A comparison of the defects introduced during plasma exposure in. high- and low-k dielectrics

Transcription

1 A comparison of the defects introduced during plasma exposure in high- and low-k dielectrics H. Ren, 1 G. Jiang, 2 G. A. Antonelli, 2 Y. Nishi, 3 and J.L. Shohet 1 1 Plasma Processing & Technology Laboratory and Department of Electrical & Computer Engineering, University of Wisconsin-Madison, Madison, Wisconsin Novellus Systems, Tualatin, Oregon Stanford University, Stanford, California ABSTRACT Defects in pristine and ultraviolet-cured low-k organosilicate glass (SiCOH) before and after electron cyclotron resonance plasma exposure were measured using electron-spin resonance. The plasma was parameterized by combinations of pressure and power to obtain a range of ion and photon fluences. The results show that for SiCOH films, ion bombardment increases the defect concentrations more than photon bombardment does. In addition, UV-cured SiCOH films have larger defect concentrations compared with pristine films. The results were confirmed with leakage-current measurements using a mercury probe. 1

2 Defects in low-k organosilicate glass (SiCOH) films have been determined to be the sources of large leakage currents 1,2 and jeopardize the use of these films for interconnects in the manufacturing of microelectronic devices. 3 Recent work using electron-spin resonance (ESR) spectroscopy shows that plasma exposure can strongly affect the defect concentrations in SiCOH. 4 A capillary-array window 5 was used as an ion filter to separate the effects from ion and photon bombardment. 6 However, synergistic effects between ion and photon bombardment may occur during plasma exposure. 7 In order to fully understand the damage mechanisms of ion and photon fluences on the defects in SiCOH films, it is critical to develop a methodology to investigate the plasma exposure with different fluences. Previous work has shown that by varying plasma parameters, charge accumulation and defect-formation mechanisms resulting from both ion and photon fluence can be found for high-k dielectrics. 8 In addition, it was found that the defects in high-k dielectrics, located in the interfacial layers, were modified primarily by the photon flux because the VUV photons, as opposed to the plasma ions, could penetrate to the dielectric-substrate interface. 8 In this Letter, we extend this work to investigate the effects of ion and photon fluences on defects in low-k (SiCOH) films. It is hypothesized that, unlike the defects in high-k dielectric films, defects in SiCOH are located in the bulk dielectric rather than at the interface. This is because in the deposition of SiCOH films, when the carbon doping process, which is used to increase porosity takes place, more silicon bonds are broken. 3 2

3 Thus, the bulk SiCOH dielectric layer is likely to have more defects than the interface. Meanwhile, because the interfacial layers are much thinner than the bulk dielectric, there are more defects in the bulk dielectric layer. As a result, both ion and photon flux can have measurable effects on the defect concentration. In order to verify the hypothesis, pressure and power in an electron cyclotron resonance (ECR) plasma were varied to obtain various combinations of ion and photon flux, as discussed in Reference 8. In addition, in order to obtain the ion energy, a Langmuir probe 9 was used to investigate the plasma potential. From these measurements, with a d.c. bias voltage of -20 V at the wafer chuck, the ion energy was estimated to be about 50 ev. After plasma exposure, the defect concentrations were measured with ESR spectroscopy. This work will show how both ion and photon fluence modify defect concentration in SiCOH. This permits the identification of the nature of the defects and how they change during plasma exposure. To conduct the investigation, SiCOH films were plasma-enhanced-chemical-vapordeposited (PECVD) on two-inch-diameter high-resistivity silicon wafers. After the deposition, the dielectric constant was measured to be 2.75 and the film thickness was 50 nm. In addition, some of the SiCOH films were UV-cured. The UV cure was made at a temperature of 400 with ambient nitrogen. The UV photon energy was between 3-6 ev with a total fluence of approximately photons/cm 2. ECR Plasma exposure 6 was then made on the SiCOH films. Argon plasma was used to minimize the creation of free radicals 10 so that ion and photon bombardment were the primary sources of potential damage. The pressure was set various values between 5 and 3

4 30 mtorr while microwave power was varied from 100 to 400 W. Plasma diagnostics, including a Langmuir probe and a VUV monochromator were used to measure the ion and photon flux. Since fluxes are functions of pressure and power, over these ranges the ion flux varied between 0.4 and ions/(cm 2 s) while the photon flux varied between 0.2 and photons/(cm 2 s). Note that the ion flux is about an order of magnitude lower than that in a typical ECR plasma, 11 due to the fact that the dielectric sample was displaced far from the resonant layer along the axis of the vacuum chamber. After plasma exposure, the defect concentrations were measured with ESR and the results are shown in Figure 1. The calculation method and calibration for absolute defect levels are given in References [4] and [12]. For comparison, Figure 1(a) shows contours of constant defect concentration of interfacial silicon dangling bonds in 20-nm-thick HfO 2 films that were atomic-layer-deposited on a silicon substrate. Figures 1 (b) and (c) show similar defect concentration contours of silicon dangling bonds in pristine and UV-cured SiCOH films. It is seen that the defect concentrations in HfO 2 are two orders of magnitude smaller than those in SiCOH. This difference between the defect concentrations comes from the fact that the defects in HfO 2 are located in the 3-nm-thick interfacial oxide layer 12 while the defects in SiCOH, as will be shown, reside in the 50- nm-thick bulk dielectric. Furthermore, comparing Figure 1(a) with Figure 1(b) and (c), it is seen that the defects in high-k HfO 2 films are modified primarily by photon bombardment, while defect 4

5 concentration in low-k SiCOH films increases more from ion than from photon bombardment. This is explained as follows. For HfO 2, the ion-penetration depth, approximately 5 nm, 13 is less than the film thickness. Thus, the ions were not able to reach and modify the interfacial defects. 14 However, photons with penetration depths that are comparable to the dielectric thickness can reach the interface and modify the defect concentration. For the results shown in Figure 1(a), plasma photons in the UV range (wavelength longer than 200 nm) penetrate through the dielectric layer and decrease the defect concentration by electron repopulation into the defect. 12,15 Thus, the Figure shows that the defect concentration for HfO 2 decreases with increasing photon fluence. On the other hand, for SiCOH, ion bombardment, together with photon bombardment, can modify the defects at the dielectric surface and beyond. Because of the amorphous structure and lower density of SiCOH, 16 the ion penetration depth in SiCOH is likely to be larger than that in HfO 2 so that effects of ion bombardment on defects in SiCOH can be deeper in the bulk dielectric. The comparison between defect modifications in pristine SiCOH (Figure 1 (b)) and UVcured SiCOH (Figure 1 (c)) shows that the defect concentration in UV-cured SiCOH is larger for the same fluence. However, the change in defect concentration is smaller in UV-cured SiCOH. It is believed that the UV curing process, although it introduces 5

6 defects in the dielectric, also enhances the chemical stability of SiCOH in the sense that fewer defects are introduced during any processing that takes place after UV curing. In order to verify the relationship between defects and leakage currents in SiCOH, current-voltage (I-V) characteristics were measured on pristine SiCOH films using a mercury probe. Figure 2 shows the I-V characteristics after plasma exposure. During exposure, some SiCOH films were covered with a capillary-array window so that only photons were incident on the dielectric sample. From Figure 2, it is seen that photon and ion bombardment increase the leakage current. Comparison between the effects of full plasma exposure with photon bombardment alone shows that photon bombardment contributes only a small part of the leakage current increase. Hence, the ion bombardment will be the main source of this damage. The results are consistent with that in Figure 1. That is, as more defects were created, higher leakage currents were observed. In contrast with SiCOH, leakage currents in HfO 2 were found to be much smaller and did not vary significantly with photon or ion exposure. 17 In summary, the changes in defect concentrations after plasma exposure in both HfO 2 and SiCOH (uncured and UV cured) have been measured. For HfO 2, the defects are located at the interface and can be modified mainly by UV photon exposure. For SiCOH, it was found that ion bombardment dominates the changes in defect concentration because even though defects are located in the bulk dielectric, the ions can more easily penetrate into the lower-density SiCOH. Although the UV-curing process introduces more defects in SiCOH, it also reduces the potential for further damage by improving the chemical 6

7 stability of the bulk dielectric. Increases in defect concentration lead to larger leakage currents regardless of the mechanism that produced the defects. This work is supported by the Semiconductor Research Corporation under Contract Number 2008-KJ-1781 and the National Science Foundation under Grant CBET

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10 Figure Captions Figure 1. Contours of the defect concentrations in (a) 20 nm HfO 2, (b) 50 nm pristine SiCOH, and (c) UV-cured SiCOH due to plasma exposure. Figure 2. I-V characteristics for 50 nm pristine SiCOH before and after plasma exposure. 10

11 H. Ren - Figure 1 11

12 H. Ren - Figure 2 12