Thin Solid Films 447 – 448 (2004) 580–585
Application of SiO2 aerogel film for interlayer dielectric on GaAs with a barrier of Si3N4 Sang-Bae Junga, Sung-Woo Parka, Jun-Kyu Yanga, Hyung-Ho Parka,*, Haecheon Kimb a Department of Ceramic Engineering, Yonsei University, 134 Shinchon-Dong, Seodaemun-Ku, Seoul 120-749, South Korea Semiconductor Technology Division, Electronics and Telecommunications Research Institute, 161 Kajong-Dong, Yusong-Ku, Taejon 305-350, South Korea
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Abstract Si3N4 film was adopted as a barrier layer for SiO2 aerogelyGaAs system. Si3 N4 yGaAs formed an almost chemically sharp interface and nearly maintained their chemical inertness during in-situ heating until 400 8C under ultra high vacuum condition. One micrometer thick SiO2 aerogel film was prepared on the Si3N4 yGaAs system using supercritical drying procedure at 250 8C ˚ The dielectric constant of all SiO2 and under 1160 psi. The thickness of Si3N4 barrier layer was varied as 400, 600 and 900 A. aerogelySi3N4 yGaAs system was measured as approximately 1.9 and almost no difference was observed from the changes in the thickness of Si3N4 barrier layer. The leakage current densities were measured as low as under 2=10y7 Aycm2 and these values are two orders smaller than that of SiO2 aerogelyGaAs system prepared without the introduction of Si3 N4 barrier layer. All the aerogelySi3N4 yGaAs systems show ohmic conduction at low applying field and space charge limited conduction at high applying field. Furthermore, a reduced leakage current behavior was observed with increased Si3 N4 layer thickness and could be explained from the electric-field depression effect due to accumulated charges in the films. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: SiO2 aerogel film; GaAs; Si3N4 barrier layer; Supercritical drying; Interlayer dielectric
1. Introduction As devices scale down to sub-micron regime for fast switching speeds, the interconnect resistance capacitance delay becomes increasingly dominant over intrinsic gate delay w1x. This problem can be overcome by introducing low dielectric constant material rather than benzocyclobutene (BCB) polymer used in GaAs-based devices w2x. One of various low-k candidate materials, SiO2 aerogel film has drawn attention due to its adequate properties for interlayer dielectric w3x. Highly porous SiO2 aerogel film is fabricated by sol–gel polymerization and subsequent supercritical drying. Supercritical drying excludes the collapse of pore structure with the aid of high temperature and pressure w4x. Our previous work revealed that deposition of ambient pressure dried SiO2 aerogel film on GaAs generates elemental As, which causes Fermi level pinning w5x. Further investigation has found that elemental As was *Corresponding author. Tel.: q82-2-2123-2853; fax: q82-2-3655882. E-mail address:
[email protected] (H.-H. Park).
generated irrespective of drying procedure. Therefore, it is necessary to introduce a chemically stable buffer layer for the fabrication of SiO2 aerogel film. In GaAs-based devices, silicon nitride (Si3N4) is widely used to passivate GaAs surface from oxidation w6x. So, stable GaAs surface can be realized by introduction of Si3N4 layer. However, introduction of Si3N4 layer with high dielectric constant (7.4) increases the total dielectric constant of SiO2 aerogelySi3N4 yGaAs system and stable GaAs surface should be preserved during high temperature aerogel processing. In this study, effectiveness in application of Si3N4 layer for the fabrication of SiO2 aerogel film on GaAs was investigated. Si3N4 yGaAs interfacial bonding state changes during synthesis of SiO2 aerogel film was estimated by annealing of thin Si3N4 film on GaAs in ultra high vacuum chamber. Also, electrical properties of SiO2 aerogelySi3N4 yGaAs systems were measured. SiO2 aerogel film on Si3N4 film with different thickness was fabricated for the elucidation of Si3N4 thickness dependence on the electrical properties.
0040-6090/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2003.07.020
S.-B. Jung et al. / Thin Solid Films 447 – 448 (2004) 580–585
2. Experimental procedure The substrate used in this study was (100) oriented and n-doped GaAs with Si to a donor level of (0.6– 1)=1017 cmy3. For the electric property measurement, low resistance-ohmic contact was formed on the back side of the wafers by e-beam evaporation of AuyGey NiyAu followed by anneal at 350 8C. Si3N4 on GaAs was deposited using plasma enhanced chemical vapor deposition (PECVD) apparatus. Precursor gases were SiH4, NH3, He and N2. Deposition temperature and gas pressure was 260 8C and 850 mTorr, respectively. Thick˚ for electrical ness of Si3N4 film was 400, 600 and 900 A ˚ property measurement and 40 A Si3N4 film was deposited on GaAs for investigation of interfacial bonding states. After Si3N4 deposition, SiO2 aerogel film was fabricated as follows. SiO2 sols were synthesized from tetraethoxysilane (TEOS) dissolved in iso-propanol (IPA) using a two-step acidybase catalyzed procedure. Final composition of TEOS: IPA: H2O: HCl: NH4OH was 1:3:4:1.8=10y3:8.13=10y3. In the optimized viscosity range, the sol was spin-deposited on each substrate at 3000 rev.ymin for 20 s. Then each spun-on film was aged in IPA and subsequently placed in an autoclave apparatus. Supercritical drying was conducted through additional solvent method using IPA. Final temperature and pressure were 250 8C and 1160 psi, respectively. Detailed experimental procedures are described elsewhere w7x. The X-ray photoelectron spectroscopy (XPS, VG Scientific, ESCALAB 220i-XL) measurement was carried out to characterize the chemical bonding state of Si3N4 and GaAs interface. The excitation source was monochromatic Al Ka radiation and narrow scan spectra of all region of interest were recorded with 20 eV of pass energy. The dielectric constant in metal–insulator– semiconductor (MIS) structure was obtained using HP 4284A, impedanceygain-phase analyzer at 1 MHz. Current–voltage (I–V) characteristic was measured using HP 4145B, semiconductor parameter analyzer. For the electrical measurements, Au was deposited on films as upper electrode with e-beam evaporator and positively biased to measure only leakage current of dielectric films. 3. Results and discussion During supercritical drying for the formation of SiO2 aerogel film, high temperature and pressure are needed to obtain supercritical fluid condition. In case of using IPA as a solvent, the supercritical drying is performed at 250 8C and 1160 psi. Under this condition, the interface between Si3N4 and substrate GaAs could be changed due to interfacial reaction. Fig. 1 shows XPS ˚ thick Si3N4 on GaAs. spectra of sputter-deposited 40-A The system was in-situ annealed in an ultra-high vacu-
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umed XPS chamber with a step of 200 8C to investigate the thermal stability of Si3N4 and GaAs interface system. Fig. 1a corresponds to the interfacial state before annealing. Just before the deposition of Si3N4, HCl-cleaned GaAs surface contains Ga–As and elemental As (As–As) bonding states w8x. However, after the deposition of Si3N4, the GaAs surface showed a possible introduction of Ga–N and As–N bonds. And because the peak shapes including these bonding states do not almost change through the anneal treatments, these bonding states might be formed at the initial stage of Si3N4 deposition. The interface bonding state does not show any difference from the one annealed before, however, after 600 8C anneal, due to an evaporation of As, the formation of elemental Ga bonding state was introduced. Fig. 2 shows the XPS spectra of Si and N obtained from the exactly same sample system as Fig. 1. In the Si 2p spectra, there is an overlap with Ga 3p1y2 and 3p3y2 peaks of 107.8 eV and 104.2 eV binding energies, respectively. The subtraction of Ga 3p peak from Si 2p spectra could be done from the fact that the areal ratio of 3p1y2 and 3p3y2 peaks shows 2:1 w9x. The remaining peak after the subtraction has a binding energy of 102.6 eV. Normally Si 2p in Si3N4 has a binding energy of 101.8 eV and that in SiO2 has 103.4 eV w10,11x. From these facts and the presence of contaminated oxygen in the system, it can be said that ˚ thick contains a partial the deposited Si3N4 film of 40-A amount of oxygen and this chemical state does not change through anneal until 600 8C. This oxygen contamination of Si-nitride film could happen easily at the initial deposition of Si3N4 film w12x. With N 1s spectra, as we can expect from the previous results, N–Si bonding state was mainly observed. From these observations, it could be concluded that the introduction of Si3N4 as a barrier for the formation of SiO2 aerogel film on GaAs substrate as an interlayer dielectric induces the presence of some N–Ga(As) bonds and elemental state of Ga(As) but generally forms a good interface with GaAs. Fig. 3 shows a variation of calculated and measured dielectric constants of Si3N4 ySiO2 aerogel system according to the thickness fraction of Si3N4 layer. For the measuring case, three sample systems were espe˚ thick Si3N4 barrier cially prepared; 400-, 600- or 900-A for 1 mm of SiO2 aerogel and their thickness fraction of Si3N4 layer are 0.0385, 0.0566 and 0.0826, respec˚ thick Si3N4 tively. Until the introduction of 900-A barrier, the dielectric constant of the system does not ˚ thick Si3N4 layer exceed 1.97. The insertion of 900-A brings the increase of dielectric constant of the system less than 10% from that of SiO2 aerogel. Then it can be said that the introduction of several hundred angstrom of Si3N4 layer as a barrier for SiO2 aerogelyGaAs system improves interfacial quality without increasing dielectric constant of the system.
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˚ thick Si3N4 film deposited on GaAs; (a) as-prepared and after annealing at (b) 200 8C, Fig. 1. Ga 3d and As 3d photoelectron spectra of 40-A (c) 400 8C and (d) 600 8C.
Fig. 4 shows leakage current behavior of 1 mm SiO2 aerogelySi3N4 yGaAs system with thickness variation of ˚ Current density increased Si3N4 as 400, 600 and 900 A. with electric field, but the increase was minimized with ˚ the thickness of Si3N4; the SiO2 aerogelySi3N4 (400 A)y GaAs system shows almost constant leakage current value with increased electric field. Also the current density is observed under 2=10y7 Aycm2 with this system. Generally, leakage current behavior of SiO2 aerogel was explained by ohmic conduction at low applying field and Poole–Frenkel conduction at high applying field w13x. To investigate the leakage current mechanism at low applying field, the current–voltage result was plotted with log J and log E and the result is given in Fig. 5. Independent with the thickness of Si3N4, all the aerogelySi3N4 yGaAs systems show a linear relationship between log J and log E at low applying field. This reveals that the leakage current mechanism of the aerogelySi3N4 yGaAs systems is ohmic conduction at low applying field. However, from the plot of current–voltage result with log(JyE) and E 1y2, an increasing linear relationship, which is a typical evidence of Poole–Frenkel conduction, was not observed at high applying field. With SiO2 aerogel system, we already observed that the leakage current
behavior was independent on the nature of electrode w13x. Then another type of bulk limited conduction mechanism could be considered for the aerogelySi3N4 y GaAs system, it is space charge limited conduction. With that, the current density plot with J and V 2 shows a linear relationship at high applying field. Fig. 6 shows the current density plot with J and V 2 of the aerogely Si3N4 yGaAs system, and we can find a linear relationship. From these results, it could be found that the introduction of Si3N4 barrier layer changes the leakage current mechanism of aerogel system from Poole– Frenkel to space charge limited conduction. About the reduction of leakage current density with the increase of the thickness of Si3N4 barrier layer as shown in Fig. 4, an electric-field depression effect inside the film due to the accumulated negative space charges could be considered as one of major reason w14x. Also, according to the increase of Si3N4 thickness, an effect of defective interfacial zone could be reduced on the leakage current density of the aerogelySi3N4 yGaAs system. However, a difference in applying field to Si3N4 and aerogel could bring no effect on the leakage current density of the aerogelySi3N4 yGaAs system because the thickness difference of Si3N4 layer does not cause any difference in applying field of the aerogely Si3N4 sys-
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˚ thick Si3 N4 film deposited on GaAs; (a) as-prepared and after annealing at (b) 200 8C, (c) Fig. 2. Si 2p and N 1s photoelectron spectra of 40-A 400 8C and (d) 600 8C.
tem. Fig. 7 shows calculated effective field distribution of each layer for Si3N4 and aerogel film where thickness of the aerogel is fixed as 1 mm and for Si3N4 they were ˚ thick, and two layers were 400-, 600- and 900-A
Fig. 3. Dielectric constant of serial capacitance of SiO2 aerogelySi3N4 system was calculated and given as a solid line according to thickness fraction of Si3N4. The measured dielectric constants ˚ were (filled square) with Si3N4 thickness of 400, 600 and 900 A compared. Here the dielectric constants of SiO2 aerogel and Si3N4 are 1.8 and 7.4, respectively, and thickness of SiO2 aerogel film is fixed as 1 mm.
Fig. 4. The leakage current behavior of SiO2 aerogelySi3N4yGaAs ˚ (b) 600 A ˚ and (c) system with varying Si3N4 thickness as (a) 400 A, ˚ 900 A.
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Fig. 5. Log J vs. log E plot of leakage current density of SiO2 aerogelySi3N4yGaAs system with varying Si3 N4 thickness as (a) 400 ˚ (b) 600 A ˚ and (c) 900 A. ˚ The linear lines represent an ohmic A, conduction at initial low field.
Fig. 7. Calculated effective electric field distribution of each layer of Si3N4 and SiO2 aerogel film with Si3N4 thickness of 400, 600 and ˚ The three calculated lines were given but hardly 900 A. distinguishable.
assumed to be normal to the applied field. From this calculated result, a distributed effective field of aerogel is found to be much higher than those of Si3N4 layers, and it could be confirmed that the difference in the distributed effective field for Si3 N4 layer which resulted from the different thickness of Si3N4 layer is not a major reason for the reduced leakage current with the thicker Si3N4 barrier layer.
4. Conclusions The investigation on the dielectric property of SiO2 aerogel on GaAs substrate using Si3N4 as a barrier layer revealed that SiO2 aerogel could be possibly used to GaAs based-devices as an inter-layer dielectric. Sputterdeposited Si3N4 showed a chemically sharp interface with GaAs and its introduction induced two orders amelioration of leakage current property in SiO2 aerogelyGaAs system, i.e. 2=10y7 Aycm2. Furthermore, although several hundred angstrom of Si3N4 layer was introduced, the dielectric constant of the system was nearly the same as that of SiO2 aerogelyGaAs system, approximately 1.9. The SiO2 aerogelySi3N4 yGaAs system showed ohmic conduction at low applying field and space charge limited conduction at high applying field. The leakage current behavior was ameliorated with increased thickness of Si3N4 barrier layer due to the electric-field depression from accumulated charges in the films. Acknowledgments This work was supported by Electronics and Telecommunications Research Institute (ETRI). References
Fig. 6. J vs. V 2 plot of leakage current density of SiO2 aerogelySi3N4yGaAs system with varying Si3 N4 thickness as (a) 400 ˚ (b) 600 A ˚ and (c) 900 A. ˚ The result shows space charge limited A, conduction is a conduction mechanism at high field region.
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