Thin Solid Films 498 (2006) 20 – 24 www.elsevier.com/locate/tsf
Characterization of sputter-induced temperature effect in fluorine doped SiO2 film deposition by high-density plasma chemical vapor deposition Wen-Chu Hsiao a, Chuan-Pu Liu a, Ying-Lang Wang b,c,* a
Department of Materials Science and Engineering, National Cheng Kung University, Tainan 701, Taiwan b College of Science and Engineering, National University of Tainan, Taiwan c Department of Applied Physics, National Chia Yi University, Chia-Yi, Taiwan Available online 10 August 2005
Abstract High-density plasma chemical vapor deposited fluorosilicate glass (FSG) has been successfully used for the inter-metal dielectric material in ultra-large semiconductor integration manufacturing due to its low-dielectric constant and stable gap-filling capability. However, temperatures rise and related effects due to sputter etch from the deposition process have become major concerns for film properties. In this paper, an independent helium-cooling system was employed to control a suitable temperature range from 410 -C to 460 -C during FSG deposition. Subsequently, film properties including fluorine concentration, distribution, refractive index, dielectric constant and gap-filling capability were thus examined as a function of He pressure used in the cooling system. The results show that both deposition rate and fluorine concentration increase with increasing helium pressure; however, more fluorine becomes inactive, which might be present as defects. We have shown that an FSG film with a dielectric constant down to 3.43 as well as good gap-filling capability can be achieved when employing this new cooling system with 9 mTorr helium pressure. D 2005 Elsevier B.V. All rights reserved. Keywords: High-density plasma CVD; Fluorosilicate glass; Low-dielectric constant
1. Introduction As the feature size in semiconductor devices shrinks to and below 0.18 um, the parasitic capacitance from the intermetal dielectrics (IMD) between metal lines becomes important in terms of RC delay in device switching [1]. Thereby, lower-dielectric constant materials with thinner thickness and better gap-filling ability are demanded for the devices of even smaller sizes. Among all the developing low-dielectric materials, fluorine-doped silicon oxide (FSG) has been recognized as a promising IMD material for its low-dielectric constant, which ranges from 3.3 to 3.9 [2– 5]. In addition, good gap-filling ability has also been demonstrated for a feature with an aspect ratio (AR) of metal line * Corresponding author. College of Science and Engineering, National University of Tainan, Taiwan. Tel.: +886 6 5052000; fax: +886 6 5052057. E-mail address:
[email protected] (Y.-L. Wang). 0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2005.07.056
height to metal line separation up to 1.35:1 [1]. In order to successfully integrate this FSG film into manufacturing for sub-0.18 um technology, high-density plasma chemical vapor deposition (HDP-CVD) is widely employed [3,4]. To prevent gaps from being sealed during deposition, sputter etch is introduced in the HDP-CVD process where the gap-filling capability is determined by the ratio of sputter etch to deposition rate. Moreover, smaller devices with deeper trench requires even faster sputter etch rate. Consequently, the HDP-CVD technique consists of both deposition and sputter etch processing, which occur simultaneously at the growing film surface. The sputter etch process is achieved by physical ion bombardment through Ar+ and O2, and chemical etch through F ions reacted with SiO2. Because of ion bombardment, substrate temperature would be raised during the sputter etch process, so that film properties may be deviated from expectation due to unstable deposition temperature, which then causes a
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bonding in the film upon decomposing overlapped peaks by a Gauss dissolution method. The distribution of fluorine in patterned wafers was measured by X-ray energy-dispersive spectrometer (EDS) in a transmission electron microscope (TEM). Dielectric constant of the FSG films was determined using a Mercury Probe at 1 MHz. Gap-filling capability of the FSG films was verified by cross-sectional scanning electron microscopy (SEM).
Temperature (˚C)
With Ion Bombarment Without Ion Bombardment
455
21
410 365 320 6
6.5
7
7.5
8
8.5
9
9.5
He pressure (mTorr) Fig. 1. Temperature of the FSG films measured during deposition by HDPCVD as a function of He pressure used in the cooling system, where the square and triangle symbols represent deposition with ion bombardment and without ion bombardment, respectively.
serious reliability issue. In a CVD process, deposition temperature is one of the major factors that determine film quality. Therefore, in order to obtain stable FSG films, a wafer backside cooling system is designed in the HDP-CVD to control wafer temperature during deposition. The cooling system is composed of a inner and outer rings of holes to inject gas, and the gas is usually helium. In this paper, we thus attempt to demonstrate that improved FSG film quality can be obtained by systematically investigating film properties as a function of helium pressure used in HDP-CVD. The film properties examined here includes fluorine dopant concentration, FSG bonding configuration, dielectric constant and gapfilling capability.
2. Experimental Fluorosilicate glass (FSG) films were deposited using an HDP-CVD system operating at 13.56 MHz with gas sources of SiH4, SiF4, O2 and Ar. The chamber pressure was kept at 5 mTorr during deposition. The helium pressure at the outer ring was varied from 6.5 to 9 mTorr with the ratio of the pressure at the inner to the outer ring being maintained at 0.62. The wafer temperature during deposition was measured in situ by an infrared thermal detector. For reference, a standard sample was also prepared using exactly the same deposition parameters except that the RF power of the bottom electrode was turned off during deposition. Consequently, no ion bombardment was involved in the standard sample and the standard deposition rate was calculated to evaluate the ratio of deposition to sputter etch for the other samples. Thickness and refractive index (RI) of the FSG films were measured by an opti-probe 5240 system. Fluorine content in the film was measured using Fourier Transformed Infrared Spectroscopy (FTIR) for active doped fluorine and using X-ray Fluorescence Spectroscopy (XRF) for total fluorine content. FTIR was also used to evaluate the ratio of Si-F (around 940 cm 1) to Si– O (around 1090 cm 1)
3. Results and discussion Since the FSG films were deposited by HDP-CVD, deposition temperature would be affected by the degree of ion bombardment and the helium pressure in the new cooling system as shown in Fig. 1, where the square and triangle symbols represent deposition with or without sputter etch, respectively. Apparently, from Fig. 1, as increasing the pressure from 6 to 9.5 mTorr, substrate temperature decreases linearly from 464 to 408 -C, which is still significantly higher than 360 -C from the standard sample heated only through a substrate heater without ion bombardment. Fig. 1 suggests that ion bombardment in the HDP-CVD process can raise substrate temperature for as much as 100 -C in this case. Owing to ion bombardment and cooling efficiency, substrate temperature varies largely even for the same heating setup. As a result, deposition rate, D/S ratio and refractive index (RI) are also affected as shown in Table 1. The reduction in film thickness at lower He pressure could result from both higher etching rate and higher film density. It has been reported [6] that FSG films become denser at high temperatures especially with less fluorine incorporated. However, we are unable to separate these two factors quantitatively in the deposition to sputter etch ratio (D/S). The apparent D/S ratio is estimated from the difference on the film thicknesses of the as-deposited and standard samples. The apparent D/S ratio increases from 2.28 to 2.44 with increasing He cooling pressure. Various D/S ratios imply either deposition rate or sputtering rate has been changed greatly, which would definitely affect fluorine distribution, resistance to bond breaking and densification in the film. Therefore, Fig. 2 shows fluorine concentration in the FSG films as a function of He pressure, for both active and total fluorine. It can be seen from Fig. 2 that the total
Table 1 Physical properties of the FSG films deposited with various He pressures He pressure (mTorr)
Thickness (nm)
D/S ratio
RI
6.5 7.0 7.5 8.0 8.5 9.0
605.8 612.1 620.9 628.5 634.7 640.9
2.28 2.31 2.35 2.38 2.42 2.45
1.448 1.445 1.444 1.443 1.442 1.441
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4.7 4.4 4.1 3.8
Si-F %
1.4
F % measured by FTIR F % measured by XRF
Absorbance (a.u)
F concentration (%)
5.0
Si-O
1.3 1.2 1.1
Si-F
1.0 6 6.5 7 7.5 8 8.5 9 9.5
Si-O
He pressure (mTorr)
3.5 6
6.5
7
7.5
8
8.5
9
3400
9.5
3000
He pressure (mTorr) Fig. 2. Fluorine concentration as a function of He pressure for active fluorine concentration measured by FTIR, and total fluorine concentration measured by XRF.
concentration of fluorine increases with increasing He pressure linearly from 3.72% to 4.79%, indicating that the incorporation rate of fluorine in the FSG film is faster at lower deposition temperature. In addition, the active fluorine in the FSG films also linearly increases but at a slower rate. In other words, when all the fluorine in the FSG films is activated at higher temperature, more fluorine becomes inactively incorporated toward lower deposition temperature. Therefore, in the lower deposition temperature, the dissolution rate of fluorine is faster, however, more fluorine becomes unbonded with Si– O probably due to high deposition rate or lower kinetic energy and this may cause some defects in IC devices such as like-bubbles, metallization damages [7,8]. Fluorine distribution in the film caused by sputter deposition and cooling is described in Fig. 3, as a function of He pressures of 7, 8 and 9 mTorr from a patterned wafer as indicated. From Fig. 3, one can see that fluorine
•
2600 2200 1800 1400 Wavenumber (cm-1)
1000
600
Fig. 4. FTIR spectra of the FSG films with various He pressures from 6.5 to 9 mTorr, where the inset shows Si – F percentage as a function of He pressure.
distributes more uniformly when processing with lower He pressure and thus higher deposition temperature. On the contrary, when processing with higher He pressure for lower deposition temperature, fluorine concentration at the bottom of the film is higher than at the top. The non-uniform distribution could relate to the temperature profile at the growing front since the efficiency of heat removal from the backside of a sample should become worse with thickness. Fig. 4 shows a typical FTIR spectrum of the FSG film deposited by HDP CVD. In general, there occurred three major peaks, which are located at 1090, 940, and 817 cm 1 corresponding to Si –O stretching mode, Si– F stretching mode, and Si– O bending mode, respectively. The peak area ratio of Si– F to Si –O in the FSG films is also calculated as the inset in Fig. 4. This result indicates that the Si– F bonds increase with increasing He pressure, which is consistent with the results of Fig. 2. In fact, the film structure can be rationalized to consist of a less dense Si –O network when more active fluorine is incorporated at lower temperature, [9] which causes a nano-porous film. The porosity of a film should be able to be identified by a shoulder peak by the main Si –O stretching peak at around 1160 cm 1 [10]. However, the shoulder peak is not clearly visible here.
Metal FSG
Dielectric Constant
Counts
3.7
9 mtorr
8 mtorr 7 mtorr
3.6
3.5
3.4
3.3
0
100
200
300
400
500
600
Position (nm) Fig. 3. EDS line profiles with a TEM for fluorine distribution from top to bottom in the FSG films with various He pressures.
6
6.5
7
7.5
8
8.5
9
9.5
He Pressure (mTorr) Fig. 5. Dielectric constant variation of the FSG films with various He pressures.
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Fig. 6. SEM cross-sectional images showing the gap-filling capability of the FSG films deposited on a patterned wafer as a function of He pressures for (a) 7, (b) 8, and (c) 9 mTorr, where the feature contains metal and gap widths of 430 nm and 240 nm, respectively.
Chou and Lee [10] and Lee and Park [11] have shown that dielectric constant is directly related to refractive index by the equations shown below [12,13] e ¼ e1 þ je2
ð1Þ
e 1 ¼ n2 k 2
ð2Þ
e2 ¼ 2nk
ð3Þ
where the (sub 1, (sub 2 are the real and imaginary part of the complex dielectric function ((). The optical constants of n and k are refractive index and extinction coefficient, respectively. In general, the extinction coefficient (k) of a SiO2 film is always positive and approaches to zero. Correspondingly, relative dielectric constant (() is proportional to the square of refractive index (n 2). This relationship is also supported by Fig. 5, which shows relative dielectric constant of the FSG films decreasing to 3.43 at 9 mTorr as increasing He pressure, analogous to RI. From the above discussion, film property especially dielectric constant becomes better in general when higher He pressure is incorporated in the cooling system, however, accompanied by more inactive fluorine. Therefore, we attempt to assess the feasibility of adopting the deposition design into a real device with the metal width and gap of 430 and 240 nm, respectively, which is used for 0.15 um technology. Fig. 6 shows SEM cross-sectional images of a patterned wafer with different He pressures of (a) 7, (b) 8 and (c) 9 mTorr, respectively, for the gap filling. The sputter etch removes the deposited materials at the gap openings and reduces sidewall deposition, achieving a ‘‘bottom-up’’ deposition mechanism [14]. From the images, we can find that a good gap filling capability is achieved from all the deposition conditions used. With the evidence from Figs. 5 and 6, we have demonstrated that a low-dielectric constant FSG film with good gap-filling capability is achieved by applying a cooling system with He in HDP-CVD to maintain lower deposition temperature. We have shown the adverse effects from the raised temperature caused by the ion bombardment during the FSG film deposition. For example, because the FSG film is denser when deposited at higher temperature, the relative dielectric constant is higher than at lower temperature. Besides, although a film deposited at higher temperature,
would be with fluorine incorporated in SiO2 network more completely than at lower temperature, film stress becomes more compressive and might cause metal film distortion [15]. Apparently, from our results, one can find that the deposition of the FSG film at the intended temperature is significant to obtain better properties.
4. Conclusion We have found that the properties of FSG films are varied greatly by the sputter etch induced temperature rise in the HDP CVD process. We have demonstrated that deposition temperature can be well controlled by a cooling system to the backside of a sample with He gas. Refractive index as well as dielectric constant of the FSG film decrease with increasing He pressure, indicating that the film density decreases and the film becomes more porous. Total fluorine concentration increases with increasing He pressure, however, total inactive fluorine also increases. It suggests that although a higher dissolution rate of fluorine occurs at a lower deposition temperature, more fluorine is not bonded with Si –O network, which would affect the reliability of the FSG films. However, they all show a good gap filling capability on a patterned wafer for 0.15 nm technology. The results suggest that there exists an optimum He pressure range, which would still provide good dielectric constant and gap filling capability and minimum effects on reliability issue.
Acknowledgements This work was supported by National Science Council of Taiwan under Contract No. NSC-93-2216-E006021 and also supported by Taiwan Semiconductor Manufacture Company.
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