nitride films that contain H atoms show poor insulating behavior due to some defects attributed to the presence of hydrogen [24, 251. (2) The memory effects of the metal-nitride-oxide semiconductors (MNOS) structures for non-volatile memory devices are associated with charge capture in the Si,N, layer. The memory traps (electron or hole localized states) exist inside the band gap of the Si,N, [5]. It has been known that these localized states are due to hydrogen in the films [26]. It is hard to control these localized states. Therefore, it is desirable to have a Si,N, film which has a low hydrogen concentration for this application. (3) When the metal-oxide-silicon field-effect transistor (MOSFET) is passivated by silicon nitride, the hydrogen in the film can cause a parameter shift in MOSFET devices [5, 271 due to the out-diffusion of the weakly bonded hydrogen [27]. (4) In the application of silicon nitride films as an encapsulant for annealing an ion-implanted compound semiconductor, hydrogen evolution induces large stresses and imperfections within the films, which leads to encapsulant failure [28].
‘,‘I 1992
Elsevier
Sequoia.
All rights
reserved
J. S. Yoon et al. / Substrate
In addition to the problems discussed above, it has been noted that residual hydrogen atoms cause longterm instabilities in transistors, change the resistance of poly load resistors (lightly doped polysilicon), change the fuse characteristics of TiW fuses in programmable read-only memory devices (PROMS), and cause blisters in large underlying Al patterns on oxides [ 171. It is, therefore, necessary to produce H-free low temperature silicon nitride films for wide ranging applications in the microelectronics industry. In this study, substrate biasing in the ARE process was explored to synthesize silicon nitride with a low concentration of hydrogen and improve the quality of ARE silicon nitride films.
2. Experimental
details
Figure I is a schematic view of a vacuum chamber and associated equipment employed in this study. The system was capable of achieving a vacuum in the low 10 - 7 Torr range. A reactive gas was introduced at a controlled rate in the region between the evaporation source and the substrates which were located in the upper chamber to maintain the desired working pressure. High purity (99.999999%) Si was evaporated in nitrogen plasma, which was excited using an inductively coupled stainless-steel mesh attached to a 13.56 MHz r.f. supply. The films were deposited onto n-type (100)
Liquid
Heating
nitrogen
copper p-e
shield
tiij Substrate holder
Heater vuvvuu Substrate
_
I
w
Stainless steel mesh /
w Matching unit L
1
Nitrogen
I -
Fig. I. Schematic silicon Initride.
diagram
of the ARE
system for the deposition
of
bius of‘ ARE silicon nitride,films
81
silicon wafers, fused silica: pieces and soda lime glass substrates placed beneath the heater. Since silicon films are nonconducting, d.c. bias of the substrate is not possible due to charge accumulation on the film surface. Therefore, r.f. bias was employed for substrate biasing in this study to avoid the problem of charge accumulation. Ions are almost immobile, whereas the electrons can follow the temporal variations in the applied potential at the typical r.f. frequency used (13.56 MHz). When the substrate is coupled to the r.f. generator, a negative voltage is developed on the insulating film due to the difference in mobility between electrons and ions. Since the insulating film constitutes a capacitor in the electrical circuit, there should be no d.c. component to the current flow. Therefore, negative voltage (self-biased) on the insulating surface is developed to compensate the difference in mobility in electron and ion and to satisfy the condition of net zero-to-peak voltage of the r.f. signal. Unfortunately, our equipment could not measure (or estimate) the effective d.c. negative bias on the surface. We measured the forward power and the reflected power. The period for the substrate to be biased positively is very short and the substrate is mostly negatively biased during the r.f. cycle. Therefore, it is possible to provide continuous bombardment of the insulating films with positive ions using r.f. bias as the film grows. The high energy flux of positive ions accelerated by the electrical field to the substrate surface is able to densify the growing film and prevent the formation of microvoids, thereby increasing the packing density. Contaminants and poorly adherent spots of the film may also be sputtered off due to ion bombardment during deposition thus producing a denser film. A copper plate cooled by liquid nitrogen was located near the substrates to condense residual water molecules in the chamber. Heat shields were placed between the cold finger and heat source to prevent heating-up of the cold finger due to radiation from the heat source. The depositions were performed with biasing the substrates at various r.f. power in the range of O-610 W. The other deposition parameters were kept at constant values, such as 400 “C substrate temperature, 2 mTorr nitrogen pressure, and 300 W of discharge power. Infrared transmission measurements were carried out with a single beam Fourier-transform IR spectrometer (Mattson Instr. CYGNUS-25) and a Perkin-Elmer 1330 IR spectrophotometer. All spectra were obtained at room temperature. Most of the transmission data were measured in the IR region from 200 cm - ’ to 4000 cm-‘. For IR transmission measurements, high resistivity ( > 80 R cm) Si wafers polished on both sides were used as substrates. UV/Vis transmission spectra for the samples deposited on fused silica pieces were recorded
82
J. S. Yoon rt ul. 1 Substrate
on a Perkin-Elmer Model 330 UV-Vis spectrophotometer. The silicon nitride films were scanned against a reference substrate in the wavelength range 200-1000 nm. SIMS (Cameca IMF 3F secondary ion mass spectrometer) technique was used mainly to determine the content of hydrogen and other impurity elements in the films. The SIMS measurement was performed with a Cs + primary ion source biased at 17 kV (around 150 nA beam current). The refractive indices of the sample deposited on the Si wafers were determined using single beam (632.8 nm) ellipsometry (Rudolf Research/Auto EL). The high-frequency C- V curves were recorded on a Hewlett Packard 4284A Precision LCR Meter at 1 MHz. The bias was automatically varied 10 times per second between minus and plus 30 V. Metal-insulator-semiconductor (MIS) structures were prepared by depositing Al dots of 350 nm thickness on silicon nitride and silicon using a thermal evaporation technique. Capacitors of 75 and 100 nm diameter on the upper electrode were formed by photolithography.
3. Results and discussion 3.1. Infrared spectroscopy The IR spectrum for the ARE silicon nitride film prepared without biasing the substrate (Fig. 2(a)) shows a broad absorption band near 3340 cm - ‘. This band was initially thought to be due to the N-H bond stretching vibration mode because of the peak position. The band minima almost coincide with those of N-H bond stretching mode (3330-3350 cm-‘) [ 18, 191. However, it was noticed that the peak shape was different from those of N-H bond stretching mode. These bands observed at around 3340 cm ’ showed a rapid decrease in transmission near 3700 cm ’ and the band-
a
b
Wavenumber
(cm -1)
Fig. 2. Infrared transmission spectra of the ARE silicon nitride films prepared without substrate biasing (a) and with substrate biasing at (b) 150 W.
bias oj ARE silicon nitrirkfiltm
widths were much larger than those of the N-H bond stretching mode. In order to explain these absorption bands, the incorporation of water molecules in the films had to be considered. These bands are believed to be due to the water molecule absorbed in samples after deposition. Pliskin [29] has reported an absorption band near 3330 cm _ ’ for electron-beam evaporated SiO, films due to adsorbed water molecules. He concluded that the electron beam gun evaporated films contained a large number of micropores and the intensities of the absorption band around 3340 cm ’ were proportional to the degree of porosity of the film. The water molecules may diffuse easily through these micropores. The microporosity also causes a larger true surface to hold and react with the absorbed water due to a high degree of bond strain in addition to porosity. Electron beam gun evaporated films exhibit very high reactivity with water at relatively low temperatures [30]. In a similar manner, water molecules are believed to be incorporated in the films through micropores of ARE silicon nitride films. The rapid decrease in transmission near 3700 cm ’ in Fig. 2(a) is due to the formation of O-H bonds in hydrogen-bonded silanol ( SiOH) groups [ 19, 291, i.e. water molecules react with the Si,N, to form SiOH. On reaction of the water with the nitride to form silanol groups, strain in the bonds may be relieved. The absorption peak near 3340 cm ~~’ due to water molecules, decreases as the r.f. biasing of the substrate increases and it disappears for substrate bias over 150 W (Fig. 2(b)). r.f. substrate bias is expected to cause continuous ion (created in the plasma) bombardment of the film as it grows. This helps to densify the growing film and prevent the formation of microvoids, thereby increasing the packing density in the film. All the spectra show that the absorption band at around 850-900 cm ~~’ is due to the asymmetric in plane SiiN bond stretching vibration mode. This absorption band shifts to lower frequencies as the substrate bias increases to 150 W. Table 1 shows that the position of the absorption band minima varies from 906 to 849 cm ’ as the r.f. bias power is changed from 0 to 150 W. This absorption band minima approaches 850 cm ~’ which is close to the value of PACVD Si3N, film, 849-857 cm ’ [ 15, 201, as the peak near 3340 cm ’ due to water molecules disappears in the IR spectra. This shift can be explained by considering the degree of oxygen contamination resulting from the reaction between moisture and nitride films discussed earlier. It is well known that oxygen incorporation in silicon nitride causes a shift in the absorption band due to SiiN bond stretching near 850 cm ’ towards higher frequencies [20]. Table 1 indicates that the minima of the absorption band due to SiiN stretching shifts towards lower wave numbers as the oxygen contamination due to the absorbed water
J. S. Yoon et (11. 1 Substrate
TABLE I. Summary of the vibration properties nitride prepared with substrate biasing RF bias
Wave
power
numbers
bius of ARE silicon nitride Jilms
83
for the ARE silicon
(cm-‘)
(W)
molecules
Water
stretching
906 853 849 858 868
3331 3330
0 50 150 310 610
Si- N bond
1.20
molecules is decreased. Even though the reason is not clear, these peak minima increase with the r.f. power of the substrate bias over 150 W, which may be attributed to increases in structural disorder with increasing substrate bias. The degree of structural disorder can be increased due to ion bombardment [ 1, 191. No absorption relating to the H atom was observed in the IR spectra for the samples prepared at and over 150 W of r.f. substrate bias power. Considering that the detection limit of H by infrared spectroscopy is 0.5-l at.% [31, 321, it can be concluded that the hydrogen content is less than 1 at.% in these films. This conclusion was confirmed by SIMS results. Table 2 shows the dependence on substrate bias power of hydrogen and oxygen content in bulk. The hydrogen and oxygen concentrations in the film increase with decrease in r.f. substrate bias power. Within the experimental uncertainties (around 5%), the samples prepared using N, gas at over 150 W of substrate power appear to be near stoichiometric Si,N,, with oxygen and hydrogen contents less than IO*’ atom cmp3. 3.2. Refractive index Figure 3 shows the relationship between refractive index and substrate bias power. Refractive index increases rapidly from 1.81 to 2.0 as the substrate bias power varies from 0 to 150 W. Figure 3 shows a slight increase in the refractive index in the range of 150-610 W. The variation of refractive index with substrate bias power can be explained by the degree of oxygen contamination and the change in density of the film.
TABLE Substrate bias (W) 615 490 310 150 0
2. Hydrogen Reactive
and oxygen
content
measured
by SIMS
H( atoms/cm’)
0( atoms/cm3)
6 6 4 3 1
7 x lO”( 6 I at.‘%,) 5 x lO’“( G 1 at.%) 7 x 102”( < 1 at.%) 1 x 102’( < 1 at.%) 4 x 102’(5 at.%)
gas N* N* N* N, N,
x x x x x
1019( 6 1 at.“/;,) 1019( < 1 at.%) 102’( < 1 at.%) lO*“( < 1 at.%) lO*‘( 1 at.‘%))
0 Substrate
Fig. 3. Plot of refractive
BE
(13
power,
index as a function
Watts)
of substrate
bias power.
The rapid increase in refractive index in the range from 0 to 150 W is mainly due to the decrease in oxygen content in the film. It is known that the presence of oxygen in the bulk of the film has a profound influence in lowering the refractive index with a linear dependence on the atom percent of oxygen [9, 321. Silicon oxynitrides have a refractive index between 2.0 (for silicon nitride) and 1.46 (for silicon dioxide). The refractive index of silicon oxynitride varies linearly with the oxygen content of the film within this range. The microporosity decreases greatly when substrate bias power is varied in the range between 0 and 150 W, thereby reducing the amount of water molecules adsorbed into the film after deposition. The degree of oxygen contamination due to the adsorbed water, therefore, decreases with the increase in substrate bias (see Table 2). The slight increase in refractive index above 150 W is believed to be due to the increase in the density of the film. The samples prepared over 150 W of substrate power have a refractive index of 2.00-2.02. These values are very close to the refractive index (2.00-2.01) of high quality CVD Si,N,films. Generally, the films deposited by plasma vapor deposition techniques, such as PACVD or the ARE process, have lower refractive indices than those of the high temperature processed films (CVD or reactive evaporation processed films). In the plasma vapor deposition processes, the surface diffusion of adatoms is much retarded due to the low substrate temperature and therefore the film normally contains more defects and has a smaller density compared to a high temperature processed film. However, the density of the film in the ARE process is believed to be increased by biasing the substrate during the deposition of the film.
84
J. S. Yom et al. 1 Substrate
TABLE 1 MHz
3. Dielectric
Substrate
bias (W)
constants
50 150 300
obtained
Reactive Nz N, Nz
gas
by C-V
measurement
Dielectric
bias of’ ARE .vilicon nitride,filtm
at
constant
6.0 7.2-7.1 7.3
3.3. Optical gap The optical energy gap was obtained by two methods, Taut rule and absorption edge method. In the Taut rule method, the optical gap is found by fitting the absorption data to the equation (NE)”
= B(E - E,)
(1)
where cc is the absorption coefficient calculated from transmission, refractive index, n, and film thickness, B is a constant, E is the photon energy, ,!$ is the energy of the optical gap in electron volts. In the absorption edge method, the position of the absorption edge was approximated by the wavelength or energy at which the ratio of absorbance, log,(&/Z), to the film thickness (in micrometers) is unity [3]. In other words, the absorption edge estimated by this method is the wavelength or energy at which the absorption coefficient is 104/e cm - ‘. This method is very simple and useful for quality control. Figure 4 shows the variation of optical gap obtained by both methods, absorption edge method and the Taut rule, with the substrate bias power. As can be seen in this figure, the two sets of values show very little difference ( co.05 eV). The optical gap of the biased ARE (BARE) silicon nitride is decreased rapidly between 0 and 150 W and then slightly as the substrate bias power varies from 150 to 610 W. The samples obtained at over 150 W of substrate bias have a gap value of 4.85-4.90 eV. The rapid decrease in optical gap in the range of 0- 150 W is believed to be mainly due to the change of oxygen content in the films. SIMS data (Table 2) shows that the oxygen content in the film decreased from around 5 at.% to the very low value ( < 1 at.‘%) as the power of the substrate bias increases from 0 to 150 W. The substrate bias effect on the optical gap for the samples which are the near stoichiometric Si,N, (prepared at over 150 W) has to be considered in two different ways. As discussed in the previous section, the density of the film increases with the substrate bias power. For near stoichiometric Si,N, films, the increase in density of the film causes an increase in the optical gap [ 331. Bauer [33] has stated that one of the reasons for the difference in the optical gap between the sputtered Si,N, film (4.55 eV) and the pyrolytic Si,N, film
QWS.O
4.70:
0
100 ’
absorption Taut rule
I
200 ’
Substrate Fig. 4. Plot of optical
0
300 ’
Bias
edge I
’
’
’
600 ’
I
700 1
(rfUiorer5atts)
gap as a function
of substrate
bias power.
(5.20 eV) is due to the difference in the density of the films obtained by two different techniques. It has been known that the density for sputtered Si,N, is smaller than that for the pyrolytic sample. Thus, the optical gap of ARE silicon nitride is likely to increase with increasing substrate bias which tends to produce denser films. The degree of structural disorder can also affect the optical gap of silicon nitride films [ 1, 3, 331. The valence and conduction states are moving into the forbidden gap as the degree of the structural disorder increases [33], producing an absorption tail at the band edges. Even though the structure of amorphous Si,N, does not have a long-range order, it shows the same short-range interatomic spacing as that for crystalline r and j? silicon nitride [ 11. Ion bombardment produces a further disordered structure thus degrading short-range order [ 1, 191. Stein [l] has reported that the degradation of short-range order for the CVD Si,N, film introduced by ion bombardment causes a decrease in the energy of the absorption edge. In our case, the number of ions bombarding the surface of the growing film and their energies are believed to increase with increase in substrate bias. This probably leads to increase in structural disorder of the sample, thereby decreasing the optical gap with the increase in substrate bias power. In Fig. 4, the slight decrease in optical gap at over 150 W may be explained by the fact that the effect of change in structural disorder dominates the effect of change in the density with the substrate biasing power on optical gap. The smaller values (4.85-4.90 eV) of optical gap for the BARE Si,N, films than those (around 5.20 eV) for the CVD S&N, films are believed to be due to the more disordered structure of the BARE S&N, films.
J. S. Yoon et al. / Substrate bias of ARE silicon nitride films
1 .o
-----150 watts 300 watts
0.8 i
:: 5
0.6 -
3 0.4 -
0.2 ,-
0.0
1
-35
I
-30
I
-25
I
I
-20
-15
Bias
I
-10
1
-5
I
0
5
:o
1
(Volts)
Fig. 5. Capacitance us. voltage curve for the ARE prepared at 150 and 300 W of the substrate.
silicon
nitrides
3.4. C - V measurements high-frequency C- V curves (1 MHz) of a capacitor with a 100 nm thick ARE silicon nitride film on n-type (1 0 0) silicon are shown in Fig. 5. All the C-V profiles obtained in this experiment exhibited injection type (clockwise) hysteresis. This is due to trapping and detrapping of charge, probably in traps near the SiN-Si interface. Gereth et al. [34] have reported that the hysteresis is caused by charging and discharging in the traps located at the interface between silicon nitride and thin native oxide on Si wafer. In this case, charges are transfered through the thin oxide layer by tunneling [ 14, 341. The data in Fig. 5 show a more negative flat band voltage than the theoretical value. This means that there may be an additional voltage drop across the nitride to overcome the band-bending produced in Si by the positive charge stored at the interface and in the bulk of the film in addition to the difference in the work functions of the metal and the silicon so that the conduction and valence bands in Si are flat. Figure 5 shows that the flat band voltage shifts to the more negative side with increasing substrate bias indicating that films deposited at higher substrate bias have larger positive stored charge than that of films at lower substrate bias. This negative C-V shift is believed to be mainly due to the increase in displacement damage with substrate bias power. Stein [35] has observed a similar C- V shift for PACVD silicon nitrideSi structure due to displacement damage induced during ion implantation. Figure 5 also shows that the C-V curve for the sample deposited with higher substrate bias is more stretched out along the gate bias. This is attributed to the increase in displacement damage at the interface with substrate bias power. The minimum capacitance of the C- V curve for the sample deposited at higher substrate bias is higher than that of the lower-biased sample (Fig. 5). This is believed due to the large amount of donor-like defects which are
85
produced in the bulk of the Si substrate at the beginning of the deposition at high substrate bias [36]. The static dielectric constant was estimated by measuring the capacitance at 1 MHz of MIS structures. The value of the dielectric constant ranges from 6.0 to 7.7 depending on the deposition conditions (Table 3). These values are appreciably greater than the optical dielectric constant (square root of the refractive index). The static dielectric constant decreases as the substrate bias power decreases. This behavior can be attributed to the increase in microporosity with decreasing substrate bias power. The values of the dielectric constant for BARE silicon nitride obtained with over 150 W of substrate bias power is close to that of high-quality CVD silicon nitride (7-7.5) [37, 381.
4. Conclusions
Typical
The biased ARE (BARE) process produces films with a quality comparable with high temperature processed CVD silicon nitride but with much lower concentrations of hydrogen. The concentrations of hydrogen and oxygen in the films were far less than 1 at.“/o. The films obtained at over 150 W of substrate bias showed a refractive index of 2.00, absorption edge of 4.9 eV, and dielectric constant of 7.2-7.7. These are close to the reported values for high quality CVD silicon nitride films.
References I H. J. Stein, J. Appl. Phys., 47 ( 1976) 3421. 2 J. W. Peters, F. L. Gebhart and T. C. Hall, Solid State Technol., September (1980) 121. 3 M. J. Rand and D. R. Wonsidler, J. Electrochem. Sot., 125 (1978) 99. 4 T. L. Chu, C. H. Lee and G. A. Gruber, J. Electrochem. Sot., I14 (1967) 717. M. Kitabatake and K. Wasa, Appl. Phys. Lett., 49 (1986) 927. A. K. Sinha, Solid State Technol., April (1980) 133. A. R. Reinberg, J. Electron. Mater., 8 (1979) 345. F. Fransen, R. V. Berghe, R. Vlaenminck, N. Hinoul, J. Remmerie and H. E. Maes, Surf. Interface Anal., 7 (1985) 79. R. Flitsch, J. A. Aboaf and W. A. Pliskin, J. 9 S. I. Raider, Electrochem. Sot., 123 (1976) 560. 10 H. J. Stein and H. A. R. Wegener, J. Electrochem. Sot., 124 (1977) 908. B. Hepp, N. Szydlo, E. Criton and J. 11 N. Proust, F. Boulitrop, Magarino, Extended Abstracts of the 18th Int. Co@ on Solid State Devices and Materials, Tokyo, 1986, p. 245. I2 H. Watanabe, K. Katoh and M. Yasui, Jpn. J. Appl. Phys., 23 (1984) 1. 13 H. Watanabe, K. Katoh and M. Yasui, Jpn. J. Appl. Phys., 21 (1982) L34l. 14 R. Hezel and R. Schorner, J. Appl. Phys., 52 (1981) 3076. I5 E. Y. Chang, G. T. Cibuzar and K. P. Pande, IEEE Trans. Electron. De&es, 35 (1988) 1412.
86
J. S. Yoon et al. / Suhstrute
I6 M. J. Helix, K. V. Vaidyanathan, B. G. Streetman, H. B. Dietrich and P. K. Chatterjee, Thin Solid Films, 55 ( 1978) 143. I7 D. Pramanik, Semiconductor Int., June (1988) 92. I8 R. Szeda, Physica B, 129 (1985) 435. 19 W. A. Landford and M. J. Rand, J. Appl. Phys., 49 ( 1978) 2473. 20 R. C. Budhani, S. Prakash, H. J. Doerr and R. F. Bunshah, J. Vat. Sci. Technol., A, 5 (1987) 1644. 21 R. C. Budhani, R. F. Bunshah and P. A. Flinn, Appl. Phys. Lett., 52 ( 1988) 284. 22 S. Yokoyama, M. Hirose and Y. Osaka, Jpn. J. Appl. Phys.. 20 (1981) L35. 23 S. Hasegawa, M. Matuura and Y. Kurata, Appl. Phys. Lett., 49 ( 1986) 1272. 24 H. Stotzel, J. Non-Cryst. Solids, 90 (1987) 267. 25 G. Lucovsky and S. Y. Lin, J. Vat. Sci. Technol., B, 3 (1985) 1122. 26 S. Fujita and A. Sasaki, J. Electrochem. Sot., 132 (1985) 398. 27 E. P. G. T. van de Ven, Solid State Technol., April (1981) 167. 28 A. Kiermasz, A. McQuarrie and J. Bhardwaj, Semiconducfor Int., Nouember (1987) 107.
bias
qf ARE silicon nitride films 29 W. A. Pliskin, J. Vat. Sci. Technol., 14 (1977) 1064. 30 W. A. Pliskin, J. Elecrrochem. Sot.. II2 (1965) 148(c). 31 B. R. Zhang, Z. Yu, G. J. Collins, T. Hwang and W. H. Ritchie. J. Vat. Sri. Technol., A, 7 ( 1989) 176. 32 S. Thomas and R. J. Mattox, J. Electrochem. Sot,., 124 (1977) 1942. 33 J. Bauer, fhys. Status Solidi A, 39 ( 1977) 411. 34 R. Gereth and W. Scherber, J. Electrochem. SOL.., I I9 ( 1972) 1249, 35 H. J. Stein, in V. J. Kapoor and H. J. Stein (Eds.), Silicon Nitride Thin hwulating Films, The Electrochemical Society, Pennington. NJ, 1983, Vol. 83-8, p. 253. 36 D. Bouchier. A. Bossebouef and G. Gautherin, in V. J. Kapoor and K. T. Hankin (Eds.), Silicon Nitride and Silicon Dioxide thin Insuhning Films, The Electrochemical Society, Pennington. NJ, 1986, Vol. 87-10, p. 527. 37 T. Y. Chiu and W. G. Oldham, in V. J. Kapoor and H. J. Stein (Eds.), Silicon Nitride Thin Insulating Films, The Electrochemical Society, Pennington. NJ, 1983, Vol. 83-8, p. 367. 38 E. H. Nicollian. in S. B. Bibyk and V. J. Kapoor (Eds.), Silicon Nitride and Silicon Dioxide Thin Insulating Films, The Electrochemical Society. Pennington, NJ, 1988, Vol. 89-7. p. 177.
Chandra Department
Design
Lahorutor):
Korea
V. Deshpandey qf Materials
Instiiute
of‘ Science and Technulog~~, P.O.
and Rointan
Science and Engineering,
Box
I3 I
Cheongrymg,
Seoul (Korerr)
F. Bunshah
School of Engineering
and Applied
Science.
Unicersify
oj’ Cuiifhrnirr.
Los Angeles,
CA, (USA)
Abstract Substrate biasing was explored to improve the quality of activated reactive evaporation processed (ARE) silicon nitride films. The samples were prepared at various r.f. (radio frequency) substrate bias power for a given electron beam current, pressure, and r.f. discharge power. The quality of the ARE silicon nitride deposited at over 150 W of substrate bias was comparable with high temperature processed CVD films. These films showed a refractive index of 2.00, an absorption edge of 4.9 eV, a dielectric constant of 7.2-7.7 and hydrogen and oxygen concentrations far less than 1 at.%.
1. Introduction There has been a long history of interest in the use of thin films of silicon nitride for various applications, especially in electronic devices. One of the most widely recognized advantages of the silicon nitride film in the applications for electron devices is its low permeability to the alkali ions, such as Na+, K+, etc. [l-3] and moisture [4-61. It has been known that silicon dioxide has an inherent structural porosity which leads to high permeability toward moisture and other impurities [4]. Furthermore, silicon nitride films are chemically inert and mechanically strong [6, 71. Therefore, they provide good physical protection against scratching during packaging, mounting and bonding operations and also help to protect the metallization from corrosion in electronic devices. Silicon nitride films have been prepared mainly by chemical vapor deposition (CVD) [4, 8-101 and plasma assisted CVD (PACVD) [3, 6, 1l- 161. It is well known that PACVD silicon nitride films possess a considerable amount of hydrogen in the films, lo-30 at.% [6, 11, 12, 17- 191. Even though high temperature CVD silicon nitride films have less hydrogen than PACVD silicon nitride films, they contain 6-8 at.% of hydrogen [ 17, 201. Hydrogen in silicon nitride films can play a positive or negative role depending on the application of the films. When silicon nitride films are used as a passivation layer, some amount of hydrogen in the film is desirable for electrical reasons. Amorphous silicon nitrides contain a significant number of dangling bonds and other types of native defects due
0040-6090/92/$5.00
to its high coordination number [20, 211. Hydrogen incorporation can passivate these defects and reduces the midgap density of states [22-241. On the other hand, potential uses of Si,N, films are prevented by the residual H atoms in the silicon nitride due to the following problems: (1) Because of its excellent barrier effect for alkali ions and its good electrical insulation, amorphous Si,N, can be an ideal candidate for gate insulators. However, silicon nitride films that contain H atoms show poor insulating behavior due to some defects attributed to the presence of hydrogen [24, 251. (2) The memory effects of the metal-nitride-oxide semiconductors (MNOS) structures for non-volatile memory devices are associated with charge capture in the Si,N, layer. The memory traps (electron or hole localized states) exist inside the band gap of the Si,N, [5]. It has been known that these localized states are due to hydrogen in the films [26]. It is hard to control these localized states. Therefore, it is desirable to have a Si,N, film which has a low hydrogen concentration for this application. (3) When the metal-oxide-silicon field-effect transistor (MOSFET) is passivated by silicon nitride, the hydrogen in the film can cause a parameter shift in MOSFET devices [5, 271 due to the out-diffusion of the weakly bonded hydrogen [27]. (4) In the application of silicon nitride films as an encapsulant for annealing an ion-implanted compound semiconductor, hydrogen evolution induces large stresses and imperfections within the films, which leads to encapsulant failure [28].
‘,‘I 1992
Elsevier
Sequoia.
All rights
reserved
J. S. Yoon et al. / Substrate
In addition to the problems discussed above, it has been noted that residual hydrogen atoms cause longterm instabilities in transistors, change the resistance of poly load resistors (lightly doped polysilicon), change the fuse characteristics of TiW fuses in programmable read-only memory devices (PROMS), and cause blisters in large underlying Al patterns on oxides [ 171. It is, therefore, necessary to produce H-free low temperature silicon nitride films for wide ranging applications in the microelectronics industry. In this study, substrate biasing in the ARE process was explored to synthesize silicon nitride with a low concentration of hydrogen and improve the quality of ARE silicon nitride films.
2. Experimental
details
Figure I is a schematic view of a vacuum chamber and associated equipment employed in this study. The system was capable of achieving a vacuum in the low 10 - 7 Torr range. A reactive gas was introduced at a controlled rate in the region between the evaporation source and the substrates which were located in the upper chamber to maintain the desired working pressure. High purity (99.999999%) Si was evaporated in nitrogen plasma, which was excited using an inductively coupled stainless-steel mesh attached to a 13.56 MHz r.f. supply. The films were deposited onto n-type (100)
Liquid
Heating
nitrogen
copper p-e
shield
tiij Substrate holder
Heater vuvvuu Substrate
_
I
w
Stainless steel mesh /
w Matching unit L
1
Nitrogen
I -
Fig. I. Schematic silicon Initride.
diagram
of the ARE
system for the deposition
of
bius of‘ ARE silicon nitride,films
81
silicon wafers, fused silica: pieces and soda lime glass substrates placed beneath the heater. Since silicon films are nonconducting, d.c. bias of the substrate is not possible due to charge accumulation on the film surface. Therefore, r.f. bias was employed for substrate biasing in this study to avoid the problem of charge accumulation. Ions are almost immobile, whereas the electrons can follow the temporal variations in the applied potential at the typical r.f. frequency used (13.56 MHz). When the substrate is coupled to the r.f. generator, a negative voltage is developed on the insulating film due to the difference in mobility between electrons and ions. Since the insulating film constitutes a capacitor in the electrical circuit, there should be no d.c. component to the current flow. Therefore, negative voltage (self-biased) on the insulating surface is developed to compensate the difference in mobility in electron and ion and to satisfy the condition of net zero-to-peak voltage of the r.f. signal. Unfortunately, our equipment could not measure (or estimate) the effective d.c. negative bias on the surface. We measured the forward power and the reflected power. The period for the substrate to be biased positively is very short and the substrate is mostly negatively biased during the r.f. cycle. Therefore, it is possible to provide continuous bombardment of the insulating films with positive ions using r.f. bias as the film grows. The high energy flux of positive ions accelerated by the electrical field to the substrate surface is able to densify the growing film and prevent the formation of microvoids, thereby increasing the packing density. Contaminants and poorly adherent spots of the film may also be sputtered off due to ion bombardment during deposition thus producing a denser film. A copper plate cooled by liquid nitrogen was located near the substrates to condense residual water molecules in the chamber. Heat shields were placed between the cold finger and heat source to prevent heating-up of the cold finger due to radiation from the heat source. The depositions were performed with biasing the substrates at various r.f. power in the range of O-610 W. The other deposition parameters were kept at constant values, such as 400 “C substrate temperature, 2 mTorr nitrogen pressure, and 300 W of discharge power. Infrared transmission measurements were carried out with a single beam Fourier-transform IR spectrometer (Mattson Instr. CYGNUS-25) and a Perkin-Elmer 1330 IR spectrophotometer. All spectra were obtained at room temperature. Most of the transmission data were measured in the IR region from 200 cm - ’ to 4000 cm-‘. For IR transmission measurements, high resistivity ( > 80 R cm) Si wafers polished on both sides were used as substrates. UV/Vis transmission spectra for the samples deposited on fused silica pieces were recorded
82
J. S. Yoon rt ul. 1 Substrate
on a Perkin-Elmer Model 330 UV-Vis spectrophotometer. The silicon nitride films were scanned against a reference substrate in the wavelength range 200-1000 nm. SIMS (Cameca IMF 3F secondary ion mass spectrometer) technique was used mainly to determine the content of hydrogen and other impurity elements in the films. The SIMS measurement was performed with a Cs + primary ion source biased at 17 kV (around 150 nA beam current). The refractive indices of the sample deposited on the Si wafers were determined using single beam (632.8 nm) ellipsometry (Rudolf Research/Auto EL). The high-frequency C- V curves were recorded on a Hewlett Packard 4284A Precision LCR Meter at 1 MHz. The bias was automatically varied 10 times per second between minus and plus 30 V. Metal-insulator-semiconductor (MIS) structures were prepared by depositing Al dots of 350 nm thickness on silicon nitride and silicon using a thermal evaporation technique. Capacitors of 75 and 100 nm diameter on the upper electrode were formed by photolithography.
3. Results and discussion 3.1. Infrared spectroscopy The IR spectrum for the ARE silicon nitride film prepared without biasing the substrate (Fig. 2(a)) shows a broad absorption band near 3340 cm - ‘. This band was initially thought to be due to the N-H bond stretching vibration mode because of the peak position. The band minima almost coincide with those of N-H bond stretching mode (3330-3350 cm-‘) [ 18, 191. However, it was noticed that the peak shape was different from those of N-H bond stretching mode. These bands observed at around 3340 cm ’ showed a rapid decrease in transmission near 3700 cm ’ and the band-
a
b
Wavenumber
(cm -1)
Fig. 2. Infrared transmission spectra of the ARE silicon nitride films prepared without substrate biasing (a) and with substrate biasing at (b) 150 W.
bias oj ARE silicon nitrirkfiltm
widths were much larger than those of the N-H bond stretching mode. In order to explain these absorption bands, the incorporation of water molecules in the films had to be considered. These bands are believed to be due to the water molecule absorbed in samples after deposition. Pliskin [29] has reported an absorption band near 3330 cm _ ’ for electron-beam evaporated SiO, films due to adsorbed water molecules. He concluded that the electron beam gun evaporated films contained a large number of micropores and the intensities of the absorption band around 3340 cm ’ were proportional to the degree of porosity of the film. The water molecules may diffuse easily through these micropores. The microporosity also causes a larger true surface to hold and react with the absorbed water due to a high degree of bond strain in addition to porosity. Electron beam gun evaporated films exhibit very high reactivity with water at relatively low temperatures [30]. In a similar manner, water molecules are believed to be incorporated in the films through micropores of ARE silicon nitride films. The rapid decrease in transmission near 3700 cm ’ in Fig. 2(a) is due to the formation of O-H bonds in hydrogen-bonded silanol ( SiOH) groups [ 19, 291, i.e. water molecules react with the Si,N, to form SiOH. On reaction of the water with the nitride to form silanol groups, strain in the bonds may be relieved. The absorption peak near 3340 cm ~~’ due to water molecules, decreases as the r.f. biasing of the substrate increases and it disappears for substrate bias over 150 W (Fig. 2(b)). r.f. substrate bias is expected to cause continuous ion (created in the plasma) bombardment of the film as it grows. This helps to densify the growing film and prevent the formation of microvoids, thereby increasing the packing density in the film. All the spectra show that the absorption band at around 850-900 cm ~~’ is due to the asymmetric in plane SiiN bond stretching vibration mode. This absorption band shifts to lower frequencies as the substrate bias increases to 150 W. Table 1 shows that the position of the absorption band minima varies from 906 to 849 cm ’ as the r.f. bias power is changed from 0 to 150 W. This absorption band minima approaches 850 cm ~’ which is close to the value of PACVD Si3N, film, 849-857 cm ’ [ 15, 201, as the peak near 3340 cm ’ due to water molecules disappears in the IR spectra. This shift can be explained by considering the degree of oxygen contamination resulting from the reaction between moisture and nitride films discussed earlier. It is well known that oxygen incorporation in silicon nitride causes a shift in the absorption band due to SiiN bond stretching near 850 cm ’ towards higher frequencies [20]. Table 1 indicates that the minima of the absorption band due to SiiN stretching shifts towards lower wave numbers as the oxygen contamination due to the absorbed water
J. S. Yoon et (11. 1 Substrate
TABLE I. Summary of the vibration properties nitride prepared with substrate biasing RF bias
Wave
power
numbers
bius of ARE silicon nitride Jilms
83
for the ARE silicon
(cm-‘)
(W)
molecules
Water
stretching
906 853 849 858 868
3331 3330
0 50 150 310 610
Si- N bond
1.20
molecules is decreased. Even though the reason is not clear, these peak minima increase with the r.f. power of the substrate bias over 150 W, which may be attributed to increases in structural disorder with increasing substrate bias. The degree of structural disorder can be increased due to ion bombardment [ 1, 191. No absorption relating to the H atom was observed in the IR spectra for the samples prepared at and over 150 W of r.f. substrate bias power. Considering that the detection limit of H by infrared spectroscopy is 0.5-l at.% [31, 321, it can be concluded that the hydrogen content is less than 1 at.% in these films. This conclusion was confirmed by SIMS results. Table 2 shows the dependence on substrate bias power of hydrogen and oxygen content in bulk. The hydrogen and oxygen concentrations in the film increase with decrease in r.f. substrate bias power. Within the experimental uncertainties (around 5%), the samples prepared using N, gas at over 150 W of substrate power appear to be near stoichiometric Si,N,, with oxygen and hydrogen contents less than IO*’ atom cmp3. 3.2. Refractive index Figure 3 shows the relationship between refractive index and substrate bias power. Refractive index increases rapidly from 1.81 to 2.0 as the substrate bias power varies from 0 to 150 W. Figure 3 shows a slight increase in the refractive index in the range of 150-610 W. The variation of refractive index with substrate bias power can be explained by the degree of oxygen contamination and the change in density of the film.
TABLE Substrate bias (W) 615 490 310 150 0
2. Hydrogen Reactive
and oxygen
content
measured
by SIMS
H( atoms/cm’)
0( atoms/cm3)
6 6 4 3 1
7 x lO”( 6 I at.‘%,) 5 x lO’“( G 1 at.%) 7 x 102”( < 1 at.%) 1 x 102’( < 1 at.%) 4 x 102’(5 at.%)
gas N* N* N* N, N,
x x x x x
1019( 6 1 at.“/;,) 1019( < 1 at.%) 102’( < 1 at.%) lO*“( < 1 at.%) lO*‘( 1 at.‘%))
0 Substrate
Fig. 3. Plot of refractive
BE
(13
power,
index as a function
Watts)
of substrate
bias power.
The rapid increase in refractive index in the range from 0 to 150 W is mainly due to the decrease in oxygen content in the film. It is known that the presence of oxygen in the bulk of the film has a profound influence in lowering the refractive index with a linear dependence on the atom percent of oxygen [9, 321. Silicon oxynitrides have a refractive index between 2.0 (for silicon nitride) and 1.46 (for silicon dioxide). The refractive index of silicon oxynitride varies linearly with the oxygen content of the film within this range. The microporosity decreases greatly when substrate bias power is varied in the range between 0 and 150 W, thereby reducing the amount of water molecules adsorbed into the film after deposition. The degree of oxygen contamination due to the adsorbed water, therefore, decreases with the increase in substrate bias (see Table 2). The slight increase in refractive index above 150 W is believed to be due to the increase in the density of the film. The samples prepared over 150 W of substrate power have a refractive index of 2.00-2.02. These values are very close to the refractive index (2.00-2.01) of high quality CVD Si,N,films. Generally, the films deposited by plasma vapor deposition techniques, such as PACVD or the ARE process, have lower refractive indices than those of the high temperature processed films (CVD or reactive evaporation processed films). In the plasma vapor deposition processes, the surface diffusion of adatoms is much retarded due to the low substrate temperature and therefore the film normally contains more defects and has a smaller density compared to a high temperature processed film. However, the density of the film in the ARE process is believed to be increased by biasing the substrate during the deposition of the film.
84
J. S. Yom et al. 1 Substrate
TABLE 1 MHz
3. Dielectric
Substrate
bias (W)
constants
50 150 300
obtained
Reactive Nz N, Nz
gas
by C-V
measurement
Dielectric
bias of’ ARE .vilicon nitride,filtm
at
constant
6.0 7.2-7.1 7.3
3.3. Optical gap The optical energy gap was obtained by two methods, Taut rule and absorption edge method. In the Taut rule method, the optical gap is found by fitting the absorption data to the equation (NE)”
= B(E - E,)
(1)
where cc is the absorption coefficient calculated from transmission, refractive index, n, and film thickness, B is a constant, E is the photon energy, ,!$ is the energy of the optical gap in electron volts. In the absorption edge method, the position of the absorption edge was approximated by the wavelength or energy at which the ratio of absorbance, log,(&/Z), to the film thickness (in micrometers) is unity [3]. In other words, the absorption edge estimated by this method is the wavelength or energy at which the absorption coefficient is 104/e cm - ‘. This method is very simple and useful for quality control. Figure 4 shows the variation of optical gap obtained by both methods, absorption edge method and the Taut rule, with the substrate bias power. As can be seen in this figure, the two sets of values show very little difference ( co.05 eV). The optical gap of the biased ARE (BARE) silicon nitride is decreased rapidly between 0 and 150 W and then slightly as the substrate bias power varies from 150 to 610 W. The samples obtained at over 150 W of substrate bias have a gap value of 4.85-4.90 eV. The rapid decrease in optical gap in the range of 0- 150 W is believed to be mainly due to the change of oxygen content in the films. SIMS data (Table 2) shows that the oxygen content in the film decreased from around 5 at.% to the very low value ( < 1 at.‘%) as the power of the substrate bias increases from 0 to 150 W. The substrate bias effect on the optical gap for the samples which are the near stoichiometric Si,N, (prepared at over 150 W) has to be considered in two different ways. As discussed in the previous section, the density of the film increases with the substrate bias power. For near stoichiometric Si,N, films, the increase in density of the film causes an increase in the optical gap [ 331. Bauer [33] has stated that one of the reasons for the difference in the optical gap between the sputtered Si,N, film (4.55 eV) and the pyrolytic Si,N, film
QWS.O
4.70:
0
100 ’
absorption Taut rule
I
200 ’
Substrate Fig. 4. Plot of optical
0
300 ’
Bias
edge I
’
’
’
600 ’
I
700 1
(rfUiorer5atts)
gap as a function
of substrate
bias power.
(5.20 eV) is due to the difference in the density of the films obtained by two different techniques. It has been known that the density for sputtered Si,N, is smaller than that for the pyrolytic sample. Thus, the optical gap of ARE silicon nitride is likely to increase with increasing substrate bias which tends to produce denser films. The degree of structural disorder can also affect the optical gap of silicon nitride films [ 1, 3, 331. The valence and conduction states are moving into the forbidden gap as the degree of the structural disorder increases [33], producing an absorption tail at the band edges. Even though the structure of amorphous Si,N, does not have a long-range order, it shows the same short-range interatomic spacing as that for crystalline r and j? silicon nitride [ 11. Ion bombardment produces a further disordered structure thus degrading short-range order [ 1, 191. Stein [l] has reported that the degradation of short-range order for the CVD Si,N, film introduced by ion bombardment causes a decrease in the energy of the absorption edge. In our case, the number of ions bombarding the surface of the growing film and their energies are believed to increase with increase in substrate bias. This probably leads to increase in structural disorder of the sample, thereby decreasing the optical gap with the increase in substrate bias power. In Fig. 4, the slight decrease in optical gap at over 150 W may be explained by the fact that the effect of change in structural disorder dominates the effect of change in the density with the substrate biasing power on optical gap. The smaller values (4.85-4.90 eV) of optical gap for the BARE Si,N, films than those (around 5.20 eV) for the CVD S&N, films are believed to be due to the more disordered structure of the BARE S&N, films.
J. S. Yoon et al. / Substrate bias of ARE silicon nitride films
1 .o
-----150 watts 300 watts
0.8 i
:: 5
0.6 -
3 0.4 -
0.2 ,-
0.0
1
-35
I
-30
I
-25
I
I
-20
-15
Bias
I
-10
1
-5
I
0
5
:o
1
(Volts)
Fig. 5. Capacitance us. voltage curve for the ARE prepared at 150 and 300 W of the substrate.
silicon
nitrides
3.4. C - V measurements high-frequency C- V curves (1 MHz) of a capacitor with a 100 nm thick ARE silicon nitride film on n-type (1 0 0) silicon are shown in Fig. 5. All the C-V profiles obtained in this experiment exhibited injection type (clockwise) hysteresis. This is due to trapping and detrapping of charge, probably in traps near the SiN-Si interface. Gereth et al. [34] have reported that the hysteresis is caused by charging and discharging in the traps located at the interface between silicon nitride and thin native oxide on Si wafer. In this case, charges are transfered through the thin oxide layer by tunneling [ 14, 341. The data in Fig. 5 show a more negative flat band voltage than the theoretical value. This means that there may be an additional voltage drop across the nitride to overcome the band-bending produced in Si by the positive charge stored at the interface and in the bulk of the film in addition to the difference in the work functions of the metal and the silicon so that the conduction and valence bands in Si are flat. Figure 5 shows that the flat band voltage shifts to the more negative side with increasing substrate bias indicating that films deposited at higher substrate bias have larger positive stored charge than that of films at lower substrate bias. This negative C-V shift is believed to be mainly due to the increase in displacement damage with substrate bias power. Stein [35] has observed a similar C- V shift for PACVD silicon nitrideSi structure due to displacement damage induced during ion implantation. Figure 5 also shows that the C-V curve for the sample deposited with higher substrate bias is more stretched out along the gate bias. This is attributed to the increase in displacement damage at the interface with substrate bias power. The minimum capacitance of the C- V curve for the sample deposited at higher substrate bias is higher than that of the lower-biased sample (Fig. 5). This is believed due to the large amount of donor-like defects which are
85
produced in the bulk of the Si substrate at the beginning of the deposition at high substrate bias [36]. The static dielectric constant was estimated by measuring the capacitance at 1 MHz of MIS structures. The value of the dielectric constant ranges from 6.0 to 7.7 depending on the deposition conditions (Table 3). These values are appreciably greater than the optical dielectric constant (square root of the refractive index). The static dielectric constant decreases as the substrate bias power decreases. This behavior can be attributed to the increase in microporosity with decreasing substrate bias power. The values of the dielectric constant for BARE silicon nitride obtained with over 150 W of substrate bias power is close to that of high-quality CVD silicon nitride (7-7.5) [37, 381.
4. Conclusions
Typical
The biased ARE (BARE) process produces films with a quality comparable with high temperature processed CVD silicon nitride but with much lower concentrations of hydrogen. The concentrations of hydrogen and oxygen in the films were far less than 1 at.“/o. The films obtained at over 150 W of substrate bias showed a refractive index of 2.00, absorption edge of 4.9 eV, and dielectric constant of 7.2-7.7. These are close to the reported values for high quality CVD silicon nitride films.
References I H. J. Stein, J. Appl. Phys., 47 ( 1976) 3421. 2 J. W. Peters, F. L. Gebhart and T. C. Hall, Solid State Technol., September (1980) 121. 3 M. J. Rand and D. R. Wonsidler, J. Electrochem. Sot., 125 (1978) 99. 4 T. L. Chu, C. H. Lee and G. A. Gruber, J. Electrochem. Sot., I14 (1967) 717. M. Kitabatake and K. Wasa, Appl. Phys. Lett., 49 (1986) 927. A. K. Sinha, Solid State Technol., April (1980) 133. A. R. Reinberg, J. Electron. Mater., 8 (1979) 345. F. Fransen, R. V. Berghe, R. Vlaenminck, N. Hinoul, J. Remmerie and H. E. Maes, Surf. Interface Anal., 7 (1985) 79. R. Flitsch, J. A. Aboaf and W. A. Pliskin, J. 9 S. I. Raider, Electrochem. Sot., 123 (1976) 560. 10 H. J. Stein and H. A. R. Wegener, J. Electrochem. Sot., 124 (1977) 908. B. Hepp, N. Szydlo, E. Criton and J. 11 N. Proust, F. Boulitrop, Magarino, Extended Abstracts of the 18th Int. Co@ on Solid State Devices and Materials, Tokyo, 1986, p. 245. I2 H. Watanabe, K. Katoh and M. Yasui, Jpn. J. Appl. Phys., 23 (1984) 1. 13 H. Watanabe, K. Katoh and M. Yasui, Jpn. J. Appl. Phys., 21 (1982) L34l. 14 R. Hezel and R. Schorner, J. Appl. Phys., 52 (1981) 3076. I5 E. Y. Chang, G. T. Cibuzar and K. P. Pande, IEEE Trans. Electron. De&es, 35 (1988) 1412.
86
J. S. Yoon et al. / Suhstrute
I6 M. J. Helix, K. V. Vaidyanathan, B. G. Streetman, H. B. Dietrich and P. K. Chatterjee, Thin Solid Films, 55 ( 1978) 143. I7 D. Pramanik, Semiconductor Int., June (1988) 92. I8 R. Szeda, Physica B, 129 (1985) 435. 19 W. A. Landford and M. J. Rand, J. Appl. Phys., 49 ( 1978) 2473. 20 R. C. Budhani, S. Prakash, H. J. Doerr and R. F. Bunshah, J. Vat. Sci. Technol., A, 5 (1987) 1644. 21 R. C. Budhani, R. F. Bunshah and P. A. Flinn, Appl. Phys. Lett., 52 ( 1988) 284. 22 S. Yokoyama, M. Hirose and Y. Osaka, Jpn. J. Appl. Phys.. 20 (1981) L35. 23 S. Hasegawa, M. Matuura and Y. Kurata, Appl. Phys. Lett., 49 ( 1986) 1272. 24 H. Stotzel, J. Non-Cryst. Solids, 90 (1987) 267. 25 G. Lucovsky and S. Y. Lin, J. Vat. Sci. Technol., B, 3 (1985) 1122. 26 S. Fujita and A. Sasaki, J. Electrochem. Sot., 132 (1985) 398. 27 E. P. G. T. van de Ven, Solid State Technol., April (1981) 167. 28 A. Kiermasz, A. McQuarrie and J. Bhardwaj, Semiconducfor Int., Nouember (1987) 107.
bias
qf ARE silicon nitride films 29 W. A. Pliskin, J. Vat. Sci. Technol., 14 (1977) 1064. 30 W. A. Pliskin, J. Elecrrochem. Sot.. II2 (1965) 148(c). 31 B. R. Zhang, Z. Yu, G. J. Collins, T. Hwang and W. H. Ritchie. J. Vat. Sri. Technol., A, 7 ( 1989) 176. 32 S. Thomas and R. J. Mattox, J. Electrochem. Sot,., 124 (1977) 1942. 33 J. Bauer, fhys. Status Solidi A, 39 ( 1977) 411. 34 R. Gereth and W. Scherber, J. Electrochem. SOL.., I I9 ( 1972) 1249, 35 H. J. Stein, in V. J. Kapoor and H. J. Stein (Eds.), Silicon Nitride Thin hwulating Films, The Electrochemical Society, Pennington. NJ, 1983, Vol. 83-8, p. 253. 36 D. Bouchier. A. Bossebouef and G. Gautherin, in V. J. Kapoor and K. T. Hankin (Eds.), Silicon Nitride and Silicon Dioxide thin Insuhning Films, The Electrochemical Society, Pennington. NJ, 1986, Vol. 87-10, p. 527. 37 T. Y. Chiu and W. G. Oldham, in V. J. Kapoor and H. J. Stein (Eds.), Silicon Nitride Thin Insulating Films, The Electrochemical Society, Pennington. NJ, 1983, Vol. 83-8, p. 367. 38 E. H. Nicollian. in S. B. Bibyk and V. J. Kapoor (Eds.), Silicon Nitride and Silicon Dioxide Thin Insulating Films, The Electrochemical Society. Pennington, NJ, 1988, Vol. 89-7. p. 177.