LETTER pubs.acs.org/Langmuir
Insight into On-Wafer Crystallization of Pure-Silica-Zeolite Films through Nutrient Replenishment Christopher M. Lew,† Yan Liu,† David Kisailus,† Grant M. Kloster,‡ Gabriel Chow,§ Boyan Boyanov,‡ Minwei Sun,† Junlan Wang,§ and Yushan Yan*,† †
Department of Chemical and Environmental Engineering, University of California, Riverside, Riverside, California 92521, United States Components Research, Intel Corporation, Hillsboro, Oregon 97124, United States § Department of Mechanical Engineering, University of Washington, Seattle, Washington 98195, United States ‡
bS Supporting Information ABSTRACT: Tetraethylorthosilicate (TEOS) is added to a pure-silica-zeolite MEL nanoparticle suspension and the mixture is subsequently used for preparing spin-on low-dielectric constant (low-k) films. The films are then characterized by ellipsometric porosimetry, transmission electron microscopy (TEM), and nanoindentation. Investigation into the film microstructure indicates that the addition of TEOS significantly increases the fraction of the crystalline domains, decreases the inter-crystal mesopore size, and narrows the pore size distribution. Films with 12% TEOS loading have a mean pore size distribution centered at 3.5 nm (diameter) with a full width at half maximum (fwhm) of 0.8 nm, while those with no TEOS have a distribution at 11.1 nm and fwhm of 7.9 nm. At 12% TEOS loading, the reduced modulus and hardness are 11.0 and 1.12 GPa, respectively; without TEOS, the values are 6.4 and 0.57 GPa.
T
he search for a new low-dielectric constant (low-k) material for future computer microprocessors has been one of the most difficult challenges in the semiconductor industry. With a DRAM 1/2 pitch of 14.2 nm expected by the year 2020, target interlevel metal insulator effective dielectric constant (k) values are between 2.3 and 2.6.1 However, according to the International Technology Roadmap for Semiconductors 2009 Edition - Interconnect, manufacturable solutions are only known for materials with a k value of 2.5-2.8.1 Thus, there is a concentrated research effort to find an alternative low-k material with suitable properties, including high mechanical strength, strong moisture and wet-etch chemical resistance, a narrow pore size distribution, and high heat conductivity. Many carbon-doped silicon dioxides, such as Black Diamond, Aurora, and Coral developed by Applied Materials, ASM International N.V., and Novellus Systems, respectively, are currently in use in commercial microprocessors. Incorporating porosity into these materials lowers the k value by taking advantage of the low dielectric constant of air (kair ≈ 1.0006). As a result, the pore size and pore size distribution of porous low-k materials must be carefully characterized and controlled to ensure uniform properties. The pore sizes should be at most 10% of the smallest feature size to prevent complications with groove etching.2 Poor pore sealing effects may also result from a wide pore size distribution, and Cu ion migration can lead to electrical breakdown.2 The recent development of on-wafer crystallization of zeolite films for low-k applications showed that crystallization of a puresilica-zeolite (PSZ) can occur in a relatively dry gel film spun onto a wafer under ambient pressure and in an open environment without humidity control.3 That work used a supposedly amorphous silica precursor solution that was spun onto a silicon wafer r 2011 American Chemical Society
and crystallized at high temperatures. As such, the on-wafer crystallization process is considered manufacturing friendly. In contrast, previous studies of zeolite low-k films investigated the use of nanoparticle spin-on PSZ MFI4,5 and MEL6,7 films in which a nanoparticle suspension was hydrothermally synthesized separately and subsequently spun onto a silicon wafer. (Zeolite framework types, such as MFI and MEL, are given a three-letter code by the International Zeolite Association.) This current study aims to decrease the pore size and narrow the pore size distribution of the on-wafer crystallization films. As a first step, we combine elements of on-wafer crystallization and nanoparticle films to investigate the conversion and crystallization of the amorphous precursor silica through the replenishment of additional silica nutrients and their impact on pore size and pore size distribution. The extra silica is shown to have a positive effect on the pore size and the pore size distribution, which is a material property that has been shown to be technologically problematic in previous zeolite nanoparticle films.8 The effect of the added silica (tetraethylorthosilicate, or TEOS) on the pore size and pore size distribution is studied using ellipsometric porosimetry. TEOS was added to hydrothermally synthesized nanoparticle zeolite suspensions; the resulting solution was spun onto a silicon wafer; and the films were calcined. As shown in Figure 1a, the porosity decreases as the added TEOS concentration increases. The pore size distribution plots in Figure 1b show that, with increasing TEOS concentration, the pore size distribution narrows and the maximum mesopore diameter peak decreases. For the films with no TEOS added, Received: February 16, 2011 Published: March 01, 2011 3283
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Figure 1. (a) Porosity as a function of added TEOS. (b) Pore size distribution for films with 0-12% TEOS: (black square) 0% TEOS; (red circle) 3% TEOS; (blue up triangle) 6% TEOS; (green down triangle) 9% TEOS; and (purple tilted square) 12% TEOS.
Table 1. Physical Properties of the PSZ MEL Low-k Films with TEOS mesopore %
porosity
diameter peak
fwhm
Er
TEOS
(%)
(nm)
(nm)
(GPa)
H (GPa)
0 3
40 38
11.1 7.0
7.9 2.1
6.4 ( 1.8 8.5 ( 1.0
0.57 ( 0.09 0.73 ( 0.11
6
29
5.3
0.9
11.7 ( 0.2
1.09 ( 0.09
9
27
4.0
0.9
11.4 ( 0.2
1.10 ( 0.08
12
23
3.5
0.8
11.0 ( 3.1
1.12 ( 0.50
the maximum peak of the pore diameter is 11.1 nm, and the peak decreases to 3.5 nm for the films with 12% TEOS (see also Table 1). The full width at half-maximum (fwhm) also decreases as the TEOS loading is increased. With no TEOS, the fwhm is 7.9 nm, decreasing to 0.8 nm for films with 12% TEOS. The addition of TEOS not only reduces the diameter of the mesopores but also narrows the distribution of the pore sizes and creates films with more uniform porosity. Moreover, the mesopore sizes are tunable simply by controlling the TEOS loading, and pore sizes can be attained that are almost 1 order of magnitude smaller than the 32 nm feature size in the current state-of-the-art chips. TEM was used to investigate the micro- and nanostructural features of the 0% and 12% TEOS films. Figures 2a and b show lower and higher magnification micrographs, respectively, of the 0% TEOS film. The zeolite lattice of the nanocrystals is visible in the darker areas of the film specimens; these are shown in the
boxed areas of Figure 2. The area surrounding the nanocrystals in Figure 2a is a porous and nonuniform silica matrix. This can be seen more clearly in Figure 2b. The large mesopores in this micrograph were measured and found to be 10-20 nm in diameter, which is consistent with the ellipsometric porosimetry data. Micrographs of 12% TEOS films are shown in Figure 2c and d. In contrast to the 0% TEOS film, the zeolitic domains in Figure 2c are tightly packed, uniform, and little void space is evident. Figure 2d is a higher magnification micrograph showing the mesoporous silica that surrounds the zeolite nanocrystals. Unlike the 0% TEOS case, this image clearly shows a highly uniform and ordered silica matrix. Measurement of the d-spacing in Figure 2d reveals pores of about 0.50 nm, which is in good agreement with the zeolite MEL pore size of 0.53 nm.9 Moreover, the large mesopore void spaces that are evident in the 0% TEOS films are absent in the 12% TEOS films. These TEM studies show that the mesoporous silica contains larger zeolite domains and more uniform mesopores after the addition of TEOS. As found in previous work, more of the precursor silica is used for crystal growth with increasing synthesis time during hydrothermal crystallization.8 The remaining material is a nutrient-deprived and highly porous silica matrix. Since the silica precursor still contains unreacted organic structure-directing agent and zeolite nuclei, replenishing the precursor with TEOS allows the film to have larger zeolite domains and smaller, more uniform intercrystalline mesopores after calcination. The mechanism for increased zeolite domains and smaller, more uniform intercrystalline mesopores is likely similar to that of the previous work pioneered by Pinnavaia10,11 and Xiao12-15 that used zeolite seeds to create ordering and thermal stability in the walls of mesoporous silica. Nanoindentation studies provide additional evidence for the smaller and narrower pore size and pore size distribution, as well as the larger zeolite domains. Reduced modulus and hardness are generally negatively correlated to porosity; thus, the films with higher TEOS loadings should exhibit better mechanical properties. Moreover, previous studies have shown that crystalline zeolite has higher reduced moduli when compared to amorphous porous silica at the same porosity.16 After the addition of TEOS, the mesoporous silica with higher crystallinity should strengthen the reduced modulus and hardness values. The results are given in Table 1. The films with 0% TEOS have a reduced modulus of 6.4 GPa, which is already above the industrially desired value of 6 GPa.17 Adding 3% TEOS increases the modulus value up to 8.5 GPa. Interestingly, the mechanical properties do not completely trend with porosity. Instead, the reduced modulus reaches a maximum of 11.7 GPa at 6% TEOS and stays relatively constant at around 11 GPa with increasing TEOS. Similarly, the hardness reaches a constant value once the TEOS loading reaches 6%. It is unclear why the mechanical properties do not continue to increase with the amount of added TEOS. Nonetheless, from a technological perspective, the films are mechanically sound. The addition of TEOS into the zeolite nanoparticle suspension successfully decreases the porosity, mesopore size, and pore size distribution of the resulting spin-on films. Furthermore, with increasing TEOS concentration, the reduced modulus and hardness increase. TEM observations confirm that this result can be explained by the microstructure of the film before and after TEOS addition. The addition of TEOS leads to lower porosity, the crystalline zeolite domains increase, and smaller and more uniform intercrystalline pores are obtained. This nutrient-replenished on-wafer crystallization provides an 3284
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Figure 2. TEM micrographs of (a, b) PSZ MEL films with 0% TEOS and (c, d) PSZ MEL films with 12% TEOS. The boxed areas highlight the zeolite lattice.
improved understanding of the film microstructure and onwafer crystallization behavior, as well as the possibility of a much improved on-wafer crystallization process.
’ ASSOCIATED CONTENT
bS
Supporting Information. Experimental procedures. This material is available free of charge via the Internet at http://pubs. acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Funding Sources
Financial support was provided by the National Science Foundation (CTS-0404376) and the Semiconductor Research Corporation (Task 1576.001 - Intel Custom Funding).
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