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Room-temperature deposition of SiNx using ECR-PECVD for IIIV semiconductor microelectronics in lift-off technique
IOURNA
ELSEVIER
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Journal of Non-Crystalline Solids 187 (1995) 334-339
Room-temperature deposition of SiNx using ECR-PECVD for III/V semiconductor microelectronics in lift-off technique A. Wiersch", C. Heedt a, S. Schneiders", R. Tildersa, F. Buchali b, W. Kuebart h, W. Prost"'*, F.J. Tegude a a Solid State Electronics Department, Sonderforschungsbereich 254, Gerhard-Mercator-University, Kommandantenstr. 60, 47057 Duisburg, Germany, b Alcatel SEL, Stuttgart, Germany
Abstract
Room-temperature deposition of silicon-nitride on InP-substrates for electronic applications is reported. A plasma enhanced chemical vapour deposition apparatus equipped with an electron cyclotron resonance source was used. Molecular nitrogen and silane diluted in helium are chosen as precursors. The dielectric films are defined by means of optical lithography and lift-off technique. C fmeasurements reveal a dielectric constant of about 9 and a dissipation factor tan 6 = 3 × 10-1 (f = 10 kHz) while the breakdown field is 2 MV/cm (I = 250 ~tA/mm2). A strong improvement of the dissipation factor by more than one order of magnitude under both electrical and thermal stress, respectively, has been observed which could not be related to a variation of Si-H or N-H bonds measured by Fourier transformed infrared spectroscopy. The influence of silicon-nitride deposition on the electrical properties of an InA1As/InGaAs heterostructure field-effect transistor is investigated. The most significant change is found as an improvement of gate leakage current by 90% while other dc- and if-properties remain unchanged.
1. Introduction
Silicon nitride (SiNx) deposited by means of plasma-enhanced (PE) chemical vapour deposition (CVD) is widely used in III/V-microelectronics. Deposited at a substrate temperature of 250-400°C with silane (Sill4) and ammonia (NH3) as precursors, it exhibits excellent dielectric properties such as low conductivity, high breakdown field strength,
* Corresponding author. Tel: +49-203 379 2989. Telefax: +49-203 379 3400.
ED, and low dissipation factor, tan 6. In microelec-
tronics fabrication, the lift-off technique based on photo-resist structures provides reliable, damagefree high resolution processing. However all films to be processed by lift-off technique have to be deposited at a substrate temperature below 120°C. For the deposition of SiNx at these low temperatures, electron cyclotron resonance P E C V D with N2 as nitrogen source is favourable [1]. This way the disadvantage of high hydrogen content in the films using NH3 and a low deposition rate is avoided [-3] because of the extremely high electron energy. In addition, the very dense electron cyclotron
0022-3093/95/$09.50 © 1995 - Elsevier Science B.V. All rights reserved SSDI 0 0 2 2 - 3 0 9 3 ( 9 5 ) 0 0 1 6 0 - 3
A. Wiersch et al. / Journal o f Non-Crystalline Solids 187 (1995) 334-339
resonance (ECR) plasma can be spatially separated from the sample allowing a high flow of ionized particles with a low kinetic energy to the sample (I = 5 mA/cm 2, E = 20-30 eV [2]). This technique has been adopted for III/V microelectronics. Room-temperature ECR P E C V D films exhibit excellent I-V characteristics [4] and the process has proven capability for passivation of InP based devices [5] and surfaces with SiOx [6]. In this work, we present process optimization of room-temperature deposited E C R - P E C V D SiNx which can be successfully processed by lift-off technique. It is shown that this material has a dielectric quality comparable to material deposited at higher temperatures (T >250°C) and that it can be used for metal-insulator-metal (MIM) capacitors. In addition, ECR PECVD deposited SiNx is well suited for a damage-free deposition on InA1As/InGaAs/InP heterostructure field-effect transistor as shown by improved dc- and unchanged rf-performance.
2. Experimental procedure For deposition of the silicon nitride layers, a PlasmaLab System 90 ECR-PECVD from Oxford Instruments has been used. No heating was applied to the system throughout this study. A detailed setup of the system is shown in Fig. 1. Molecular nitrogen (N2) and silane (Sill4) diluted in helium (5%/95%) are used. The magnetic field of the ECR region is provided by two electro magnets using a current of 180 and 120 A for the top and the bottom magnet, respectively. The vacuum system consists of a roots/rotary combination and a turbomolecular pump providing a base pressure better than 1 × 10 - 7 mbar. Pressure during process is controlled with the use o f a baratron and a pressure controller (APC) in the range of 1 up to 100 mTorr. The process pressure studied is 5-10 mTorr. MIM capacitors have been fabricated with optical lithography. The capacitance has been measured using a Hewlett Packard 4275 A Multi-Frequency LCR
Magnetron N 2 - SiI-I 4 / H e - -
335
Magnet coils Substrate
Fig. 1. Schematic description of the E C R - P E C V D system for SiNk deposition at room temperature.
A. Wiersch et al. /Journal o f Non-Crystalline Solids 187 (1995) 334-339
336
bridge in the parallel equivalent circuit mode for the complex resistance. With the measured capacitance and conductance the relative dielectric constant, e,, and the dielectric dissipation factor, tan 6, have been deduced. The bond characteristic of the SiN~ was deduced by absorption measurements in the infrared wavelength range using a Biorad FTIR. Peaks in the absorption characteristic can be correlated with bonds in the layer, while the substrate was subtracted.
3. Results and discussion
In the first step, the applicability of the ECRPECVD deposited SiNx layer for lift-off has been evaluated. A photo resist structure with a negative undercut has been prepared using an image reversal resist. This resist lip (cf. Fig. 2) is known to be very sensitive to enhanced temperatures. After deposition of 200 nm SiNx, no damage can be detected. The variation of process parameters has been checked over a wide range and resist damage oc-
curs only at very high power levels of more than 450 W. Despite the widely isotropic deposition, a reliable lift-off process can be obtained in boiling acetone without any mechanical aid. However this process becomes critical if very small areas of resist are covered within large areas where the SiNx layer is directly deposited on the substrate. The composition of the SiNx layers can be estimated from the refractive index, n [7]. In Fig. 3 the dependence of the refractive index deduced from ellipsometry measurements is given as a function of the gas flow ratio silane to nitrogen. A SiH4/N2 ratio close to unity corresponds to n ~ 2, indicating normally stoichiometric Si3N4. However we found that a ratio SiH4/N2 of 0.24).4 (n ~ 1.7) is recommended for improved dielectric properties attributed to reduced hydrogen incorporation. Metal-insulator-metal capacitors with an area of 2 × 10 4 cm 2 have been produced using the lift-off technique for patterning the silicon nitride films. I-V measurements of all devices revealed a breakdown field strength above 1 MV/cm and a standard value of 2 MV/cm. The dielectric properties were
SiNx AZ
5214 resist
GaAs substrate
Fig. 2. SEM picture of a photo resist edge with undercut after deposition of 200 nm SiNx at a thermocouple temperature of 40°C. (SiH4/N2 = 0.4, p = 6 mTorr, P = 250 W.)
A. Wiersch et al. /Journal o f Non-Crystalline Solids 187 (1995) 334 339
analyzed by means of C-f measurements without any dc bias. For the investigated deposition conditions, the silane to nitrogen ratio has been varied from 0.2 to 0.9 resulting in dielectric constants between e~ ~ 7 (nitrogen rich) to e~ ~ 12.5 (silicon rich). A low dielectric dissipation factor is obtained for high nitrogen flow only. Silicon-rich samples, however, exhibit a high dissipation, tan 3. Both electrical and temperature stress was studied with respect to the dissipation factor. In Fig. 4 the dissipation factor is plotted as a function of frequency for an unstressed device (first measurement, curve (a)). At 10 kHz, tan 3 was measured to 1.3 x 10while at 1 M H z the dissipation factor drops down to 4.1 x 10 -2. The second measurement revealed a drastic reduction of tan 6 at low frequencies which is in agreement with the observation of Jeon et al. who found a strongly reduced leakage current in the second I V measurement at low D C bias [4]. It is worth noting that in our case no dc bias has been applied and the ac voltage is below 100 mV in all cases. A thermal annealing of M I M test structures was carried out at 200°C for 2.5 h using the same wafer as for curve (a) and the result is shown in curve (b) of Fig. 4. Even electrically unstressed devices exhibit a strong reduction of the dissipation factor comparable to the data obtained by electrical stress. The quality of room-temperature-deposited silicon nitride layers and the influence of thermal stress was further studied by F T I R measurements. Results of our analysis are shown in Fig. 5 where the absorbance of the different layers are
2.4
¢
iO o
o x ....
~P
¢ 10"1
lsl measurement 2nd measurement eredat 473
1°2 C
3¢"
- ' l ~ " "ID
10-310 °i . . . . . . . .I0 . . .~. . . . . . . . . . I0 . . .z. . . . . . . . . . .10 . . 3. . . . . frequency f / k H z - -
I0 ~
I0 s
Fig. 4. Stress investigation of the dielectric dissipation factor, tan 3 as a function of frequency for differently stressed M I M capacitors fabricated with room-temperature-deposited SiNx (SiH,/N2 = 0.4, dc bias = 0V): (a) without stress, first measurement; (b) second measurement of the same device; (c) first measurement of a device stressed at 200°C for 2.5 h under nitrogen ambient.
Si-N ¢',
Si-O ! ! N-H/,~!
Si-H N-H
)
C dr" - - ~ ' ' " ,a~,v,"~"
"
}"/!':
/iJ I~'
0.14 0.12
o.1 =
0.08
~
0.06 0.04 0.02
2.2
337
/ ,, ;
~,~,,,:
0 -0.02 4000
3000
2000
1000
Wavenumbers
°1.4 '~1.2
16 , 'o:3'
'o16'
'0:9'
'112'
'1.5
SiH¢ / N z ratio -~ Fig. 3. The refractive index of RT deposited SiNx as a function of silane to nitrogen ratio flow (SiHa/N2).
Fig. 5. Infrared absorption spectra of different SiNx layers deposited on boron doped St-wafers. The absorption of the substrate is subtracted. (a) Reference sample deposited by means of a standard P E C V D system at a substrate temperature of 250°C. (b) Low nitrogen content (SiH,/N2 = 0.2, n = 1.7) RT deposited SiNx. (c) Same sample shown in (b) but post annealed at 250°C in nitrogen ambient for 30 min. (d) High nitrogen content (SiH¢/N 2 = 0.9, n = 2.0) RT deposited SiNx.
338
A. Wiersch et al. /Journal of Non-Crystalline Solids 187 (1995) 334-339
plotted as a function of the wave number. For comparison, a layer deposited at 250°C in a standard P E C V D system with parallel plate electrode configuration without ECR was measured (curve (a) in Fig. 5). According to the analysis in Refs. [8-11], absorbance peaks are correlated with specific bonds in the material. All samples exhibit the main absorbance peak due to Si-N bonds. Samples deposited at room temperature show a weak peak or shoulder at the high wave number side of the main Si-N peak. This feature is attributed to a Si-Ox bond. Oxygen may originate from residual water in the system because no additional heating is applied to the sample prior to deposition and no load-lock is used. The second shoulder is correlated with a N - H bond and is somewhat higher than in the case of a higher deposition rate. The influence of deposition conditions can be associated to the occurrence of the Si-H bonds at a wave number of about 2200, only. In the nitrogen-rich sample (curve (a) in Fig. 5) with a silane to nitrogen ratio of about 0.2, this peak is absent, while in a silane-richdeposited layer (curve (b) in Fig. 5) with a silane to nitrogen ration of about 0.9, this peak can easily be seen. Even after thermal stress (T = 250°C, 30 min) the Si-H bond related peak is not changed (curve (c)). Finally an increased N-H-bond-related peak at a wave number of about 3300 can be seen for the nitrogen rich deposited layer (curve (d)). In summary, changing the deposition conditions from silane rich (b, c) to nitrogen rich (d) clearly results in a decreased Si-H bond related peak towards an
enhanced N - H bond related peak. The Si-H-bondrelated peak cannot be reduced by thermal stress in the investigated range but by means of a higher nitrogen flow. In addition, we have observed that the deposition rate does not increase with higher nitrogen flow. Hence we can assume that nitrogen replaces hydrogen in the layer. Finally the influence of room temperaturedeposited SiNx on the electrical performance of InA1As/InGaAs/InP heterostructure field effect transistors (HFET) grown by MBE has been investigated. This material system is of major interest for future high frequency and opto-electronic applications. A 300 nm passivation layer has been deposited using nitrogen rich conditions (SiH4/N2 = 0.4, p = 10mTorr, P = 185 W) on top of fully processed transistors. The thermocouple temperature never exceeded 25°C. The bond pads of the devices were covered with photo resist which was removed after deposition using the lift-off technique. MBE growth and device fabrication procedure is described elsewhere 1-12]. Table 1 shows the results of the statistical dc characterization on 25 devices across a quarter of a two inch wafer prior to and after deposition of the SiNx layer. With respect to the threshold voltage, V~, output conductance, 9~, and drain current, ID, no degradation was observed. A reduction of the maximum output conductance of 6.3% was found. The strongest improvement has been achieved for the gate-leakage current of the transistors in agreement with [5]. A reduction from IG = 28 laA/mm prior to SiNx
Table 1 Dc-parameter deviations of 25 InGaAs/InAIAs HFETs before and after room-temperature deposition of 300 nm SiNx passivation layer InGaAs/InAlAs H F E T Lg = 1 l.tm, WK= 50 lain (25 devices) Threshold voltage Vr (mS/mm) : Vos = 2.5 V Transconductance gm.max (mS/ram) : Drain current, ID (mA/mm) : VGS = 0 V, VDS = 2.5 V Output conductance, gd (mS/mm) : Vcs = 0 V, VDS = 2.5 V Gate current, 1~ (laA/rnm) : VGS = 0 V, VDs = 2.5 V
Prior to passivation
-0.667
After deposition of 300 nm SIN:, --0.657
Deviation (%)
+ 1.5
355
333
-6.3
151
146
-3.3
10.2
0.2
-29.5
2.54
0 --91
A. Wiersch et al. /Journal of Non-Crystalline Solids 187 (1995) 334 339
deposition down to I~ = 2.54 ~tA/mm after SiNx deposition can be observed. Following the explanation from Trew and Mishra [13], the silicon nitride layer could provide a sufficiently high trap concentration at the gate edge to accommodate injected electrons from the gate contact. This leads to a reduced electric field and thereby reduced gateleakage current. The SiNx deposition shows no impact on the RF performance. Devices with a gate length of about 1 p.m exhibit a transit frequency (current gain h21 = 1) of f x ~ 4 0 G H z and a maximum frequency of oscillation (unilateral gain GU = 1)fmax ~ 115 GHz before and after deposition.
4. Conclusion
SiNx deposited by ECR-PECVD at roomtemperature is shown to be suitable for lift-off processing. The layers exhibit quality comparable to silicon nitride deposited at enhanced temperatures. We have found that nitrogen rich deposition conditions resulting in non-stoichiometric SiNx with a refractive index of 1.7 are highly recommended for a low dissipation factor and reduced hydrogen incorporation in the material. Both thermal and electric stress, respectively, improves the dissipation factor of room temperature ECR-PECVD deposited silicon nitride. Finally the deposition of SiNx on InA1As/InGaAs/InP HFET does degrades
339
neither dc nor rf performance but improves the gate leakage current. References [1] S. Matsuo and I. Kiachi, in: Proc. Symp. on VeryLarge-Scale Integration Science and Technology (Electrochemical Society, Punington, N J, 1982). [2] S. Matsuo, in: Handbook of Thin-Film Deposition Processes and Techniques, ed. K. Schuegraf (Noyes, Park Ridge, N J, 1988) ch. 5. [3] H. Dun, P. Pan, F.R. White and R.W. Douse, J. Electrochem. Soc. 128 (1981) 1555. 1-4] Y.C. Jeon, H.Y. Lee and S.K. Joo, J. Appl. Phys. 75 (1991) 979. 1,5] D.J. Newson, A.J. Murrell, R.C. Grimwood and I.D. Henning, Electron. Lett. 29 (1993) 472. [6] Y.Z. Hu, M. Li, Y. Wang, E.A. Irene, M. Rowe and H.C. Casey, Appl. Phys. Lett. 63 (1993) 1113 [7] W.A.P. Claasen, W.G.J.N. Valkenburg, M.F.C. Willemsen, and W.M.c.d. Wijgert, J. Electrochem. Soc. 130 (1983) 2419. 1-8] S.E. Hicks and R.A.G. Gibson, Plasma Chem. Plasma Proc. 11 (1991) 455. 1-9] M. Boudreau, M. Boumerzoug, R.V. Kruzelecky, P. Mascher, P.E. Jessop and D.A. Thompson, Can. J. Phys. 70 (1992) 1404 [10] P.G. Pai, S.S. Chao and Y. Takagi, J. Vac. Sci. Technol. A4 (1986) 689 [11] A.S. Harrus and E.P. van de Ven, Semicond. Int. (May 1990) 124. [12] C. Heedt, P. Gottwald, F. Buchali, W. Prost, H. Kiinzel, F.J. Tegude, presented at 4th InP and Related Materials Conference, Apr. 1992, Newport, RI, IEEE Catalog # 92CH3104-7, WD 4, p. 238. 1-13] R. Trew and U.K. Mishra, IEEE Electron Dev. Lett. 12 (1991) 524.
ELSEVIER
L OF
Journal of Non-Crystalline Solids 187 (1995) 334-339
Room-temperature deposition of SiNx using ECR-PECVD for III/V semiconductor microelectronics in lift-off technique A. Wiersch", C. Heedt a, S. Schneiders", R. Tildersa, F. Buchali b, W. Kuebart h, W. Prost"'*, F.J. Tegude a a Solid State Electronics Department, Sonderforschungsbereich 254, Gerhard-Mercator-University, Kommandantenstr. 60, 47057 Duisburg, Germany, b Alcatel SEL, Stuttgart, Germany
Abstract
Room-temperature deposition of silicon-nitride on InP-substrates for electronic applications is reported. A plasma enhanced chemical vapour deposition apparatus equipped with an electron cyclotron resonance source was used. Molecular nitrogen and silane diluted in helium are chosen as precursors. The dielectric films are defined by means of optical lithography and lift-off technique. C fmeasurements reveal a dielectric constant of about 9 and a dissipation factor tan 6 = 3 × 10-1 (f = 10 kHz) while the breakdown field is 2 MV/cm (I = 250 ~tA/mm2). A strong improvement of the dissipation factor by more than one order of magnitude under both electrical and thermal stress, respectively, has been observed which could not be related to a variation of Si-H or N-H bonds measured by Fourier transformed infrared spectroscopy. The influence of silicon-nitride deposition on the electrical properties of an InA1As/InGaAs heterostructure field-effect transistor is investigated. The most significant change is found as an improvement of gate leakage current by 90% while other dc- and if-properties remain unchanged.
1. Introduction
Silicon nitride (SiNx) deposited by means of plasma-enhanced (PE) chemical vapour deposition (CVD) is widely used in III/V-microelectronics. Deposited at a substrate temperature of 250-400°C with silane (Sill4) and ammonia (NH3) as precursors, it exhibits excellent dielectric properties such as low conductivity, high breakdown field strength,
* Corresponding author. Tel: +49-203 379 2989. Telefax: +49-203 379 3400.
ED, and low dissipation factor, tan 6. In microelec-
tronics fabrication, the lift-off technique based on photo-resist structures provides reliable, damagefree high resolution processing. However all films to be processed by lift-off technique have to be deposited at a substrate temperature below 120°C. For the deposition of SiNx at these low temperatures, electron cyclotron resonance P E C V D with N2 as nitrogen source is favourable [1]. This way the disadvantage of high hydrogen content in the films using NH3 and a low deposition rate is avoided [-3] because of the extremely high electron energy. In addition, the very dense electron cyclotron
0022-3093/95/$09.50 © 1995 - Elsevier Science B.V. All rights reserved SSDI 0 0 2 2 - 3 0 9 3 ( 9 5 ) 0 0 1 6 0 - 3
A. Wiersch et al. / Journal o f Non-Crystalline Solids 187 (1995) 334-339
resonance (ECR) plasma can be spatially separated from the sample allowing a high flow of ionized particles with a low kinetic energy to the sample (I = 5 mA/cm 2, E = 20-30 eV [2]). This technique has been adopted for III/V microelectronics. Room-temperature ECR P E C V D films exhibit excellent I-V characteristics [4] and the process has proven capability for passivation of InP based devices [5] and surfaces with SiOx [6]. In this work, we present process optimization of room-temperature deposited E C R - P E C V D SiNx which can be successfully processed by lift-off technique. It is shown that this material has a dielectric quality comparable to material deposited at higher temperatures (T >250°C) and that it can be used for metal-insulator-metal (MIM) capacitors. In addition, ECR PECVD deposited SiNx is well suited for a damage-free deposition on InA1As/InGaAs/InP heterostructure field-effect transistor as shown by improved dc- and unchanged rf-performance.
2. Experimental procedure For deposition of the silicon nitride layers, a PlasmaLab System 90 ECR-PECVD from Oxford Instruments has been used. No heating was applied to the system throughout this study. A detailed setup of the system is shown in Fig. 1. Molecular nitrogen (N2) and silane (Sill4) diluted in helium (5%/95%) are used. The magnetic field of the ECR region is provided by two electro magnets using a current of 180 and 120 A for the top and the bottom magnet, respectively. The vacuum system consists of a roots/rotary combination and a turbomolecular pump providing a base pressure better than 1 × 10 - 7 mbar. Pressure during process is controlled with the use o f a baratron and a pressure controller (APC) in the range of 1 up to 100 mTorr. The process pressure studied is 5-10 mTorr. MIM capacitors have been fabricated with optical lithography. The capacitance has been measured using a Hewlett Packard 4275 A Multi-Frequency LCR
Magnetron N 2 - SiI-I 4 / H e - -
335
Magnet coils Substrate
Fig. 1. Schematic description of the E C R - P E C V D system for SiNk deposition at room temperature.
A. Wiersch et al. /Journal o f Non-Crystalline Solids 187 (1995) 334-339
336
bridge in the parallel equivalent circuit mode for the complex resistance. With the measured capacitance and conductance the relative dielectric constant, e,, and the dielectric dissipation factor, tan 6, have been deduced. The bond characteristic of the SiN~ was deduced by absorption measurements in the infrared wavelength range using a Biorad FTIR. Peaks in the absorption characteristic can be correlated with bonds in the layer, while the substrate was subtracted.
3. Results and discussion
In the first step, the applicability of the ECRPECVD deposited SiNx layer for lift-off has been evaluated. A photo resist structure with a negative undercut has been prepared using an image reversal resist. This resist lip (cf. Fig. 2) is known to be very sensitive to enhanced temperatures. After deposition of 200 nm SiNx, no damage can be detected. The variation of process parameters has been checked over a wide range and resist damage oc-
curs only at very high power levels of more than 450 W. Despite the widely isotropic deposition, a reliable lift-off process can be obtained in boiling acetone without any mechanical aid. However this process becomes critical if very small areas of resist are covered within large areas where the SiNx layer is directly deposited on the substrate. The composition of the SiNx layers can be estimated from the refractive index, n [7]. In Fig. 3 the dependence of the refractive index deduced from ellipsometry measurements is given as a function of the gas flow ratio silane to nitrogen. A SiH4/N2 ratio close to unity corresponds to n ~ 2, indicating normally stoichiometric Si3N4. However we found that a ratio SiH4/N2 of 0.24).4 (n ~ 1.7) is recommended for improved dielectric properties attributed to reduced hydrogen incorporation. Metal-insulator-metal capacitors with an area of 2 × 10 4 cm 2 have been produced using the lift-off technique for patterning the silicon nitride films. I-V measurements of all devices revealed a breakdown field strength above 1 MV/cm and a standard value of 2 MV/cm. The dielectric properties were
SiNx AZ
5214 resist
GaAs substrate
Fig. 2. SEM picture of a photo resist edge with undercut after deposition of 200 nm SiNx at a thermocouple temperature of 40°C. (SiH4/N2 = 0.4, p = 6 mTorr, P = 250 W.)
A. Wiersch et al. /Journal o f Non-Crystalline Solids 187 (1995) 334 339
analyzed by means of C-f measurements without any dc bias. For the investigated deposition conditions, the silane to nitrogen ratio has been varied from 0.2 to 0.9 resulting in dielectric constants between e~ ~ 7 (nitrogen rich) to e~ ~ 12.5 (silicon rich). A low dielectric dissipation factor is obtained for high nitrogen flow only. Silicon-rich samples, however, exhibit a high dissipation, tan 3. Both electrical and temperature stress was studied with respect to the dissipation factor. In Fig. 4 the dissipation factor is plotted as a function of frequency for an unstressed device (first measurement, curve (a)). At 10 kHz, tan 3 was measured to 1.3 x 10while at 1 M H z the dissipation factor drops down to 4.1 x 10 -2. The second measurement revealed a drastic reduction of tan 6 at low frequencies which is in agreement with the observation of Jeon et al. who found a strongly reduced leakage current in the second I V measurement at low D C bias [4]. It is worth noting that in our case no dc bias has been applied and the ac voltage is below 100 mV in all cases. A thermal annealing of M I M test structures was carried out at 200°C for 2.5 h using the same wafer as for curve (a) and the result is shown in curve (b) of Fig. 4. Even electrically unstressed devices exhibit a strong reduction of the dissipation factor comparable to the data obtained by electrical stress. The quality of room-temperature-deposited silicon nitride layers and the influence of thermal stress was further studied by F T I R measurements. Results of our analysis are shown in Fig. 5 where the absorbance of the different layers are
2.4
¢
iO o
o x ....
~P
¢ 10"1
lsl measurement 2nd measurement eredat 473
1°2 C
3¢"
- ' l ~ " "ID
10-310 °i . . . . . . . .I0 . . .~. . . . . . . . . . I0 . . .z. . . . . . . . . . .10 . . 3. . . . . frequency f / k H z - -
I0 ~
I0 s
Fig. 4. Stress investigation of the dielectric dissipation factor, tan 3 as a function of frequency for differently stressed M I M capacitors fabricated with room-temperature-deposited SiNx (SiH,/N2 = 0.4, dc bias = 0V): (a) without stress, first measurement; (b) second measurement of the same device; (c) first measurement of a device stressed at 200°C for 2.5 h under nitrogen ambient.
Si-N ¢',
Si-O ! ! N-H/,~!
Si-H N-H
)
C dr" - - ~ ' ' " ,a~,v,"~"
"
}"/!':
/iJ I~'
0.14 0.12
o.1 =
0.08
~
0.06 0.04 0.02
2.2
337
/ ,, ;
~,~,,,:
0 -0.02 4000
3000
2000
1000
Wavenumbers
°1.4 '~1.2
16 , 'o:3'
'o16'
'0:9'
'112'
'1.5
SiH¢ / N z ratio -~ Fig. 3. The refractive index of RT deposited SiNx as a function of silane to nitrogen ratio flow (SiHa/N2).
Fig. 5. Infrared absorption spectra of different SiNx layers deposited on boron doped St-wafers. The absorption of the substrate is subtracted. (a) Reference sample deposited by means of a standard P E C V D system at a substrate temperature of 250°C. (b) Low nitrogen content (SiH,/N2 = 0.2, n = 1.7) RT deposited SiNx. (c) Same sample shown in (b) but post annealed at 250°C in nitrogen ambient for 30 min. (d) High nitrogen content (SiH¢/N 2 = 0.9, n = 2.0) RT deposited SiNx.
338
A. Wiersch et al. /Journal of Non-Crystalline Solids 187 (1995) 334-339
plotted as a function of the wave number. For comparison, a layer deposited at 250°C in a standard P E C V D system with parallel plate electrode configuration without ECR was measured (curve (a) in Fig. 5). According to the analysis in Refs. [8-11], absorbance peaks are correlated with specific bonds in the material. All samples exhibit the main absorbance peak due to Si-N bonds. Samples deposited at room temperature show a weak peak or shoulder at the high wave number side of the main Si-N peak. This feature is attributed to a Si-Ox bond. Oxygen may originate from residual water in the system because no additional heating is applied to the sample prior to deposition and no load-lock is used. The second shoulder is correlated with a N - H bond and is somewhat higher than in the case of a higher deposition rate. The influence of deposition conditions can be associated to the occurrence of the Si-H bonds at a wave number of about 2200, only. In the nitrogen-rich sample (curve (a) in Fig. 5) with a silane to nitrogen ratio of about 0.2, this peak is absent, while in a silane-richdeposited layer (curve (b) in Fig. 5) with a silane to nitrogen ration of about 0.9, this peak can easily be seen. Even after thermal stress (T = 250°C, 30 min) the Si-H bond related peak is not changed (curve (c)). Finally an increased N-H-bond-related peak at a wave number of about 3300 can be seen for the nitrogen rich deposited layer (curve (d)). In summary, changing the deposition conditions from silane rich (b, c) to nitrogen rich (d) clearly results in a decreased Si-H bond related peak towards an
enhanced N - H bond related peak. The Si-H-bondrelated peak cannot be reduced by thermal stress in the investigated range but by means of a higher nitrogen flow. In addition, we have observed that the deposition rate does not increase with higher nitrogen flow. Hence we can assume that nitrogen replaces hydrogen in the layer. Finally the influence of room temperaturedeposited SiNx on the electrical performance of InA1As/InGaAs/InP heterostructure field effect transistors (HFET) grown by MBE has been investigated. This material system is of major interest for future high frequency and opto-electronic applications. A 300 nm passivation layer has been deposited using nitrogen rich conditions (SiH4/N2 = 0.4, p = 10mTorr, P = 185 W) on top of fully processed transistors. The thermocouple temperature never exceeded 25°C. The bond pads of the devices were covered with photo resist which was removed after deposition using the lift-off technique. MBE growth and device fabrication procedure is described elsewhere 1-12]. Table 1 shows the results of the statistical dc characterization on 25 devices across a quarter of a two inch wafer prior to and after deposition of the SiNx layer. With respect to the threshold voltage, V~, output conductance, 9~, and drain current, ID, no degradation was observed. A reduction of the maximum output conductance of 6.3% was found. The strongest improvement has been achieved for the gate-leakage current of the transistors in agreement with [5]. A reduction from IG = 28 laA/mm prior to SiNx
Table 1 Dc-parameter deviations of 25 InGaAs/InAIAs HFETs before and after room-temperature deposition of 300 nm SiNx passivation layer InGaAs/InAlAs H F E T Lg = 1 l.tm, WK= 50 lain (25 devices) Threshold voltage Vr (mS/mm) : Vos = 2.5 V Transconductance gm.max (mS/ram) : Drain current, ID (mA/mm) : VGS = 0 V, VDS = 2.5 V Output conductance, gd (mS/mm) : Vcs = 0 V, VDS = 2.5 V Gate current, 1~ (laA/rnm) : VGS = 0 V, VDs = 2.5 V
Prior to passivation
-0.667
After deposition of 300 nm SIN:, --0.657
Deviation (%)
+ 1.5
355
333
-6.3
151
146
-3.3
10.2
0.2
-29.5
2.54
0 --91
A. Wiersch et al. /Journal of Non-Crystalline Solids 187 (1995) 334 339
deposition down to I~ = 2.54 ~tA/mm after SiNx deposition can be observed. Following the explanation from Trew and Mishra [13], the silicon nitride layer could provide a sufficiently high trap concentration at the gate edge to accommodate injected electrons from the gate contact. This leads to a reduced electric field and thereby reduced gateleakage current. The SiNx deposition shows no impact on the RF performance. Devices with a gate length of about 1 p.m exhibit a transit frequency (current gain h21 = 1) of f x ~ 4 0 G H z and a maximum frequency of oscillation (unilateral gain GU = 1)fmax ~ 115 GHz before and after deposition.
4. Conclusion
SiNx deposited by ECR-PECVD at roomtemperature is shown to be suitable for lift-off processing. The layers exhibit quality comparable to silicon nitride deposited at enhanced temperatures. We have found that nitrogen rich deposition conditions resulting in non-stoichiometric SiNx with a refractive index of 1.7 are highly recommended for a low dissipation factor and reduced hydrogen incorporation in the material. Both thermal and electric stress, respectively, improves the dissipation factor of room temperature ECR-PECVD deposited silicon nitride. Finally the deposition of SiNx on InA1As/InGaAs/InP HFET does degrades
339
neither dc nor rf performance but improves the gate leakage current. References [1] S. Matsuo and I. Kiachi, in: Proc. Symp. on VeryLarge-Scale Integration Science and Technology (Electrochemical Society, Punington, N J, 1982). [2] S. Matsuo, in: Handbook of Thin-Film Deposition Processes and Techniques, ed. K. Schuegraf (Noyes, Park Ridge, N J, 1988) ch. 5. [3] H. Dun, P. Pan, F.R. White and R.W. Douse, J. Electrochem. Soc. 128 (1981) 1555. 1-4] Y.C. Jeon, H.Y. Lee and S.K. Joo, J. Appl. Phys. 75 (1991) 979. 1,5] D.J. Newson, A.J. Murrell, R.C. Grimwood and I.D. Henning, Electron. Lett. 29 (1993) 472. [6] Y.Z. Hu, M. Li, Y. Wang, E.A. Irene, M. Rowe and H.C. Casey, Appl. Phys. Lett. 63 (1993) 1113 [7] W.A.P. Claasen, W.G.J.N. Valkenburg, M.F.C. Willemsen, and W.M.c.d. Wijgert, J. Electrochem. Soc. 130 (1983) 2419. 1-8] S.E. Hicks and R.A.G. Gibson, Plasma Chem. Plasma Proc. 11 (1991) 455. 1-9] M. Boudreau, M. Boumerzoug, R.V. Kruzelecky, P. Mascher, P.E. Jessop and D.A. Thompson, Can. J. Phys. 70 (1992) 1404 [10] P.G. Pai, S.S. Chao and Y. Takagi, J. Vac. Sci. Technol. A4 (1986) 689 [11] A.S. Harrus and E.P. van de Ven, Semicond. Int. (May 1990) 124. [12] C. Heedt, P. Gottwald, F. Buchali, W. Prost, H. Kiinzel, F.J. Tegude, presented at 4th InP and Related Materials Conference, Apr. 1992, Newport, RI, IEEE Catalog # 92CH3104-7, WD 4, p. 238. 1-13] R. Trew and U.K. Mishra, IEEE Electron Dev. Lett. 12 (1991) 524.