- Email: [email protected]
Novel Parylene-N films deposited at liquid nitrogen temperatures
June 1999
Materials Letters 39 Ž1999. 339–342 www.elsevier.comrlocatermatlet
Novel Parylene-N films deposited at liquid nitrogen temperatures James Erjavec, John Sikita, Stephen P. Beaudoin, Gregory B. Raupp
)
Department of Chemical, Bio, Materials Engineering, Arizona State UniÕersity, Mail Code 6006, Tempe, AZ 85283-6006, USA Received 14 December 1998; accepted 20 December 1998
Abstract Vapor deposition polymerization of Parylene-N films at y1968C produces optically opaque, highly porous films. Film density averaged 0.195 grcm3, more than five times less than that of conventional non-porous Parylene-N films deposited near room temperature, which have densities of 1.11 grcm3. Deposition rates, measured as the change in film thickness per unit time, averaged 8.3 mmrmin. This rate is more than two orders of magnitude greater than the deposition rates of nonporous films near room temperature, at otherwise fixed conditions of monomer delivery rate and deposition chamber pressure. On a mass deposition rate basis, the new low temperature film deposition rate is more than an order of magnitude faster than the corresponding rate at room temperature. q 1999 Elsevier Science B.V. All rights reserved. PACS: 77.84 y s; 77.84 P gd Keywords: Vapor deposition polymerization; Low k dielectric; Polymer thin films; Parylene-N
1. Introduction Many polymers have been considered as new options for interlayer dielectrics ŽILD. in integrated circuit ŽIC. technology w1–3x. These low dielectric constant materials may be used to replace or complement the present use of SiO 2 w2–4x. From a processing viewpoint, research is focused either on conventional spin-on polymers or on vapor deposited polymers such as parylene w5–8x. The family of parylenes possesses many properties that are well suited for use as an ILD. For example, Parylene-N has a dielectric constant of 2.65, with high thermal stability, and low water absorption. Perhaps the most
)
Corresponding author. Tel.: q1-602-9652828; Fax: q1-6029650037; E-mail: [email protected]
attractive quality of parylene is its ability to vapor deposit conformally in features such as via holes and trenches. In addition, the vapor deposition process produces less waste and greater film thickness control than their spin-on process counterparts. In this note we report on the production of porous Parylene-N films on substrates held near liquid nitrogen temperatures. Because these films are highly porous, they should yield a substantially lower dielectric constant than conventional dense, nonporous Parylene-N films.
2. Experiment Gorham w5x first developed the parylene vapor deposition polymerization ŽVDP. process. In this process, a dimer of di-p-xylylene is sublimed at
00167-577Xr99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 9 9 . 0 0 0 3 1 - 2
340
J. ErjaÕec et al.r Materials Letters 39 (1999) 339–342
1758C. The dimer then flows through a pyrolysis furnace held at 6008C, where it cracks into two monomers. Monomer then flows into a reactor chamber and deposits on surfaces held below a threshold temperature, which is 308C for Parylene-N. In our deposition process carried out in a custombuilt research scale vapor deposition polymerization tool, the dimer was sublimed at approximately 1408C and the pyrolysis furnace temperature was held at 6858C. The substrate was cooled to y1968C by continuously recirculating liquid nitrogen through the sample holder during the course of each deposition. The substrate temperature was monitored by a thermocouple mounted in the cooling block just below the sample. The pressure throughout each deposition cycle was held constant at 0.09 Torr by a butterfly valve pressure controller. Substrates were cut from polished silicon Ž100. wafers obtained from Silica Source. Samples were approximately 3r4 in.= 2 1r2 in., and were employed without pretreatment Že.g., no adhesion promoter was employed.. The samples were weighed before and after each run to determine the weight gain due to film deposition. Following deposition, the substrate was freeze fractured by immersing it in a container of liquid nitrogen until it reached liquid nitrogen temperature, then removing it and cleaving it with a diamond scribe. Cross-sectional SEMs of the as-deposited films were taken with a JEOL JSM-840 scanning microscope operating at an accelerating voltage of 15 keV.
Samples were coated with gold in a Denton sputter coater for 180 s at 20 mA and 2.5 kV in argon in order to minimize electron beam charging during SEM imaging.
3. Results and discussion Both the deposition rate and the physical characteristics of films deposited at liquid nitrogen temperatures are significantly different than those for films deposited at higher temperatures. Fig. 1 shows representative SEM cross-sectional images of films deposited at liquid nitrogen temperature. Fig. 1Ža. is an SEM micrograph of a freestanding Parylene-N film that was peeled from the substrate. The left side of the film was originally flush with the substrate. Fig. 1Žb,c. shows Parylene-N films that are attached to the Si substrate. The films are highly porous, and exhibit a general increase in pore size with increasing distance from the film–substrate interface. Pores range in size from approximately one micron up to 15 to 20 mm. The general appearance is that of a honeycomb. In contrast, Surendran et al. w6x deposited films at LN2 temperatures and obtained a cauliflower-type morphology. The porous films are optically opaque, which is caused by light diffraction by the porous structure. Fig. 2 shows an SEM of a Parylene-N film cross-section deposited at y12.58C in our VDP tool. The film is dense and nonporous.
Fig. 1. Representative cross-sectional SEM micrographs of porous Parylene-N film deposited at a substrate temperature of y1968C. Ža. The film is free standing. The length of the lines is 36 mm. Žb. The porous Parylene-N film is on the right side, and the left side is the Si substrate. The line length is 35 mm. Žc. The porous Parylene-N film is on the right side, and the left side is a Si wafer. The line length is 43 mm.
J. ErjaÕec et al.r Materials Letters 39 (1999) 339–342
Fig. 2. SEM cross-section of a dense Parylene-N film deposited at y508C. The line length is 6 mm. The right side is the Parylene-N film, the left side is the Si wafer.
Table 1 compares our deposition rate data with the literature values of Ganguli et al. w8x for Parylene-N deposition at various temperatures. The reported deposition rates for the ASU values are averaged over the course of each run, obtained from the average thickness of the Parylene-N films as determined from SEM micrographs, divided by the deposition run time. The total pressure during deposition for both the ASU runs and literature runs was 0.09 Torr. The pyrolysis furnace temperature for all the literature values was 6508C. The furnace temperatures for the ASU values were 7008C for the room temperature runs, 6258C for the y12.58C runs, and 6858C for the LN2 runs. On a mmrmin basis, films deposited at LN2 temperatures grow more than two orders of magnitude faster than those deposited near
341
room temperature. At least a portion of this increased rate is due to the film porosity. To extract the intrinsic chemical reaction rate increase, we estimated film densities and calculated mass-based deposition rates. The densities of the low temperature films were determined through weight gain and film thickness measurements. The average density of the films Parylene-N deposited at LN2 temperature was 0.195 grcm3, which is much lower than the 1.11 grcm3 density of non-porous Parylene-N films w7x. Massbased deposition rates were calculated by multiplying the thickness-based deposition rate by the film density. Table 1 shows that the mass deposition rates of the films deposited at liquid nitrogen temperature are more than an order of magnitude higher than corresponding deposition rates of films deposited near room temperature. The origin of increased deposition rate with decreasing temperature can be at least partially explained by Flory’s theory w9x. During the VDP process, monomer is adsorbed on the surface, after which it may desorb from the surface, react on the surface, or diffuse and react in the bulk of the film. Equilibrium is reached between the rates of adsorption and desorption. Assuming that the gas phase is composed mainly of monomer molecules the equilibrium concentration M0 of monomer in the film is estimated by
r M0 s
P
Ž 1.
K H P0
where r is the film density, K H is Henry’s law constant for the monomer in the growing polymer film, P is the monomer partial pressure in the gas phase, and P0 is the vapor pressure of the monomer at the substrate temperature w9x. Vapor pressure P0
Table 1 Comparison of deposition rates as a function of substrate temperature Substrate temperature Ž8C.
25 y12.5 y20 y196
Deposition rate This work Žmmrmin.
This work Žmgrmin.
Ganguli et al. w8x Žmmrmin.
Ganguli et al. w8x Žmgrmin.
0.020 0.040 – 8.3
0.0022 0.0044
0.01 – 0.20 10
0.0011 – 0.022 ?
0.16
342
J. ErjaÕec et al.r Materials Letters 39 (1999) 339–342
decreases sharply as the substrate temperature decreases, and therefore the concentration of the monomer in the film increases for constant monomer overpressure, leading to higher deposition rates. The estimated dielectric constant of these films, based on the densities and the dielectric constants of nonporous films and an estimated porosity of 80%, approximately 1.3. According to the National Technology Roadmap for Semiconductors ŽNTRS. w10x, advanced generation of microchips will require ILDs with dielectric constants below 2.0. Teflon or polyŽtetrafluoroethylene. has the lowest dielectric constant of the candidate polymer ILDs, with a dielectric constant near 2. If polymers are to be employed, this physical property limitation will require that the films be porous. For parylene to be a viable candidate porous polymer, the porosity will need to be uniformly distributed, pore sizes will need to be smaller, and the process will need to be reliable. We are now exploring process modifications to achieve these goals.
4. Conclusion We have shown that Parylene-N films deposited at liquid nitrogen temperatures are opaque, highly porous films. The deposition rates of these films on a thickness per unit time basis are more than two orders of magnitude higher than those deposited at room temperatures. The film densities averaged 0.195 grcm3, more than five times less than conventional
Parylene-N films. On a mass deposition rate basis, the porous, low temperature films were found to deposit almost an order of magnitude faster than room temperature, higher density films. Acknowledgements This work was supported by a grant from the National Science Foundation and U.S. EPA, grant no. CTS-9613377. We also thank Kevin Cooper for his assistance with the SEM. References w1x C.L. Lang, G.R. Yang, J.A. Moore, T.-M. Lu, Mater. Res. Soc. Symp. Proc. 381 Ž1995. 45. w2x J.V. Crivello, Mater. Res. Soc. Symp. Proc. 381 Ž1995. 51. w3x W.F. Beach, T.M. Austin, 2nd International SAMPE Electronics Conference, 1988, p. 25. w4x G.A. Dixit, K.J. Taylor, A. Singh, C.K. Lee, G.B. Shinn, A. Konecni, W.Y. Hsu, K. Brennan, M. Chang, 1996 Symposium on VLSI Technology Digest of Technical Papers, 1996, p. 86. w5x W.F. Gorham, J. Polym. Sci., Part A-1 4 Ž1966. 3027. w6x G. Surendran, M. Gazicki, W.J. James, H. Yasuda, J. Polym. Sci., Part A: Polymer Chemistry 25 Ž1987. 1481. w7x W.F. Beach, C. Lee, D. Basset, T. Austin, R. Olson, Encyclopedia of Polymer Science 17 Ž1988. 990. w8x S. Ganguli, H. Agrawal, B. Wang, J.F. McDonald, T.-M. Lu, G.-R. Yang, W.N. Gill, J. Vac. Sci. Technol. A 15 Ž1997. 3138. w9x P.J. Flory, Principles of Polymer Chemistry, Cornell Univ. Press, Ithaca, NY, 1953, p. 511. w10x National Technology Roadmap for Semiconductors, SIA, 1997.
Materials Letters 39 Ž1999. 339–342 www.elsevier.comrlocatermatlet
Novel Parylene-N films deposited at liquid nitrogen temperatures James Erjavec, John Sikita, Stephen P. Beaudoin, Gregory B. Raupp
)
Department of Chemical, Bio, Materials Engineering, Arizona State UniÕersity, Mail Code 6006, Tempe, AZ 85283-6006, USA Received 14 December 1998; accepted 20 December 1998
Abstract Vapor deposition polymerization of Parylene-N films at y1968C produces optically opaque, highly porous films. Film density averaged 0.195 grcm3, more than five times less than that of conventional non-porous Parylene-N films deposited near room temperature, which have densities of 1.11 grcm3. Deposition rates, measured as the change in film thickness per unit time, averaged 8.3 mmrmin. This rate is more than two orders of magnitude greater than the deposition rates of nonporous films near room temperature, at otherwise fixed conditions of monomer delivery rate and deposition chamber pressure. On a mass deposition rate basis, the new low temperature film deposition rate is more than an order of magnitude faster than the corresponding rate at room temperature. q 1999 Elsevier Science B.V. All rights reserved. PACS: 77.84 y s; 77.84 P gd Keywords: Vapor deposition polymerization; Low k dielectric; Polymer thin films; Parylene-N
1. Introduction Many polymers have been considered as new options for interlayer dielectrics ŽILD. in integrated circuit ŽIC. technology w1–3x. These low dielectric constant materials may be used to replace or complement the present use of SiO 2 w2–4x. From a processing viewpoint, research is focused either on conventional spin-on polymers or on vapor deposited polymers such as parylene w5–8x. The family of parylenes possesses many properties that are well suited for use as an ILD. For example, Parylene-N has a dielectric constant of 2.65, with high thermal stability, and low water absorption. Perhaps the most
)
Corresponding author. Tel.: q1-602-9652828; Fax: q1-6029650037; E-mail: [email protected]
attractive quality of parylene is its ability to vapor deposit conformally in features such as via holes and trenches. In addition, the vapor deposition process produces less waste and greater film thickness control than their spin-on process counterparts. In this note we report on the production of porous Parylene-N films on substrates held near liquid nitrogen temperatures. Because these films are highly porous, they should yield a substantially lower dielectric constant than conventional dense, nonporous Parylene-N films.
2. Experiment Gorham w5x first developed the parylene vapor deposition polymerization ŽVDP. process. In this process, a dimer of di-p-xylylene is sublimed at
00167-577Xr99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 9 9 . 0 0 0 3 1 - 2
340
J. ErjaÕec et al.r Materials Letters 39 (1999) 339–342
1758C. The dimer then flows through a pyrolysis furnace held at 6008C, where it cracks into two monomers. Monomer then flows into a reactor chamber and deposits on surfaces held below a threshold temperature, which is 308C for Parylene-N. In our deposition process carried out in a custombuilt research scale vapor deposition polymerization tool, the dimer was sublimed at approximately 1408C and the pyrolysis furnace temperature was held at 6858C. The substrate was cooled to y1968C by continuously recirculating liquid nitrogen through the sample holder during the course of each deposition. The substrate temperature was monitored by a thermocouple mounted in the cooling block just below the sample. The pressure throughout each deposition cycle was held constant at 0.09 Torr by a butterfly valve pressure controller. Substrates were cut from polished silicon Ž100. wafers obtained from Silica Source. Samples were approximately 3r4 in.= 2 1r2 in., and were employed without pretreatment Že.g., no adhesion promoter was employed.. The samples were weighed before and after each run to determine the weight gain due to film deposition. Following deposition, the substrate was freeze fractured by immersing it in a container of liquid nitrogen until it reached liquid nitrogen temperature, then removing it and cleaving it with a diamond scribe. Cross-sectional SEMs of the as-deposited films were taken with a JEOL JSM-840 scanning microscope operating at an accelerating voltage of 15 keV.
Samples were coated with gold in a Denton sputter coater for 180 s at 20 mA and 2.5 kV in argon in order to minimize electron beam charging during SEM imaging.
3. Results and discussion Both the deposition rate and the physical characteristics of films deposited at liquid nitrogen temperatures are significantly different than those for films deposited at higher temperatures. Fig. 1 shows representative SEM cross-sectional images of films deposited at liquid nitrogen temperature. Fig. 1Ža. is an SEM micrograph of a freestanding Parylene-N film that was peeled from the substrate. The left side of the film was originally flush with the substrate. Fig. 1Žb,c. shows Parylene-N films that are attached to the Si substrate. The films are highly porous, and exhibit a general increase in pore size with increasing distance from the film–substrate interface. Pores range in size from approximately one micron up to 15 to 20 mm. The general appearance is that of a honeycomb. In contrast, Surendran et al. w6x deposited films at LN2 temperatures and obtained a cauliflower-type morphology. The porous films are optically opaque, which is caused by light diffraction by the porous structure. Fig. 2 shows an SEM of a Parylene-N film cross-section deposited at y12.58C in our VDP tool. The film is dense and nonporous.
Fig. 1. Representative cross-sectional SEM micrographs of porous Parylene-N film deposited at a substrate temperature of y1968C. Ža. The film is free standing. The length of the lines is 36 mm. Žb. The porous Parylene-N film is on the right side, and the left side is the Si substrate. The line length is 35 mm. Žc. The porous Parylene-N film is on the right side, and the left side is a Si wafer. The line length is 43 mm.
J. ErjaÕec et al.r Materials Letters 39 (1999) 339–342
Fig. 2. SEM cross-section of a dense Parylene-N film deposited at y508C. The line length is 6 mm. The right side is the Parylene-N film, the left side is the Si wafer.
Table 1 compares our deposition rate data with the literature values of Ganguli et al. w8x for Parylene-N deposition at various temperatures. The reported deposition rates for the ASU values are averaged over the course of each run, obtained from the average thickness of the Parylene-N films as determined from SEM micrographs, divided by the deposition run time. The total pressure during deposition for both the ASU runs and literature runs was 0.09 Torr. The pyrolysis furnace temperature for all the literature values was 6508C. The furnace temperatures for the ASU values were 7008C for the room temperature runs, 6258C for the y12.58C runs, and 6858C for the LN2 runs. On a mmrmin basis, films deposited at LN2 temperatures grow more than two orders of magnitude faster than those deposited near
341
room temperature. At least a portion of this increased rate is due to the film porosity. To extract the intrinsic chemical reaction rate increase, we estimated film densities and calculated mass-based deposition rates. The densities of the low temperature films were determined through weight gain and film thickness measurements. The average density of the films Parylene-N deposited at LN2 temperature was 0.195 grcm3, which is much lower than the 1.11 grcm3 density of non-porous Parylene-N films w7x. Massbased deposition rates were calculated by multiplying the thickness-based deposition rate by the film density. Table 1 shows that the mass deposition rates of the films deposited at liquid nitrogen temperature are more than an order of magnitude higher than corresponding deposition rates of films deposited near room temperature. The origin of increased deposition rate with decreasing temperature can be at least partially explained by Flory’s theory w9x. During the VDP process, monomer is adsorbed on the surface, after which it may desorb from the surface, react on the surface, or diffuse and react in the bulk of the film. Equilibrium is reached between the rates of adsorption and desorption. Assuming that the gas phase is composed mainly of monomer molecules the equilibrium concentration M0 of monomer in the film is estimated by
r M0 s
P
Ž 1.
K H P0
where r is the film density, K H is Henry’s law constant for the monomer in the growing polymer film, P is the monomer partial pressure in the gas phase, and P0 is the vapor pressure of the monomer at the substrate temperature w9x. Vapor pressure P0
Table 1 Comparison of deposition rates as a function of substrate temperature Substrate temperature Ž8C.
25 y12.5 y20 y196
Deposition rate This work Žmmrmin.
This work Žmgrmin.
Ganguli et al. w8x Žmmrmin.
Ganguli et al. w8x Žmgrmin.
0.020 0.040 – 8.3
0.0022 0.0044
0.01 – 0.20 10
0.0011 – 0.022 ?
0.16
342
J. ErjaÕec et al.r Materials Letters 39 (1999) 339–342
decreases sharply as the substrate temperature decreases, and therefore the concentration of the monomer in the film increases for constant monomer overpressure, leading to higher deposition rates. The estimated dielectric constant of these films, based on the densities and the dielectric constants of nonporous films and an estimated porosity of 80%, approximately 1.3. According to the National Technology Roadmap for Semiconductors ŽNTRS. w10x, advanced generation of microchips will require ILDs with dielectric constants below 2.0. Teflon or polyŽtetrafluoroethylene. has the lowest dielectric constant of the candidate polymer ILDs, with a dielectric constant near 2. If polymers are to be employed, this physical property limitation will require that the films be porous. For parylene to be a viable candidate porous polymer, the porosity will need to be uniformly distributed, pore sizes will need to be smaller, and the process will need to be reliable. We are now exploring process modifications to achieve these goals.
4. Conclusion We have shown that Parylene-N films deposited at liquid nitrogen temperatures are opaque, highly porous films. The deposition rates of these films on a thickness per unit time basis are more than two orders of magnitude higher than those deposited at room temperatures. The film densities averaged 0.195 grcm3, more than five times less than conventional
Parylene-N films. On a mass deposition rate basis, the porous, low temperature films were found to deposit almost an order of magnitude faster than room temperature, higher density films. Acknowledgements This work was supported by a grant from the National Science Foundation and U.S. EPA, grant no. CTS-9613377. We also thank Kevin Cooper for his assistance with the SEM. References w1x C.L. Lang, G.R. Yang, J.A. Moore, T.-M. Lu, Mater. Res. Soc. Symp. Proc. 381 Ž1995. 45. w2x J.V. Crivello, Mater. Res. Soc. Symp. Proc. 381 Ž1995. 51. w3x W.F. Beach, T.M. Austin, 2nd International SAMPE Electronics Conference, 1988, p. 25. w4x G.A. Dixit, K.J. Taylor, A. Singh, C.K. Lee, G.B. Shinn, A. Konecni, W.Y. Hsu, K. Brennan, M. Chang, 1996 Symposium on VLSI Technology Digest of Technical Papers, 1996, p. 86. w5x W.F. Gorham, J. Polym. Sci., Part A-1 4 Ž1966. 3027. w6x G. Surendran, M. Gazicki, W.J. James, H. Yasuda, J. Polym. Sci., Part A: Polymer Chemistry 25 Ž1987. 1481. w7x W.F. Beach, C. Lee, D. Basset, T. Austin, R. Olson, Encyclopedia of Polymer Science 17 Ž1988. 990. w8x S. Ganguli, H. Agrawal, B. Wang, J.F. McDonald, T.-M. Lu, G.-R. Yang, W.N. Gill, J. Vac. Sci. Technol. A 15 Ž1997. 3138. w9x P.J. Flory, Principles of Polymer Chemistry, Cornell Univ. Press, Ithaca, NY, 1953, p. 511. w10x National Technology Roadmap for Semiconductors, SIA, 1997.