Effect of vacuum break after the barrier layer ... - Science Direct

Microelectronics Reliability 45 (2005) 1449–1454 www.elsevier.com/locate/microrel

Effect of vacuum break after the barrier layer deposition on the electromigration performance of aluminum based line interconnects Cher Ming Tana, *, Arijit Roya, Kok Tong Tanb, Derek Sim Kwang Yeb, Frankie Lowb a

School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639 798 Failure Analysis and Reliability Section, Systems on Silicon Manufacturing Co. Pte. Ltd., 70 Pasir Ris Industrial Drive 1, Singapore 519 527

b

Abstract Electromigration (EM) experiments have been performed for aluminum (Al) based interconnect of samples with and without air exposure after barrier layer deposition and prior to aluminum deposition. The intention of air exposure is normally performed to improve the efficiency of barrier layer against the Al diffusion. However, the EM performance of such air exposed samples is found to be controversial. It has been found that in the case of highly accelerated tests, the EM life-time decreases and failure rate increases for the samples with air exposure, while these variations has been found to be negligible in the case of moderate accelerated tests. Finite element analyses reveal that high temperature gradient exists in the metallization at highly accelerated test and this gradient enhances the atomic flux divergence due to triple point formed by titanium nitride, titanium oxide and aluminum. Ó 2005 Elsevier Ltd. All rights reserved.

1. Introduction Dramatic improvement in EM life-time is achieved by the incorporation of barrier layer into the interconnect metallization. Barrier layer is used in interconnect metallization for two purposes. Firstly, the barrier layer protects metal diffusion into the surrounding dielectric and secondly, the barrier layer serves as an alternative current path during the void growth in EM. Titanium (Ti) is one of the promising metal used for barrier layer in Al based interconnection. Since TiN and Al are both of fcc lattice structure and of similar lattice constant (e.g., TiN: 0.424 nm, Al:

0.405 nm) [1,2], TiN layer is usually formed on top of Ti layer to prevent Ti and Al interdiffusion. However, oxygen incorporation into the TiN layer is needed to effectively protect this interdiffusion [3]. As a result, chamber vacuum break prior to Al deposition was proposed to create an effective diffusion barrier layer. On the other hand, to improve EM life-time, the grain orientation of TiN layer should be so that Al can grow on it in the preferred orientation normal to the film surface [4,5]. However, the preferred texture quality of Al is found to be difficult to achieve in the presence of TiN as underlayer [6]. Atakov [7] reported that TiN underlayer reduces Al-alloy EM resistance but it improves EM resistance

* Corresponding author. [email protected] Tel: +(65) 6790 4567; Fax: +(65) 6792 0415 0026-2714/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.microrel.2005.07.045

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C.M. Tan et al. / Microelectronics Reliability 45 (2005) 1449–1454

when TiN layer is exposed to air prior to Al deposition. But Avinun et al. [8] studied the detail of the Al texture quality on various types of TiN underlayer, and they observed that upon air exposure, the proportion of the Al grain in the preferred orientation reduces and thus it would expect to have a shorter EM life-time as compared to the non air exposure scheme. Also for Cu based metallization, reduction in EM life-time has been reported due to the vacuum break after nitridation of Ti layer [9]. Thus a conflicting message is being generated about the suitability of vacuum break process. It is the purpose of this work to investigate the impact of vacuum break process on the EM reliability of Al interconnects. EM experiments are performed at two different stress conditions, and different EM behaviors are observed. Finite element analysis is performed to examine the root cause of the different EM behaviors of Al metallization fabricated with and without vacuum break.

Fig. 2. Cross sectional diagram of the structure.

2. Experimental 2.1. Sample fabrication ‘NIST like EM test structure’ samples are fabricated using standard 0.18 ȝm Al/oxidetechnology-node processes. A total of four level metallization is patterned on the wafers. Each level of metallization consists of TiN/ AlCu/ TiN metal stack, and the metallization width is 3 µm and 0.4 µm thick. The cross section of the test structure is shown schematically in Figure 1. Two types of samples are fabricated in this work with their process flow as shown in Figure 2. Sample type II is the samples with vacuum break. 2.2 EM characterization Package level EM test are carried out at two different stress levels, namely levels A and B. Their stress conditions are shown in Table 1. The EM stress levels A and B fall in the regime of highly and moderately accelerated test conditions. A 10% resistance change was used as the failure criteria. 14 sets of EM tests were performed for type I samples and 9 sets for type II samples using test stress level A. The EM test results with test condition A reveal that both the median-time-to-failure (t50) and lognormal ‘sigma’ (ı) decreases significantly for the

Fig. 1. Process flow for two types of samples.

Table 1 EM Stress condition Level

Temperature (in °C)

Current (mA)

A B

200 175

75 30


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type II samples in comparison to type I samples. The test results are shown in Fig. 3 and Fig. 4. On the other hand, test conducted using stress level B shows no different between the two types of samples. 2.3 Failure analysis Extensive physical analysis on stressed and unstressed samples is performed using SEM, TEM and optical microscope. ‘Slit’ like voids in Al are observed for failed type II samples as shown in Fig. 5. As it is suspected that the failure is related to the TiN barrier layer with and without vacuum break, H3PO4 (85%) etch at 75°C was performed to completely remove the Al as confirmed by the EDX. Upon exposure of TiN barrier layer, crack-like lines are observed on TiN barrier layer for the samples exposed to air (both stressed and unstressed) while no such crack lines are observed for samples which were not exposed to air (both stressed and unstressed). The SEM micrographs of the samples with and without air exposure are shown in Fig. 6 and Fig. 7 respectively. Fig. 8 shows the TEM of the crack-like line on the bottom of the TiN layer, revealing clearly that the crack-like line is the results of TiO2 begin etched away during the Al removal.

Fig. 3. t50 variation for samples with and without air exposure.

3. Finite element model (FEM) In order to understand the different EM behaviors for the samples with and without vacuum break at different stress levels, a finite element model is developed. The discritization of the structure for FEM is shown in Fig. 9. The presence of TiO2 lines in the bottom TiN layer due to vacuum break is schematically shown in Fig. 10. The dimensions of the various layers are according to the actual samples, and the typical dimension of TiO2 is 0.28 ȝm ɯ 2 ȝm as measured from the TEM micrographs. The EM stress levels A & B are applied as the boundary conditions and current-temperature coupled field analysis is performed. The localized temperature gradient distributions are obtained for the structure with and without vacuum break as shown in Fig. 11.

Fig. 4. ı variation for the samples with and without air exposure.

4. Results and discussion The EM tests at stress level A confirmed that both the EM life-time and the ‘log normal sigma’ (ı)

Fig. 5. SEM micrograph of failed samples exposed to air.

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decrease for the type II samples fabricated with intentional vacuum break prior to Al deposition as compared to the type I samples fabricated without the vacuum break. Also, the t50’s for all the four levels of metallization for type II samples (i.e. with vacuum break) are found to be similar while shorter t50 is found for the lower level metallization in type I samples. These observations clearly indicate that the physical mechanisms of EM are different for the two types of samples. Due to the presence of TiO2, there exists an inherent weak triple point sites at the interfaces of Al, TiN and TiO2. From FEM analysis it is found that very high temperature gradient (339730°K/m) occur at the triple points under the stress level A. As temperature gradient and the subsequent induced hydrostatic stress gradient are the main driving forces for EM, besides the electron-wind force [10-12], we can see that EM performance at condition A will be significantly affected by the presence of TiO2. In fact, the high temperature gradient coupled with the inherent weak triple points accelerates the failure rate process under stress level A. Hence, a significant decrease in t50 is observed experimentally. Also, since the void nucleation is likely to be at the triple points, the time to failure variation will be less as compared to the case without TiO2 where the void nucleation will be dependent on the defect in the metal line in this case, and this is highly process dependent. Thus, the σ of the EM test for samples with vacuum break decreases under stress level A. On the other hand, the temperature gradient of the type II samples under stress level B is 7 times lesser than that under stress level A. Such a low value of temperature gradient may not be enough to enhance the atomic flux divergence at the triple points, and hence no significant changes in t50 and σ are observed for both types of samples.

Fig. 6. Micrographs of unstressed samples after Al removal (a) with air exposure and (b) without air exposure. Cracks like lines are found for the samples exposed to air.

Fig. 7. Micrographs of stressed samples after Al removal (a) with air exposure and (b) without air exposure. Cracks like lines are found for the samples exposed to air.

Fig. 8. Rough TiN surface upon exposure to air.

5. Conclusion Effect of intentional vacuum break in Al based interconnect fabrication process prior to Al deposition on EM reliability is presented. It is shown that, for samples with vacuum break, the EM life-time and its variation decreases when tested at highly accelerated stress, while negligible variations of these quantities are observed when tested by moderate accelerated stress. Temperature gradient distribution obtained by

Fig. 9. Structure disctritization for FEA.


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finite element model reveals that the observed difference in EM behaviors are likely due to the existence of triple points at the interfaces of Al, TiN and TiO2 and the temperature gradient at the triple

Fig. 10. Schematic representation of the presence of TiO2 for the samples fabricated with vacuum break.

points. It is therefore expected that interconnect EM life-time at normal operating condition will not be affected by the vacuum break. Thus the intentional chamber vacuum break should be preferred in the Al(Cu)/TiN interconnection fabrication.

6. Future work Experimental determination of the activation energy and current density exponent for the two types of samples in the regime of moderate and highly accelerated test will definitely give more insight about the failure mechanisms. These test results can then be used to predict the EM life-time at normal interconnect operating condition via extrapolation, and a clear decision can then be made.

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