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Properties of liquid-phase-deposited SiO2 films for interlayer dielectrics in ultralarge-scale integrated circuit multilevel interconnections
Thin Solid Films, 249 (1994) 15-21
15
Properties of liquid-phase-deposited SiOe films for interlayer dielectrics in ultralarge-scale integrated circuit multilevel interconnections Tetsuya Homma and Yukinobu Murao ULSI Device Development Laboratories, NEC Corp., 1120 Shimokuzawa, Sagamihara, Kanagawa 229 (Japan) (Received September 1, 1993; accepted February 16, 1994)
Abstract Properties of a new fluorinated SiO2 film for interlayer dielectrics in multilevel interconnections of ultralarge-scale integrated circuits (ULSIs) are investigated. The fluorinated SiO2 films are formed at 35 °C by a liquid phase deposition (LPD) technique using a supersaturated hydrofluosilicic acid (H2SiF6) aqueous solution. The LPD SiO2 film surface profiles on polysilicon and aluminum wirings are flat enough, indicating that the LPD technique has good capability for the surface planarization of interlayer dielectric films. The compositions of as-deposited LPD SiOz films and those annealed at 400 and 900 °C are SiOt.ssF0.~5, SiOz.ssF0.ts and SiOi.90F0.1o respectively. The LPD SiO 2 film deposition mechanism is explained as follows: (i) fluorosilanols [F,, Si(OH)4_n] formation; (ii) fluorosilanol oligomer formation by a catalytic reaction in the solution; (iii) oligomer adsorption onto the substrate surface; (iv) oligomer polymerization by a catalytic reaction. The absorption peak position, full width at half-maximum (FWHM) and absorption coefficient for the Si-O bond in the Fourier transform infrared spectra for the as-deposited LPD SiO2 films are 9.17 pm, 0.83 pm and 1.19 x 106 m -j, respectively, indicating that the films are formed by tightly bonded Si-O networks. The as-deposited LPD SiO 2 films have a refractive index of 1.433, a density of 2.19 x 103 kg m -j, an etching rate (measured using 1:30 buffered hydrofluoric acid (HF) solution) of 83 nm min -t, and a residual stress of 20 MPa (tensile). The film shrinkages after annealing at 400 and 900 °C are 0.8% and 2.0% respectively. Although these properties are changed by annealing at 400 and 900 °C, these values are still better than those of SiO 2 films deposited by chemical vapor deposition (CVD) at 400 °C for use as interlayer dielectric films. The LPD SiO, films have better electrical properties, such as lower leakage current, higher dielectric breakdown strength (>6.3 x 108Vm -~) and lower dielectric constant (<3.9 at 1 MHz), than the CVD SiO 2 films.
1. Introduction Interlayer dielectric film formation technology is essential for multilevel interconnection fabrication in ultralarge-scale integrated circuit (ULSI) devices. Low temperature deposition is required for interlayer dielectric films of multilevel interconnections, because thermal stress degrades the device characteristics and wiring reliability [1, 2]. Low temperature dielectric film formation techniques, such as plasma-enhanced chemical vapor deposition (PECVD) using tetraethoxysilane (TEOS) [3], atmospheric pressure CVD (APCVD) using TEOS and ozone [4, 5], and biased electron cyclotron resonance (ECR) CVD [6], have been investigated. However, these techniques cannot reduce the deposition temperature enough, because a substrate temperature above 300 °C is required for polymerization and dehydration reactions. It is well known that deposition at a low temperature without residual O H is difficult to achieve. A method for the formation of SiO2 films at room temperature using a liquid phase deposition (LPD) technique has been investigated by Nagayama
0040-6090/94[$7.00 SSDI 0040-6090(94)06109-X
and coworkers [7-10] to be used as passivation films for a liquid crystal display (LCD). The basic properties of the LPD SiO2 films have been reported [7]. We also have reported an application of the LPD technique to selective SiO2 film deposition for the full planarization of interlayer dielectric films [11, 12]. The purpose of this paper is to reveal the LPD SiO2 film properties and to study the feasibility of using the LPD SiO2 films for the interlayer dielectric films of ULSI multilevel interconnections. The capability of LPD SiOz films to allow planarization of the interlayer dielectric films will also be discussed.
2. Experimental procedure The LPD technique utilizes a hydrofluosilicic acid (H2SiF6) aqueous solution with a concentration of 40 wt.% which is saturated with SIO2. To maintain the H2SiF 6 solution in a supersaturated state, a boric acid (H3BO3) aqueous solution of concentration 0.1 mol 1-t was continuously added at a rate of 1 x 10 -21 h -t per
© 1994 - - Elsevier Science S.A. All rights reserved
16
T. Homma, Y. Murao / LPD Si02 fibns fi~r ULSI inter&yet dielectrics
liter of H2SiF 6 aqueous solution during the film deposition. The experimental set-up was described in our previous paper [12]. The film deposition was achieved by immersing substrates in the solution at 35 °C. The deposition rate on silicon substrates 0.15 m in diameter was about 20 nm min -t. To study the planarization capability of the LPD SiO2 films, an LPD SiO2 film 0.8 lain thick was formed on a substrate which had polysilicon patterns 0.9 p.m thick with 0.9 jam wide and spacings of 0.6 lain. An LPD SiO2 film 0.8 jam thick was also formed on a substrate which had aluminum patterns 1.0 jam thick with 1.0 jam wide and spacings of 0.7 jam. These substrate with aluminum patterns were previously covered with a PECVD SiO2 film 0.2 jam thick. Cross-sectional observations by scanning electron microscopy (SEM) were used to study the surface profiles. The chemical, physical and electrical properties of the LPD SiO2 films, such as the composition, chemical bonding structure, refractive index, density, etching rate, shrinkage, residual stress, leakage current, dielectric breakdown strength and dielectric constant, were evaluated using LPD SiO2 films 0.2 jam thick formed on silicon 0.15m in diameter. The film non-uniformity was under 3%. These properties were compared with those of conventional thermal CVD SiO2 films formed at 400°C using silane and oxygen as gas sources. The changes in the film properties after annealing at 400 and 900 °C for 60 min in a nitrogen atmosphere were also investigated, because these temperatures are the maximum temperatures for the thermal treatments for the poly-metal and inter-metal dielectric films respectively. To determine the film composition, which can be represented as SIO2_, F,., the concentrations of silicon, oxygen and fluorine atoms were analyzed by X-ray photoemission spectroscopy (XPS). The refractive index and thickness of the films were evaluated from the average of 10 point measurements, by using the ellipsometric system Auto EL IV (Rudolph Research) and a Nano Spec/AFT Model 200 (Nanometrics) respectively. The chemical bonding structures were investigated using Fourier transform IR ( F T I R ) spectra measured using a Model QS-100 (Bio-Rad). The peak absorption coefficient A was calculated from A = d-'
log(T/To)
at 400 and 900 °C, relative to the as-deposited film thickness. The residual stress value was calculated from Sf = h~2E/6rhr( 1 - v)
(2)
where Sf, h~, h,-, r, E and v are the film stress, silicon wafer thickness, film thickness, radius of curvature, Young's modulus and Poisson's ratio for silicon respectively [ 13]. The leakage current, dielectric breakdown strength and dielectric constant were measured using metal-insulator-semiconductor (MIS) (A1/LPD SiO2 film/p-Si) structure samples, where the aluminum electrode area was 1 x 10 -6 m -2. The dielectric breakdown strength was determined as the electric field at a leakage current density of 1 × 10 -2 A m -2. The dielectric constant was calculated from the maximum capacitance obtained by capacitance-voltage ( C - V ) measurement at 1 MHz.
3. Results and discussion
Figure 1 shows SEM cross-sectional views of the LPD SiO2 films 0.8 lain thick which are deposited on the
(a) On PolysUicon Wirings
(l)
where d, T and To are the film thickness, and the top and bottom values of the transmittance respectively. The density of the films was calculated from their weight and volume. The etching rate of the films was measured using 1:30 buffered hydrofluoric acid (HF) aqueous solution. The film shrinkage was determined by taking the differential film thickness, after annealing
(b) On AI Wirings Fig. 1. SEM cross-sectionalviewsof the LPD SiO2 films deposited on the substrates with (a) polysilicon patterns (lines, 0.9 :am; spacing, 0.6 jam; thickness, 0.9 :am) and (b) aluminum patterns (lines, 0.7 jam: spacing, 0.7 jam;thickness, 1.0 :am),wherethe substrate with aluminum patterns was first covered with a PECVD SiO2 film 0.2 jam thick.
T. Homma, Y. Murao / LPD SiO 2 films f o r ULSI interlayer dielectrics (a)
100 7s
S¢-OH OH
...........
FI,,
40
80
i-F
,
/_~ ,;',,, ~,',
s~ap
0
S~-o
t
L..o::
so
C 0
17
~----._.~00 "C e-
=.
"5
£ v
120 160 200 240
O C
E t c ? d ~ T ~ e (rain)
=_
Fig. 2. XPS depth profiles for as-deposited LPD SiO z film, LPD SiO 2 films annealed at 400 °C and L P D SiO 2 films annealed at 900 °C, where the full lines are for the as-deposited film and that annealed at 400 °C, and the broken lines are for the film annealed at 900 °C.
substrates with polysilicon patterns 0.9 p.m thick and aluminum patterns 1.0 ~tm thick, where the aluminumpatterned substrate was first covered with a PECVD SiO2 film 0.2 ~tm thick. The surface profiles of the deposited LPD SiO2 films were flat enough, without any crack or keyhole formation. The results indicate that the LPD technique has a good capability to achieve the surface planarization of interlayer dielectric films. Figure 2 shows XPS depth profiles of silicon, oxygen and fluorine across the as-deposited LPD SiO2 films and those annealed at 400 and 900 °C. The silicon, oxygen and fluorine atoms were almost uniformly distributed across the as-deposited films and those annealed at 400 °C. The fluorine atomic concentrations in the as-deposited films and those annealed at 400 °C were approximately 5.6%. The results indicate that the fluorine atoms were not decomposed from the films by annealing at 400 °C. For the LPD SiO2 film annealed at 900 °C, the fluorine atomic concentrations near the silicon substrate and the film surface were approximately 5.6% and 0%, respectively, indicating that the fluorine atoms were decomposed from the film surface. The average concentration of the fluorine atoms was approximately 3.5%. The compositions calculated from the XPS depth profiles for the as-deposited LPD SiO2 films and those annealed at 400 and 900°C are SiOl.~sFo.~5, SiOl.ssFol5 and SiOi.9oFo.lo respectively. The LPD SiO2 film deposition mechanism can be explained as follows. Since the silicon halide can be hydrolyzed and changed into A, Si(OH)4_n (A, halogen atoms; n = 1, 2, 3) in aqueous solutions, the LPD SiO2 film deposition mechanism can be speculated as being as follows: (i) fluorosilanols (F.Si(OH)4 .... ; n = 1, 2, 3) are formed in the supersaturated H2SiF6 aqueous solution; (ii) fluorosilanol oligomers (F,,,Si(OH) 3.... {F, SiO~4_,)/2}y-F,,Si(OH)3_,,; m 1, 2;y > 0) are then formed in the solution; (iii) next, the fluorosilanol oligomers are adsorbed onto the substrate surface; (iv)
=
(b) SJ-OH OH
~' ~.
As-depo.
S~O
400 *C
21. 310 ,i0 ;.o ;0 2; Wavelength (.um) Fig. 3. F T I R spectra for (a) the LPD SiO 2 and (b) CVD SiO 2 films as a function of annealing temperature.
finally, the oligomers are polymerized at the substrate surface. The fluorosilanol oligomer formation and polymerization occur by catalytic reactions, where the fluorine acts as a catalyst [14]. The LPD SiO2 films are deposited by these consequent reactions. This speculated LPD SiO2 film deposition mechanism can provide a good explanation of why the LPD SiO,_ films contain many fluorine atoms. Figure 3 shows FTIR spectra for the LPD SiO2 and CVD SiO2 films as a function o f the annealing temperature. For the as-deposited LPD SiO: films, there are four absorption peakS, corresponding to Si-O, Si-F, Si-OH bonds and an OH group at the wavelengths around 9.17, 10.75, 2.74 and 2.94pm respectively [8, 15]. For the as-deposited CVD SiO2 films, there are three absorption peaks, corresponding to Si-O, Si-OH bonds and an OH group at the wavelengths around 9.35, 2.74 and 2.94 jam respectively. The absorption peak intensities of the Si-OH bonds and OH group for the as-deposited LPD SiO2 films were smaller than those for the as-deposited CVD SiO2 films. Although the absorption peaks of the Si-OH bonds and OH group for the LPD SiO2 films annealed at 400 °C almost fully disappeared, those for the CVD SiO 2 films annealed at 400 °C still remained. The results for the CVD SiO2 films annealed at 400 °C can be explained, however, because the deposition temperature is the same as the annealing temperature, resulting in the temperature having an insufficient effect for reductions of the Si-OH bonds and OH group from the CVD SiO2 films. For the LPD SiO2 and CVD SiO2
7". Homma, Y. Murao /LPD SiO, films for ULSI interlayer dielectrics
18 (a) As-depo.
,
• U g:
Ib ) As-depo.
E C
i
~ " Si-O
Wavelength (pm) Fig. 4. Expanded FTIR spectra around the Si-O bond absorption peaks for (a) the LPD SiO2 and (b) CVD S i Q films.
films annealed at 900 °C, the absorption peaks of the Si-OH bond and OH group disappeared fully. Figure 4 shows the expanded FTIR spectra around the Si-O bond absorption peaks for the LPD SiO2 and CVD SiO2 films. To clarify the nature of the Si-O and Si-F bonds in the LPD SiOz films, the changes in absorption peak position, FWHM and absorption coefficients were measured from the spectra. Table 1 summarizes the FTIR spectral analysis results, and other chemical and physical properties for the LPD SiO2 and CVD SiO2 films. For the as-deposited LPD SiOz films, the wavelength of the Si-O bond peak position is lower than that for the as-deposited CVD SiO2 films. The FWHM of the
Si-O bond absorption peak for the as-deposited LPD SiO2 films is narrower than that for the as-deposited CVD SiO2 films. These results are due to the effects of fluorine atoms, which have a higher electronegativity (4.0) than that of oxygen atoms (3.5). Also, the Si-O bond peak absorption coefficient for the as-deposited LPD SiO2 films is about 20% higher than that for the as-deposited CVD SiO2 films. The FTIR spectral analysis results indicate that the LPD SiO2 films are formed by tightly bonded Si-O networks. The refractive index for the as-deposited LPD SiO2 films was 1.433 and is close to that of the CVD SiO2 films (1.432). The density of the as-deposited LPD SiO2 films was about 2.19x 103kgm -3, which is higher than that of the as-deposited CVD SiO2 films (2.11 x 103 kg m-3). Although the density of the as-deposited LPD SiO2 films is higher than that for the CVD SiO2 films, the refractive index for the LPD SiO2 films is almost the same as that for the CVD SiO2 films. This is as a result of the fluorine contained in the LPD SiO2 films [16]. The higher density of the as-deposited LPD SiO2 films can be explained as being because the films are formed by tightly bonded Si-O networks, and contain fluorine atoms which are heavier than oxygen atoms, For the LPD SiO2 and CVD SiO2 films annealed at 400 °C, the Si-O bond absorption peak positions and the absorption coefficients were not changed by this annealing. Although the FWHM for the LPD SiO2 films did not change, that for the CVD SiO2 films decreased after annealing at 400 °C. The refractive index for the LPD SiO2 films annealed at 400 °C decreased and is lower than that for the CVD SiO2 films annealed at 400 °C. Since the absorption peak position and coefficient for the Si-F bonds were not changed by annealing at 400 °C the reduction in the refractive index for the LPD SiO2 films annealed at 400 °C results from dehydration of the films.
T A B L E 1. S u m m a r y of the F T I R spectrum analysis results, and other chemical and physical properties for the LPD-SiO 2 and CVD-SiO 2 films LPD-SiO 2 film As-deposited
CVD-SiO 2 film 400 °C
900 °C
As-deposited
400 °C
900 °C
9.35 1.32 1.00
9.35 1.28 1.00
9.23 1.08 1.1 I
F T I R ( S i - O bond) Peak position (p.m) F W H M (lain) Absorption coefficient ( x 106 m -I)
9.17 0.83 1.19
9.17 0.83 1.19
9.24 0.86 1.16
F T I R ( S i - F bond) Peak position (lam) Absorption coefficient ( x 104 m-~)
10.75 9.0
10.75 9.0
10.75 7.1
---
---
---
1.433 2.19 83
1.424 2.15 70 0.8 50 (T)
1.430 2.24 50 2.0 50 (C)
1.432 2.11 80 -50 (T)
1.434 -65 0.95 65 (T)
1.445 -48 3.5 30 (C)
Refractive index Density ( x 103 k g m -3) Etching rate ( n m min -~) Shrinkage (%) Residual stress ~ (MPa) T, tensile; C, compressive.
20 (T)
T. Homma, Y. Murao / LPD Si02films for ULSI interlayer dielectrics
The small change in the refractive index for the CVD SiO2 films annealed at 400 °C can be explained as being because the dehydration reaction was insufficient on annealing at 400 °C. These results are consistent with the FTIR spectral analysis results. The density of the LPD SiO 2 films annealed at 400 °C also decreased slightly to 2.15 × 103 k g m -3, as a result of dehydration of the films. For the LPD SiO2 films annealed at 900 °C, the absorption peak position and FWHM shifted to higher values than those for the as-deposited LPD SiO2 films and the films annealed at 400 °C. This was because of the decomposition of fluorine from the Si-F bond in the LPD SiO2 films. This result is consistent with the reduction of the absorption coefficient for the Si-F bond peak. In contrast, for the CVD SiO2 films annealed at 900 °C, the peak position and FWHM shifted to lower values than those for the CVD SiO2 films annealed at 400 °C, owing to the films' densification. Although the absorption coefficient decreased for LPD SiO2 films annealed at 900 °C, that for CVD SiO2 films annealed at 900 °C increased. The results arise because of the increment and reduction of the FWHM for the LPD SiO2 and CVD SiO2 films respectively. The refractive index for the LPD SiO2 films annealed at 900 °C increased to 1.430, as a result of the films' densification and because of the decomposition of the fluorine from the films. The refractive index is lower than that for the CVD SiO2 films annealed at 900 °C (1.445) and is almost the same as that for the as-deposited LPD SiO 2 films. This can be explained as resulting from the balance of degree in densification and reduction of the amount of fluorine atoms by decomposition from the LPD SiO2 films. The density of LPD SiOz films annealed at 900 °C increased to 2.24 × 103 kg m -3, owing to the films' densification. The etching rate for the as-deposited LPD SiO2 films etched by 1:30 buffered HF solution was about 83 nm min-1, which is close to that for the as-deposited CVD SiO2 films, owing to high film density, despite low temperature deposition. The etching rates for the LPD SiOz films annealed at 400 and 900 °C are also close to those for the CVD SiO2 films annealed at these temperatures. These results indicate that the LPD SiO2 films are formed by tightly bonded Si-O networks, as mentioned in the FTIR spectrum analysis results. The shrinkage for LPD SiO2 films annealed at 400 °C was about 0.8%, owing to dehydration of the films; this is lower than that for the CVD SiO2 films. The shrinkages for the LPD SiO,_and CVD SiO2 films annealed at 900 °C were about 2.0% and 3.5% respectively. The lower shrinkages for the LPD SiO2 films can be explained as resulting from the contents of the Si-OH bonds and OH groups in the as-deposited LPD SiO2 films being lower
19
than those in the as-deposited CVD SiO2 films, as discussed in the FTIR spectrum analysis results. The residual tensile stress for the as-deposited LPD SiO2 films was about 20 MPa, which is lower than that for the as-deposited CVD SiO2 films (50 MPa). This is because of the lower deposition temperature than that for the CVD SiQ films and because of the fluorine contained in the films [17]. The residual tensile stress for the LPD SiO2 films was increased to 50 MPa by annealing at 400 °C, as a result of shrinkage of the films caused by the reduction of water components, such as Si-OH bonds and OH groups. For the CVD SiO2 films, the residual tensile stress was also increased to 65 MPa by annealing at 400 °C. The low residual stress increment for the CVD SiO2 films annealed at 400 °C can be explained as being because the reduction in water components is less than that for the LPD SiO2 films, as mentioned in the FTIR spectrum analysis results. The residual stress for the films annealed at 900 °C was converted to compressive stress, as a result of film densification and because of the difference in the coefficients of thermal expansion between silicon substrates and SiO2 films. The residual compressive stress value for the LPD SiO2 films annealed at 900 °C was 50 MPa, which is higher than that for the CVD SiO2 films annealed at 900 °C (30 MPa). The high stress value for the LPD SiO2 films annealed at 900 °C results from there being less film shrinkage than that for the CVD SiO2 films annealed at this temperature. These values are sufficiently low for interlayer dielectric films. Figure 5 shows the leakage current characteristics for the LPD SiO2 and CVD SiOz films as a function of the
10.5
E >,
/O/A ,/A
O~
10 .6 - a /, , ' / ~/-it' TM. .-,
O
--
0~0~0~0 O
-" - "- ",7-',-',
n-A--a--
~
C 0 ~/~rl ~ ' r l W
~ 10 .7 Q
-J
10"80
0.5 '
1.0 '
1.5 '
2i 0
2.5
Electric Field ( x l 0 a Vm") Fig. 5. Leakage current characteristics through the LPD SiO 2 (open symbols) and CVD SiO 2 films (full symbols) as a function of annealing temperature: ©, 0 , as-deposited; z~, A, annealed at 400 °C; D, I , annealed at 900 °C.
20
T. Homma, Y. Murao / LPD SiO z films for ULSI interho'er dielectrics
TABLE 2. Dielectricbreakdown strength at the leakage current density of 1 x 10--' A m-2, and dielectricconstant at 1 MHz for the LPD-SiO2 and CVD-SiO2 films LPD-SiO2
CVD-SiO2
As-deposited
400 °C
900 °C
As-deposited
400 °C
900 °C
Dielectric breakdown strength ( x 108 V m-1)
6.3
7.2
7.2
5.7
6.1
7.0
Dielectric constant at 1 MHz
3,9
3.7
3.7
4.4
4.3
4.1
annealing temperature. The leakage current densities for the as-deposited LPD SiO2 films were lower than for the as-deposited CVD SiO2 films, owing to the high quality of the Si-O bonds with less Si-OH bonds and OH groups, as mentioned in the F T I R spectrum analysis results. The leakage current densities for the LPD SiO2 films decreased with increasing annealing temperature. They were even lower than those for the CVD SiO2 films. F r o m the result obtained for LPD SiO 2 films annealed at 900 °C, the decomposition of fluorine from the films does not affect the leakage current characteristics at electric fields lower than 2.5 x 108 V m - 2. Table 2 summarizes the results for the dielectric breakdown strength and the dielectric constant. It can be seen that the dielectric breakdown strength for the as-deposited LPD SiO= films was about 6.3 x 108 V m -~, which is higher than that for the as-deposited CVD SiO2 films (5.7 x 108 V m-l). The dielectric breakdown strengths for LPD SiO2 and CVD SiO2 films annealed at 400°C increased to 7.2 x l0 s and 6.1 x 108Vm -~ respectively. Although the dielectric breakdown strength for CVD SiO2 films annealed at 900 °C increased to 7.0 x 10sV m -~, the dielectric breakdown strength for LPD SiO 2 films annealed at this temperature was still the same as that for the LPD SiO2 films annealed at 400 °C. This may be because of the change in chemical bonding structure caused by the decomposition of fluorine from the LPD SiO2 films annealed at 900 °C. These results indicate that the decomposition of fluorine affected the dielectric breakdown strength. The dielectric constant for the as-deposited LPD SiO2 films was about 3.9 at 1 MHz, which is lower than that for the as-deposited CVD SiO2 films (4.4). This lower dielectric constant results from the fluorine contained in the films and from there being less O H groups in the films [ 18]; this is consistent with the low refractive index. The dielectric constant decreased for the LPD SiO2 and CVD SiO2 films annealed at 400 °C, owing to the dehydration of the films. Although the dielectric constant for CVD SiO2 films annealed at 900 °C decreased further, the dielectric constant for the LPD SiO2 films annealed at 900 °C was not changed. The low dielectric constant for the LPD SiO2 films is
very useful for the reduction of capacitance among wirings, resulting in a reduction in the signal delay time through the multilevel interconnections. These electrical property evaluation results indicate that the LPD SiO2 films can be used for the interlayer dielectric films in ULSI multilevel interconnections.
4. Conclusions The properties of a new fluorinated SiO2 film formed by the LPD technique at 35 °C and used as the interlayer dielectrics of ULSI multilevel interconnections have been investigated. It has been revealed that the LPD technique has a good capability for the surface planarization of interlayer dielectric films. The LPD SiO2 film deposition mechanism can be explained as follows: (i) fluorosilanol formation; (ii) fluorosilanol oligomer formation by a catalytic reaction in the supersaturated HzSiF6 solution; (iii)adsorption of the oligomers onto the substrate surface; (iv) polymerization by a catalytic reaction at the substrate surface. The properties of LPD SiO2 films deposited at 35 °C are better than those of CVD SiO2 films when used as interlayer dielectrics. Although the LPD SiOz film properties are changed by annealing at 400 and 900 °C, the values are still better than those of the CVD SiO2 films for both the poly-metal and inter-metal dielectric films. This fluorinated SiO2 film deposition technique is useful for interlayer dielectric film formation in USLI multilevel interconnection fabrication.
Acknowledgment The authors would like to thank Dr. M. Kamoshida, Dr. M. Ogawa, Dr. O. Kudoh, Dr. A. Ishitani, Dr. H. Tsuya, Dr. K. Hamano, and Dr. D. T. C. Huo from AT&T Bell Laboratories for their encouragement and useful advice. They also would like to express appreciation to Mr. S. Kamiyama and Mr. T. Ishijima for their aid in sample preparation.
T. Homma, Y. Murao / LPD SiO 2 films for ULSI interlayer dielectrics
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15
Properties of liquid-phase-deposited SiOe films for interlayer dielectrics in ultralarge-scale integrated circuit multilevel interconnections Tetsuya Homma and Yukinobu Murao ULSI Device Development Laboratories, NEC Corp., 1120 Shimokuzawa, Sagamihara, Kanagawa 229 (Japan) (Received September 1, 1993; accepted February 16, 1994)
Abstract Properties of a new fluorinated SiO2 film for interlayer dielectrics in multilevel interconnections of ultralarge-scale integrated circuits (ULSIs) are investigated. The fluorinated SiO2 films are formed at 35 °C by a liquid phase deposition (LPD) technique using a supersaturated hydrofluosilicic acid (H2SiF6) aqueous solution. The LPD SiO2 film surface profiles on polysilicon and aluminum wirings are flat enough, indicating that the LPD technique has good capability for the surface planarization of interlayer dielectric films. The compositions of as-deposited LPD SiOz films and those annealed at 400 and 900 °C are SiOt.ssF0.~5, SiOz.ssF0.ts and SiOi.90F0.1o respectively. The LPD SiO 2 film deposition mechanism is explained as follows: (i) fluorosilanols [F,, Si(OH)4_n] formation; (ii) fluorosilanol oligomer formation by a catalytic reaction in the solution; (iii) oligomer adsorption onto the substrate surface; (iv) oligomer polymerization by a catalytic reaction. The absorption peak position, full width at half-maximum (FWHM) and absorption coefficient for the Si-O bond in the Fourier transform infrared spectra for the as-deposited LPD SiO2 films are 9.17 pm, 0.83 pm and 1.19 x 106 m -j, respectively, indicating that the films are formed by tightly bonded Si-O networks. The as-deposited LPD SiO 2 films have a refractive index of 1.433, a density of 2.19 x 103 kg m -j, an etching rate (measured using 1:30 buffered hydrofluoric acid (HF) solution) of 83 nm min -t, and a residual stress of 20 MPa (tensile). The film shrinkages after annealing at 400 and 900 °C are 0.8% and 2.0% respectively. Although these properties are changed by annealing at 400 and 900 °C, these values are still better than those of SiO 2 films deposited by chemical vapor deposition (CVD) at 400 °C for use as interlayer dielectric films. The LPD SiO, films have better electrical properties, such as lower leakage current, higher dielectric breakdown strength (>6.3 x 108Vm -~) and lower dielectric constant (<3.9 at 1 MHz), than the CVD SiO 2 films.
1. Introduction Interlayer dielectric film formation technology is essential for multilevel interconnection fabrication in ultralarge-scale integrated circuit (ULSI) devices. Low temperature deposition is required for interlayer dielectric films of multilevel interconnections, because thermal stress degrades the device characteristics and wiring reliability [1, 2]. Low temperature dielectric film formation techniques, such as plasma-enhanced chemical vapor deposition (PECVD) using tetraethoxysilane (TEOS) [3], atmospheric pressure CVD (APCVD) using TEOS and ozone [4, 5], and biased electron cyclotron resonance (ECR) CVD [6], have been investigated. However, these techniques cannot reduce the deposition temperature enough, because a substrate temperature above 300 °C is required for polymerization and dehydration reactions. It is well known that deposition at a low temperature without residual O H is difficult to achieve. A method for the formation of SiO2 films at room temperature using a liquid phase deposition (LPD) technique has been investigated by Nagayama
0040-6090/94[$7.00 SSDI 0040-6090(94)06109-X
and coworkers [7-10] to be used as passivation films for a liquid crystal display (LCD). The basic properties of the LPD SiO2 films have been reported [7]. We also have reported an application of the LPD technique to selective SiO2 film deposition for the full planarization of interlayer dielectric films [11, 12]. The purpose of this paper is to reveal the LPD SiO2 film properties and to study the feasibility of using the LPD SiO2 films for the interlayer dielectric films of ULSI multilevel interconnections. The capability of LPD SiOz films to allow planarization of the interlayer dielectric films will also be discussed.
2. Experimental procedure The LPD technique utilizes a hydrofluosilicic acid (H2SiF6) aqueous solution with a concentration of 40 wt.% which is saturated with SIO2. To maintain the H2SiF 6 solution in a supersaturated state, a boric acid (H3BO3) aqueous solution of concentration 0.1 mol 1-t was continuously added at a rate of 1 x 10 -21 h -t per
© 1994 - - Elsevier Science S.A. All rights reserved
16
T. Homma, Y. Murao / LPD Si02 fibns fi~r ULSI inter&yet dielectrics
liter of H2SiF 6 aqueous solution during the film deposition. The experimental set-up was described in our previous paper [12]. The film deposition was achieved by immersing substrates in the solution at 35 °C. The deposition rate on silicon substrates 0.15 m in diameter was about 20 nm min -t. To study the planarization capability of the LPD SiO2 films, an LPD SiO2 film 0.8 lain thick was formed on a substrate which had polysilicon patterns 0.9 p.m thick with 0.9 jam wide and spacings of 0.6 lain. An LPD SiO2 film 0.8 jam thick was also formed on a substrate which had aluminum patterns 1.0 jam thick with 1.0 jam wide and spacings of 0.7 jam. These substrate with aluminum patterns were previously covered with a PECVD SiO2 film 0.2 jam thick. Cross-sectional observations by scanning electron microscopy (SEM) were used to study the surface profiles. The chemical, physical and electrical properties of the LPD SiO2 films, such as the composition, chemical bonding structure, refractive index, density, etching rate, shrinkage, residual stress, leakage current, dielectric breakdown strength and dielectric constant, were evaluated using LPD SiO2 films 0.2 jam thick formed on silicon 0.15m in diameter. The film non-uniformity was under 3%. These properties were compared with those of conventional thermal CVD SiO2 films formed at 400°C using silane and oxygen as gas sources. The changes in the film properties after annealing at 400 and 900 °C for 60 min in a nitrogen atmosphere were also investigated, because these temperatures are the maximum temperatures for the thermal treatments for the poly-metal and inter-metal dielectric films respectively. To determine the film composition, which can be represented as SIO2_, F,., the concentrations of silicon, oxygen and fluorine atoms were analyzed by X-ray photoemission spectroscopy (XPS). The refractive index and thickness of the films were evaluated from the average of 10 point measurements, by using the ellipsometric system Auto EL IV (Rudolph Research) and a Nano Spec/AFT Model 200 (Nanometrics) respectively. The chemical bonding structures were investigated using Fourier transform IR ( F T I R ) spectra measured using a Model QS-100 (Bio-Rad). The peak absorption coefficient A was calculated from A = d-'
log(T/To)
at 400 and 900 °C, relative to the as-deposited film thickness. The residual stress value was calculated from Sf = h~2E/6rhr( 1 - v)
(2)
where Sf, h~, h,-, r, E and v are the film stress, silicon wafer thickness, film thickness, radius of curvature, Young's modulus and Poisson's ratio for silicon respectively [ 13]. The leakage current, dielectric breakdown strength and dielectric constant were measured using metal-insulator-semiconductor (MIS) (A1/LPD SiO2 film/p-Si) structure samples, where the aluminum electrode area was 1 x 10 -6 m -2. The dielectric breakdown strength was determined as the electric field at a leakage current density of 1 × 10 -2 A m -2. The dielectric constant was calculated from the maximum capacitance obtained by capacitance-voltage ( C - V ) measurement at 1 MHz.
3. Results and discussion
Figure 1 shows SEM cross-sectional views of the LPD SiO2 films 0.8 lain thick which are deposited on the
(a) On PolysUicon Wirings
(l)
where d, T and To are the film thickness, and the top and bottom values of the transmittance respectively. The density of the films was calculated from their weight and volume. The etching rate of the films was measured using 1:30 buffered hydrofluoric acid (HF) aqueous solution. The film shrinkage was determined by taking the differential film thickness, after annealing
(b) On AI Wirings Fig. 1. SEM cross-sectionalviewsof the LPD SiO2 films deposited on the substrates with (a) polysilicon patterns (lines, 0.9 :am; spacing, 0.6 jam; thickness, 0.9 :am) and (b) aluminum patterns (lines, 0.7 jam: spacing, 0.7 jam;thickness, 1.0 :am),wherethe substrate with aluminum patterns was first covered with a PECVD SiO2 film 0.2 jam thick.
T. Homma, Y. Murao / LPD SiO 2 films f o r ULSI interlayer dielectrics (a)
100 7s
S¢-OH OH
...........
FI,,
40
80
i-F
,
/_~ ,;',,, ~,',
s~ap
0
S~-o
t
L..o::
so
C 0
17
~----._.~00 "C e-
=.
"5
£ v
120 160 200 240
O C
E t c ? d ~ T ~ e (rain)
=_
Fig. 2. XPS depth profiles for as-deposited LPD SiO z film, LPD SiO 2 films annealed at 400 °C and L P D SiO 2 films annealed at 900 °C, where the full lines are for the as-deposited film and that annealed at 400 °C, and the broken lines are for the film annealed at 900 °C.
substrates with polysilicon patterns 0.9 p.m thick and aluminum patterns 1.0 ~tm thick, where the aluminumpatterned substrate was first covered with a PECVD SiO2 film 0.2 ~tm thick. The surface profiles of the deposited LPD SiO2 films were flat enough, without any crack or keyhole formation. The results indicate that the LPD technique has a good capability to achieve the surface planarization of interlayer dielectric films. Figure 2 shows XPS depth profiles of silicon, oxygen and fluorine across the as-deposited LPD SiO2 films and those annealed at 400 and 900 °C. The silicon, oxygen and fluorine atoms were almost uniformly distributed across the as-deposited films and those annealed at 400 °C. The fluorine atomic concentrations in the as-deposited films and those annealed at 400 °C were approximately 5.6%. The results indicate that the fluorine atoms were not decomposed from the films by annealing at 400 °C. For the LPD SiO2 film annealed at 900 °C, the fluorine atomic concentrations near the silicon substrate and the film surface were approximately 5.6% and 0%, respectively, indicating that the fluorine atoms were decomposed from the film surface. The average concentration of the fluorine atoms was approximately 3.5%. The compositions calculated from the XPS depth profiles for the as-deposited LPD SiO2 films and those annealed at 400 and 900°C are SiOl.~sFo.~5, SiOl.ssFol5 and SiOi.9oFo.lo respectively. The LPD SiO2 film deposition mechanism can be explained as follows. Since the silicon halide can be hydrolyzed and changed into A, Si(OH)4_n (A, halogen atoms; n = 1, 2, 3) in aqueous solutions, the LPD SiO2 film deposition mechanism can be speculated as being as follows: (i) fluorosilanols (F.Si(OH)4 .... ; n = 1, 2, 3) are formed in the supersaturated H2SiF6 aqueous solution; (ii) fluorosilanol oligomers (F,,,Si(OH) 3.... {F, SiO~4_,)/2}y-F,,Si(OH)3_,,; m 1, 2;y > 0) are then formed in the solution; (iii) next, the fluorosilanol oligomers are adsorbed onto the substrate surface; (iv)
=
(b) SJ-OH OH
~' ~.
As-depo.
S~O
400 *C
21. 310 ,i0 ;.o ;0 2; Wavelength (.um) Fig. 3. F T I R spectra for (a) the LPD SiO 2 and (b) CVD SiO 2 films as a function of annealing temperature.
finally, the oligomers are polymerized at the substrate surface. The fluorosilanol oligomer formation and polymerization occur by catalytic reactions, where the fluorine acts as a catalyst [14]. The LPD SiO2 films are deposited by these consequent reactions. This speculated LPD SiO2 film deposition mechanism can provide a good explanation of why the LPD SiO,_ films contain many fluorine atoms. Figure 3 shows FTIR spectra for the LPD SiO2 and CVD SiO2 films as a function o f the annealing temperature. For the as-deposited LPD SiO: films, there are four absorption peakS, corresponding to Si-O, Si-F, Si-OH bonds and an OH group at the wavelengths around 9.17, 10.75, 2.74 and 2.94pm respectively [8, 15]. For the as-deposited CVD SiO2 films, there are three absorption peaks, corresponding to Si-O, Si-OH bonds and an OH group at the wavelengths around 9.35, 2.74 and 2.94 jam respectively. The absorption peak intensities of the Si-OH bonds and OH group for the as-deposited LPD SiO2 films were smaller than those for the as-deposited CVD SiO2 films. Although the absorption peaks of the Si-OH bonds and OH group for the LPD SiO2 films annealed at 400 °C almost fully disappeared, those for the CVD SiO 2 films annealed at 400 °C still remained. The results for the CVD SiO2 films annealed at 400 °C can be explained, however, because the deposition temperature is the same as the annealing temperature, resulting in the temperature having an insufficient effect for reductions of the Si-OH bonds and OH group from the CVD SiO2 films. For the LPD SiO2 and CVD SiO2
7". Homma, Y. Murao /LPD SiO, films for ULSI interlayer dielectrics
18 (a) As-depo.
,
• U g:
Ib ) As-depo.
E C
i
~ " Si-O
Wavelength (pm) Fig. 4. Expanded FTIR spectra around the Si-O bond absorption peaks for (a) the LPD SiO2 and (b) CVD S i Q films.
films annealed at 900 °C, the absorption peaks of the Si-OH bond and OH group disappeared fully. Figure 4 shows the expanded FTIR spectra around the Si-O bond absorption peaks for the LPD SiO2 and CVD SiO2 films. To clarify the nature of the Si-O and Si-F bonds in the LPD SiOz films, the changes in absorption peak position, FWHM and absorption coefficients were measured from the spectra. Table 1 summarizes the FTIR spectral analysis results, and other chemical and physical properties for the LPD SiO2 and CVD SiO2 films. For the as-deposited LPD SiOz films, the wavelength of the Si-O bond peak position is lower than that for the as-deposited CVD SiO2 films. The FWHM of the
Si-O bond absorption peak for the as-deposited LPD SiO2 films is narrower than that for the as-deposited CVD SiO2 films. These results are due to the effects of fluorine atoms, which have a higher electronegativity (4.0) than that of oxygen atoms (3.5). Also, the Si-O bond peak absorption coefficient for the as-deposited LPD SiO2 films is about 20% higher than that for the as-deposited CVD SiO2 films. The FTIR spectral analysis results indicate that the LPD SiO2 films are formed by tightly bonded Si-O networks. The refractive index for the as-deposited LPD SiO2 films was 1.433 and is close to that of the CVD SiO2 films (1.432). The density of the as-deposited LPD SiO2 films was about 2.19x 103kgm -3, which is higher than that of the as-deposited CVD SiO2 films (2.11 x 103 kg m-3). Although the density of the as-deposited LPD SiO2 films is higher than that for the CVD SiO2 films, the refractive index for the LPD SiO2 films is almost the same as that for the CVD SiO2 films. This is as a result of the fluorine contained in the LPD SiO2 films [16]. The higher density of the as-deposited LPD SiO2 films can be explained as being because the films are formed by tightly bonded Si-O networks, and contain fluorine atoms which are heavier than oxygen atoms, For the LPD SiO2 and CVD SiO2 films annealed at 400 °C, the Si-O bond absorption peak positions and the absorption coefficients were not changed by this annealing. Although the FWHM for the LPD SiO2 films did not change, that for the CVD SiO2 films decreased after annealing at 400 °C. The refractive index for the LPD SiO2 films annealed at 400 °C decreased and is lower than that for the CVD SiO2 films annealed at 400 °C. Since the absorption peak position and coefficient for the Si-F bonds were not changed by annealing at 400 °C the reduction in the refractive index for the LPD SiO2 films annealed at 400 °C results from dehydration of the films.
T A B L E 1. S u m m a r y of the F T I R spectrum analysis results, and other chemical and physical properties for the LPD-SiO 2 and CVD-SiO 2 films LPD-SiO 2 film As-deposited
CVD-SiO 2 film 400 °C
900 °C
As-deposited
400 °C
900 °C
9.35 1.32 1.00
9.35 1.28 1.00
9.23 1.08 1.1 I
F T I R ( S i - O bond) Peak position (p.m) F W H M (lain) Absorption coefficient ( x 106 m -I)
9.17 0.83 1.19
9.17 0.83 1.19
9.24 0.86 1.16
F T I R ( S i - F bond) Peak position (lam) Absorption coefficient ( x 104 m-~)
10.75 9.0
10.75 9.0
10.75 7.1
---
---
---
1.433 2.19 83
1.424 2.15 70 0.8 50 (T)
1.430 2.24 50 2.0 50 (C)
1.432 2.11 80 -50 (T)
1.434 -65 0.95 65 (T)
1.445 -48 3.5 30 (C)
Refractive index Density ( x 103 k g m -3) Etching rate ( n m min -~) Shrinkage (%) Residual stress ~ (MPa) T, tensile; C, compressive.
20 (T)
T. Homma, Y. Murao / LPD Si02films for ULSI interlayer dielectrics
The small change in the refractive index for the CVD SiO2 films annealed at 400 °C can be explained as being because the dehydration reaction was insufficient on annealing at 400 °C. These results are consistent with the FTIR spectral analysis results. The density of the LPD SiO 2 films annealed at 400 °C also decreased slightly to 2.15 × 103 k g m -3, as a result of dehydration of the films. For the LPD SiO2 films annealed at 900 °C, the absorption peak position and FWHM shifted to higher values than those for the as-deposited LPD SiO2 films and the films annealed at 400 °C. This was because of the decomposition of fluorine from the Si-F bond in the LPD SiO2 films. This result is consistent with the reduction of the absorption coefficient for the Si-F bond peak. In contrast, for the CVD SiO2 films annealed at 900 °C, the peak position and FWHM shifted to lower values than those for the CVD SiO2 films annealed at 400 °C, owing to the films' densification. Although the absorption coefficient decreased for LPD SiO2 films annealed at 900 °C, that for CVD SiO2 films annealed at 900 °C increased. The results arise because of the increment and reduction of the FWHM for the LPD SiO2 and CVD SiO2 films respectively. The refractive index for the LPD SiO2 films annealed at 900 °C increased to 1.430, as a result of the films' densification and because of the decomposition of the fluorine from the films. The refractive index is lower than that for the CVD SiO2 films annealed at 900 °C (1.445) and is almost the same as that for the as-deposited LPD SiO 2 films. This can be explained as resulting from the balance of degree in densification and reduction of the amount of fluorine atoms by decomposition from the LPD SiO2 films. The density of LPD SiOz films annealed at 900 °C increased to 2.24 × 103 kg m -3, owing to the films' densification. The etching rate for the as-deposited LPD SiO2 films etched by 1:30 buffered HF solution was about 83 nm min-1, which is close to that for the as-deposited CVD SiO2 films, owing to high film density, despite low temperature deposition. The etching rates for the LPD SiOz films annealed at 400 and 900 °C are also close to those for the CVD SiO2 films annealed at these temperatures. These results indicate that the LPD SiO2 films are formed by tightly bonded Si-O networks, as mentioned in the FTIR spectrum analysis results. The shrinkage for LPD SiO2 films annealed at 400 °C was about 0.8%, owing to dehydration of the films; this is lower than that for the CVD SiO2 films. The shrinkages for the LPD SiO,_and CVD SiO2 films annealed at 900 °C were about 2.0% and 3.5% respectively. The lower shrinkages for the LPD SiO2 films can be explained as resulting from the contents of the Si-OH bonds and OH groups in the as-deposited LPD SiO2 films being lower
19
than those in the as-deposited CVD SiO2 films, as discussed in the FTIR spectrum analysis results. The residual tensile stress for the as-deposited LPD SiO2 films was about 20 MPa, which is lower than that for the as-deposited CVD SiO2 films (50 MPa). This is because of the lower deposition temperature than that for the CVD SiQ films and because of the fluorine contained in the films [17]. The residual tensile stress for the LPD SiO2 films was increased to 50 MPa by annealing at 400 °C, as a result of shrinkage of the films caused by the reduction of water components, such as Si-OH bonds and OH groups. For the CVD SiO2 films, the residual tensile stress was also increased to 65 MPa by annealing at 400 °C. The low residual stress increment for the CVD SiO2 films annealed at 400 °C can be explained as being because the reduction in water components is less than that for the LPD SiO2 films, as mentioned in the FTIR spectrum analysis results. The residual stress for the films annealed at 900 °C was converted to compressive stress, as a result of film densification and because of the difference in the coefficients of thermal expansion between silicon substrates and SiO2 films. The residual compressive stress value for the LPD SiO2 films annealed at 900 °C was 50 MPa, which is higher than that for the CVD SiO2 films annealed at 900 °C (30 MPa). The high stress value for the LPD SiO2 films annealed at 900 °C results from there being less film shrinkage than that for the CVD SiO2 films annealed at this temperature. These values are sufficiently low for interlayer dielectric films. Figure 5 shows the leakage current characteristics for the LPD SiO2 and CVD SiOz films as a function of the
10.5
E >,
/O/A ,/A
O~
10 .6 - a /, , ' / ~/-it' TM. .-,
O
--
0~0~0~0 O
-" - "- ",7-',-',
n-A--a--
~
C 0 ~/~rl ~ ' r l W
~ 10 .7 Q
-J
10"80
0.5 '
1.0 '
1.5 '
2i 0
2.5
Electric Field ( x l 0 a Vm") Fig. 5. Leakage current characteristics through the LPD SiO 2 (open symbols) and CVD SiO 2 films (full symbols) as a function of annealing temperature: ©, 0 , as-deposited; z~, A, annealed at 400 °C; D, I , annealed at 900 °C.
20
T. Homma, Y. Murao / LPD SiO z films for ULSI interho'er dielectrics
TABLE 2. Dielectricbreakdown strength at the leakage current density of 1 x 10--' A m-2, and dielectricconstant at 1 MHz for the LPD-SiO2 and CVD-SiO2 films LPD-SiO2
CVD-SiO2
As-deposited
400 °C
900 °C
As-deposited
400 °C
900 °C
Dielectric breakdown strength ( x 108 V m-1)
6.3
7.2
7.2
5.7
6.1
7.0
Dielectric constant at 1 MHz
3,9
3.7
3.7
4.4
4.3
4.1
annealing temperature. The leakage current densities for the as-deposited LPD SiO2 films were lower than for the as-deposited CVD SiO2 films, owing to the high quality of the Si-O bonds with less Si-OH bonds and OH groups, as mentioned in the F T I R spectrum analysis results. The leakage current densities for the LPD SiO2 films decreased with increasing annealing temperature. They were even lower than those for the CVD SiO2 films. F r o m the result obtained for LPD SiO 2 films annealed at 900 °C, the decomposition of fluorine from the films does not affect the leakage current characteristics at electric fields lower than 2.5 x 108 V m - 2. Table 2 summarizes the results for the dielectric breakdown strength and the dielectric constant. It can be seen that the dielectric breakdown strength for the as-deposited LPD SiO= films was about 6.3 x 108 V m -~, which is higher than that for the as-deposited CVD SiO2 films (5.7 x 108 V m-l). The dielectric breakdown strengths for LPD SiO2 and CVD SiO2 films annealed at 400°C increased to 7.2 x l0 s and 6.1 x 108Vm -~ respectively. Although the dielectric breakdown strength for CVD SiO2 films annealed at 900 °C increased to 7.0 x 10sV m -~, the dielectric breakdown strength for LPD SiO 2 films annealed at this temperature was still the same as that for the LPD SiO2 films annealed at 400 °C. This may be because of the change in chemical bonding structure caused by the decomposition of fluorine from the LPD SiO2 films annealed at 900 °C. These results indicate that the decomposition of fluorine affected the dielectric breakdown strength. The dielectric constant for the as-deposited LPD SiO2 films was about 3.9 at 1 MHz, which is lower than that for the as-deposited CVD SiO2 films (4.4). This lower dielectric constant results from the fluorine contained in the films and from there being less O H groups in the films [ 18]; this is consistent with the low refractive index. The dielectric constant decreased for the LPD SiO2 and CVD SiO2 films annealed at 400 °C, owing to the dehydration of the films. Although the dielectric constant for CVD SiO2 films annealed at 900 °C decreased further, the dielectric constant for the LPD SiO2 films annealed at 900 °C was not changed. The low dielectric constant for the LPD SiO2 films is
very useful for the reduction of capacitance among wirings, resulting in a reduction in the signal delay time through the multilevel interconnections. These electrical property evaluation results indicate that the LPD SiO2 films can be used for the interlayer dielectric films in ULSI multilevel interconnections.
4. Conclusions The properties of a new fluorinated SiO2 film formed by the LPD technique at 35 °C and used as the interlayer dielectrics of ULSI multilevel interconnections have been investigated. It has been revealed that the LPD technique has a good capability for the surface planarization of interlayer dielectric films. The LPD SiO2 film deposition mechanism can be explained as follows: (i) fluorosilanol formation; (ii) fluorosilanol oligomer formation by a catalytic reaction in the supersaturated HzSiF6 solution; (iii)adsorption of the oligomers onto the substrate surface; (iv) polymerization by a catalytic reaction at the substrate surface. The properties of LPD SiO2 films deposited at 35 °C are better than those of CVD SiO2 films when used as interlayer dielectrics. Although the LPD SiOz film properties are changed by annealing at 400 and 900 °C, the values are still better than those of the CVD SiO2 films for both the poly-metal and inter-metal dielectric films. This fluorinated SiO2 film deposition technique is useful for interlayer dielectric film formation in USLI multilevel interconnection fabrication.
Acknowledgment The authors would like to thank Dr. M. Kamoshida, Dr. M. Ogawa, Dr. O. Kudoh, Dr. A. Ishitani, Dr. H. Tsuya, Dr. K. Hamano, and Dr. D. T. C. Huo from AT&T Bell Laboratories for their encouragement and useful advice. They also would like to express appreciation to Mr. S. Kamiyama and Mr. T. Ishijima for their aid in sample preparation.
T. Homma, Y. Murao / LPD SiO 2 films for ULSI interlayer dielectrics
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