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Journal of Scientific and Technological Studies, 41(1), 65-72(2007)

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Optimized Process Improvement for Tungsten CMP Throughput Kei-Wei Chen, Ying-Lang Wang and Yungder Juang Department of Material Scientific, National University of Tainan

Abstract Tungsten (W) chemical-mechanical planarization (CMP) characteristics are studied systematically for the optimization of process throughput and integration. By the advance surface analysis of tungsten film and polishing pad, the influence and deviation of surface roughness of tungsten film during CMP upon different hardness of pads are investigated. From the result of experiment, this article attempts to address the key point of non-linear tungsten polishing performance, which is different from linear polishing phenomena of oxide film. In addition, the phenomena provides the hint to fulfill optimized procedure to improve the throughput and cost, that is, multiple steps and pads could be required to resolve the redundant process and promote the throughput. Finally, the marathon test would prove the stability and flexibility with the optimized process. There is over 25 % throughput improvement compared with vendor’s bestknown method. About 30% ~ 50% efficiency of extended pad life and cost of reduced slurry would be enhanced. Details of the developed and optimized theory are demonstrated and appeared to be reproducible on tungsten CMP. Key words: Tungsten chemical-mechanical planarization (WCMP), throughput and combination of multiple pads

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Journal of Scientific and Technological Studies, 41(1), 65-72(2007)

1. Introduction Tungsten chemical-mechanical planarization (WCMP) is rapidly applied in sub-deep micro W plug formation. It is very advantageous for semiconductor process integration by minimizing plug recess and local step heights. Using CMP to remove the tungsten, however, has been shown to produce higher yields through improved defects and superior final plug morphology. To obtain the production-worthy W CMP process, the advanced challenge now lies in achieving a manufacturable process with higher throughput and low cost while maintaining process integrity. However, the practice of WCMP application still remains at an empirical level owing to the numerous process parameters involved and the lack of a systematic methodology and the way for characterizing and optimizing the process. For example, the requirement of high throughput often experiments with high down force and platen rotation speed, and alternative parameters. This result is further complicated with occasional wafer broken and slipped when it comes to optimize the numerous WCMP process parameters in order to broaden the process and throughput latitude. Through the investigation of each parameter, the tungsten removal rate could provide the complex interactive variables as the index. Besides, it does decide throughput and cost, such as longer pad life 1 and lower slurry usage . Optimized removal rate would be approached with design of experiment (DOE) 2 or robust method . However, these optimized removal rate and process are based on the overall WCMP process, which is lack of the sequential analysis and each pad optimization in a multi-platen system. Several key issues must be also induced in mass production, including the PM cycle, throughput, slurry usage, and defects (especially, scratch). The article attempts to analyze the deviation of removal thickness of tungsten film with time increasing and multiple platens polishing. It should be implemented and resolved with these key issues and multiple steps optimization for WCMP process.

2. Experiment Experiments were carried out on an Applied Material Mirra CMP system with a multi-platen and multi-head configuration. There are a sequential two-platen polisher and the following oxide buff step on the third platen. The optical in-situ rate monitor (ISRM) used to control the endpoint and over-polish time on all wafers with the tungsten and barrier (TiN/Ti) stacked films. Various hardness types of pads using in the multi-platen system are compared in the marathon test of 4K Å tungsten blanket wafers. A diamond disk conditioner was employed prior to wafer polish. SEM and atomic force microscopy (AFM) were used to analyze the pads and surface roughness of tungsten film after CMP polish. To demonstrate the thickness of removal tungsten with time increasing, a four-point probe (RS-75) measures the sheet resistance converted to film thickness.

Optimized Process Improvement for Tungsten CMP Throughput

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3. Results and Discussion Non-linear tungsten polishing performance: The characteristic of non-linear removal thickness with polishing time increasing, shown in Fig. 1, is one of critical points to control the deviation of removal rate while tungsten blanket wafer keeps on polishing in the dedicated platen. From experiment data, there does almost not remove any thickness from initial 5K Å tungsten film during the first 25 seconds. Then, most of tungsten thicknesses are polished in the sequential 25 seconds. It is different from the phenomena of oxide polish, which is the linear relationship of removal thickness with time accumulated. To study the phenomenon, SEM and atomic force microscopy (AFM) techniques would be prepared to observe the derivations between thickness and surface roughness. From the results of SEM and AFM, presented in Fig. 2 and 3, we found the initial rough surface of tungsten film makes the lower removal rate. Meanwhile, the degree of surface roughness decreases with polishing time and removal thickness increases. By the phenomena and observation, we figured out the perspective of surface roughness of tungsten film in Fig. 4. On the top surface, it presents the initial rough degree after WCVD deposition. Then, the bottom of film, under rough parts, means the smooth crystal structure after 25 seconds polishing. The initial rough surface suspects to be the main factor of pad support 3 damaged because the hardness of tungsten film is harder than pad material, such as polyurethane . It seems to be like the diamond conditioner to wear the pad life during polishing process. Besides, in the marathon test, we compared the pad life in the two-platen system. Pad life in the first platen is always shorter than the second pad, shown in Fig. 5. That is the evidence to explain the phenomena of shrinking pad life due to tungsten. initial rough surface of tungsten. 5000

100 ThW -Removal THK (5K) ThW -Removal THK (4K ) IC1000-Removal TH K (5K)

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Fig. 1Fig. Time course of of removal thickness with various thickness and 1 Time course removaltungsten tungsten thickness with various initialinitial thickness and pads. pads.

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Journal of Scientific and Technological Studies, 41(1), 65-72(2007)

Fig. 1 Time course of removal tungsten thickness with various initial thickness and pads.

5 sec

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Fig. 2Fig. Time course ofoftungsten roughness by AFM 2 Time course tungsten surface surface roughness by AFM checkcheck As the initial rough layer removed, the following smooth surface contacts to the

As the initial rough layer removed, the following smooth surface contacts to the second pad in our secondHence, pad inthe our system. padIn life would In lack second system. second pad Hence, life wouldthe be second prolonged. second pad, be thatprolonged. is pad support of server mechanical but enhancing chemical polish. If we intend to prolong and optimize pad life in the pad, that iswearing pad support lack ofthe server mechanical wearing but enhancing the chemical first platen, the initial rough surface of tungsten film should be treated with harder pad, and the softer pad is used to polish the smooth parts. Hence, we take advantage of combination of different hardness of pads in order to improve the removal rate and throughput.

polish. If we intend to prolong and optimize pad life in the first platen, the initial rough surface of tungsten film should be treated with harder pad, and the softer pad is used to polish the smooth parts. Hence, we take advantage of combination of different hardness of pads in orderProcess to improve the removal rate andCMP throughput. Optimized Improvement for Tungsten Throughput

5 sec

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Fig. 3 Time course of tungsten surface roughness by SEM check.

Fig. 3 Time course of tungsten surface roughness by SEM check.

Softer pad tretment

Smooth film (Layer II)

Harder pad tretment

Rough surface (Layer I)

tangstem film W film

Substrate 4 The perspective of surface roughness of tungsten film and pad treatment Fig. 4Fig. The perspective of surface roughness of tangsten film and pad treatment

H1 Removal Rate (A/min)

H2 Removal Rate (A/min)

H3 Removal Rate (A/min)

H4 Removal Rate (A/min)

H1 Uniformity(%)

H2 Uniformity(%)

H3 Uniformity(%)

H4 Uniformity(%)

(Thruput = 55 WPH)

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Substrate

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Fig. 4 The perspective of surface roughness of tangsten film and pad treatment Journal of Scientific and Technological Studies, 41(1), 65-72(2007)

H1 Removal Rate (A/min)

H2 Removal Rate (A/min)

H3 Removal Rate (A/min)

H4 Removal Rate (A/min)

H1 Uniformity(%)

H2 Uniformity(%)

H3 Uniformity(%)

H4 Uniformity(%)

(Thruput = 55 WPH)

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Fig. result of soft (WWP3000, Rodel, USA) in platen I and Fig. 55 Marathon Marathon result of soft padspads (WWP3000, Rodel, USA) applied in applied platen I and II of WCMP. II of WCMP.

Combination of different hardness of pads: To improve the removal rate and throughput, the optimization process would be approached to multiple steps and combination of different hardness of pads, which the scheme is shown in Fig. 6. The first platen adapts to harder pads and the second one does the softer. In one hand, the soft pad in the worldwide could not almost use to the ex-situ diamond disk to refresh the pad surface. The surface support of soft pad is refreshed and worn by the polish film. In the other hand, support of hard pad could be hardly damaged during polishing cycle, shown in Fig. 7. Thus, the hard pad should be treated with the initial rough surface of tungsten film, and the sequential smooth layer does with soft pad. In the two-platen system to polish tungsten film, the benefit of combination of different hardness of pads is to have higher throughput than the same hardness of pads applied in such a system. In case, the hard pads applied in two-platen system, the throughput could be lower than system of two soft pads applied without extra ex-situ conditioning time. However, the throughput of combination of different pads can be comparable to the system of two soft pads. Of course, such high throughput must be developed and optimized with multiple polish steps based on the combination of different hardness of pads. Several parameters like slurry flows and down force would be investigated to treat as the initial rough surfaces and promote overall removal rate. For example, the higher down force and harder pad are applied in the initial steps to treat rapidly rough surface of tungsten film. The softer pad and more slurry flow treat the smooth parts. In advance, the DOE or Tauguchi methods can implement the optimization recipe for each platen. That is approached to the optimization of production and process, which is different from the overall optimization with the unique kind of hardness pads.

Optimized Process Improvement for Tungsten CMP Throughput

Platen 3 PlatenBuff 3 Buff

Load Loadcup cup

Platen 2 PlatenPolish 2 Polish

Platen 1 PlatenPolish 1 Polish

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Softer Pad Softer Pad Harder Pad Harder Pad PC 1 condition PC 1

condition

Water tank Water tank

Fig. 6 Scheme of combination of hard and soft pads in WCMP application Fig. 6 Scheme of combination of hard andsoft softpads pads in in WCMP Fig. 6 Scheme of combination of hard and WCMPapplication application

(a) (a)

(b) (b)

(c)

(d) (d)

(d) (c) (d) Fig. 7 SEM comparisons of various new and used pads. (a)new WWP3000 pad, US. Fig. 7 Fig. SEM comparisons of ofvarious new andused used pads. 7 SEM comparisons various newRodel, and pads. (b)used WWP3000 pad, Rodel, US. (a)new(a)new WWP3000 pad,pad, Rodel, WWP3000 Rodel,US. US. (c)new IC1000 pad, Rodel, US. (b)used(b)used WWP3000 pad, Rodel, WWP3000 pad, Rodel,US. US. (d)used IC1000 pad, Rodel, (c)new IC1000 pad, Rodel, US. US. (c)new IC1000 pad, Rodel, US.

(d)used IC1000 pad, Rodel, IC1000 Rodel, US. US. throughput of combination of Observation from our(d)used system and pad, experiment, Observation system andblanket experiment, different padsfrom with our 4K Å tungsten wafer throughput polished canofbecombination implementedofto 52

Observation from our system and experiment, throughput of combination of different pads with 4K Å different pads with 4K Å tungsten blanket wafer canthebelimitation implemented to WPH, which limitation of throughput is 55which WPH, i.e. limitation of 52 Robot tungsten blanket wafer the polished can be implemented to 52 polished WPH, of throughput is 55 WPH, i.e. limitation of Robot transferring wafer. In addition, pad life in the first and second platen WPH, which thewafer. limitation of throughput i.e. limitation of prolong Robot does transferring In addition, pad life in is the55 firstWPH, and second platen does one prolong one to two times compared to original status. The result of marathon test has been presented in transferring wafer. In addition, pad life in the first and second platen does prolong one Fig.8.to two times compared to original status. The result of marathon test has been

to two times in compared presented Fig.8. to original status. The result of marathon test has been

presented Then, in Fig.8. by the optimization of recipe for each platen, pad and slurry usage could Then, by the optimization ofvender’s recipe for each platen, pad and slurry usage could be reduced to the 20~30% of best-known method. be reduced to the 20~30% of vender’s best-known method.

Journal of Scientific and Technological Studies, 41(1), 65-72(2007)

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Then, by the optimization of recipe for each platen, pad and slurry usage could be reduced to the 20~30% of vender’s best-known method. P1H1 Removal Rate (A/min) P2H4 Removal Rate (A/min) P1H3 Uniformity(%)

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P2H2 Removal Rate (A/min) P1H1 Uniformity(%) P2H4 Uniformity(%)

P1H3 Removal Rate (A/min) P2H2 Uniformity(%)

(Thruput = 52 WPH)

Heads rebuilt, no lap(no sand paper), heads break-in in 15 min on previous IC1000, BKM pad break in for Platen 1 only.

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Fig. 8 Result of Marathon test with the method of combination of different pads.

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Fig. 8 Result of Marathon test with the method of combination of different pads.

4. Conclusion

the non-linear tungsten polish performance, we develop the method of combination of different 4.ByConclusion hardness of pads in order to improve the throughput. In multi-platen system, the first platen is set with By the non-linear tungsten polish performance, we develop the method of harder pad to reduce the pad support worn by polishing film. The second platen does attach with the combination of conditioning different hardness of pads in order to polish improve In softer pad to save the time. Besides, the multiple steps recipethe for throughput. each platen should be optimized by the characteristics of tungsten For initial rough surface, is necessary multi-platen system, the first platen ispolishing set withfilm. harder pad to reduce the itpad support to provide higher down force and platen rotation speed to truncate and smooth the film. Then, the more slurry polishing film. second does attach withand thecombination softer padoftodifferent save the flowworn is, thebyhigher removal rate The will be. Theseplaten optimizations of recipes pads could implement the higher throughput, cost down, and stable status in WCMP. conditioning time. Besides, the multiple steps polish recipe for each platen should be

optimized by the characteristics of tungsten polishing film. For initial rough surface, it

5. References

is necessary to provide higher down force and platen rotation speed to truncate and [1] smooth Wijekoon Lin R., Fishkin Yang S., Redeker F., the Amico G., and Nanjangud theK.,film. Then, the B., more slurry flow is, higher removal rate S.(1998). will be. Tungsten These CMP process developed. Solid State Technol., Vol.4, 53-56. optimizations of recipes and combination of different pads could implement the [2] Kim I., Murella K., Schlueter J., Nikkel E., Traut J., and Castleman G..(1996). Optimized Process Developed for Tungsten CMP. Semicond. Int., Vol.11, higher throughput, cost down, and stable status 119-123. in WCMP. [3] Steigerwald J. M., Murarka S. P., and Gutmann R. J. (1997). Chemical Mechanical Planarization of Microelectronic Materials. New York, John Wiley & Sons, Ch6, 192-206.

5. References

Submission Date: 2006/10/18

[1] [1] Kim I., Murella K., Schlueter J., Nikkel E., Traut J., and Revision Date: Castleman 2007/2/1 Date:2007/3/5 G..(1996). Optimized Process Developed for Tungsten Acceptance CMP. Semicond. Int.,

Vol.11, 119-123.