抛光参数对铜

内容摘要：钨CMP工艺对生成铜-钨金属层连接塞(plug)至关重要。由于电化学侵蚀效应，钨塞凹陷在钨CMP工艺中是一种常见的现象。因此，如何减少这类缺陷对于提高合格率就显得尤为重要。本文介绍了钨塞凹陷的形成机制，研究了抛光参数对这一缺陷的影响。通过实验，我们发现在使用相同的抛光耗材条件下，增加过抛时间将导致钨塞凹陷数量的上升而增加氧化缓抛的时间则降低钨塞凹陷的数量。

INTRODUCTION

In IC manufacturing, Tungsten is widely used in the connection of device and Cu line, and W-CMP is the critical process of forming CT W plug. Along with the evolution of IC manufacturing process, the diminishing line width is raising the requirement for precision, thus W plug recess defect affecting more on yield. How to reduce W plug recess defect turns to be increasingly important.

W plug recess is typical phenomenon because of the existence of photo assistant Tungsten plug corrosion (PATC) effect. Generally, PATC arose on W plug connecting NMOS. When W plug exposed in electrolyte with light, it caused electron migration and generated electron hole. In W-CMP process, when light irradiating on the wafer surface is strong enough, electron hole pair would be generated in N/P junction depletion region. Electron hole moves to P-well direction, while electron moves to N-well direction. In the built-in field of junction depletion region, relative to N-well, P-well became positive pole, which led anode corrosion on W plug connecting P-well. 

Based on the electrochemistry reaction mechanism of W plug, P/N junction depletion caused the generation of anodic oxidation WO3 film on the surface of W plug, which located at N-well and P-well. Because of the existence of PATC effect, only the W plug connecting NMOS would be corroded, and such corrosion directly reduced the yield of wafer. In fact, according to the understanding of PATC principle, we found that W plug recess cannot be avoided completely. Following study is just on how to reduce the probability of occurrence of W plug recess. 

 

Fig.1 PTCA mechanism

EXPERIMENTAL DETAILS

W-CMP process platform is based on AMAT Reflexion LK multi head and platen polishing system, using Tri-step process to form W plug. As below Picture 2 shows, it is the work-flow chart of forming W plug. The first platen used higher polish rate to remove bulk of Tungsten. The second platen used lower polish rate to remove the Tungsten remained on the wafer surface, while generate W plug and carry certain over polishing, to ensure Tungsten on the wafer surface is cleared without any residue. The third platen used as buff step to recondition dielectric material. Using high polish rate for dielectric material and low polish rate for Tungsten is to ensure the final formed W plug has certain protrusion relative to the surface of dielectric material. Such protrusion of W plug is formed by different polish rate of CMP Slurry for Tungsten and dielectric material. 

Because the PATC effect is brought by the inherent character of device, which made W plug an objective existence. In existing technological system, it cannot be eliminated completely.

 

Fig.2 WCMP process work flow

Therefore, we started with the process parameters. By means of readjusting and optimizing the process parameters, to reduce the amount of W plug recess. According to above mentioned W-CMP process flow, we can see in the third step of the process, polish rate for dielectric material is much higher than for Tungsten. As polish time goes by, because polish rate for dielectric material is higher than for Tungsten, W plug gradually protrude from the surface of dielectric material and formed W plug protrusion. This experiment based on the adjustment of polish time of different steps in process parameters, attempt at different time ratio of over polishing time of the second polish table and adjusted polish time for dielectric material of the third platen, to reduce the amount of W plug recess.

The experiment in turns tested several time combinations: to increase OP time and OX buff time together, to increase OP time while decrease OX buff time, to decrease OP time while increase OX buff time, etc.

RESULTS AND DISCUSSIONS

Under same polish condition, like same pressure, flux, and rpm, for different material, same CMP Slurry caused different polish speed. W-CMP Slurry has different polish selection ratio for Tungsten and dielectric material. This experiment used AMAT LK platform, using Tri-step polish to form CT W plug.

Fig.3 Over polishing time & buff time split

In which, the first step and second step of polish process used same CMP Slurry against Tungsten, while the third step used CMP Slurry against dielectric material. When the second polish process finished, relative to dielectric material on the wafer surface, W plug is in recess condition. Different over polishing time caused different W plug recess. The long over polishing time, the sharp W plug recess was caused. The third polish process used CMP Slurry against dielectric material, which cause higher polish speed for dielectric material and lower polish speed for metal. Different OX buff time has effect on if the W plug protrudes and how much it protruded.

 

a

b

Fig.4 Optimized OP & Buff time results

(a). Before optimized, W recess;

(b). After optimized, W protrusion;

Based on the polish process flow, we found that when increase P3 OX buff time and decrease P2 Over polishing time, W plug became protrusion instead of recess.

 

Fig.5 Defect performance

Based on defect statistics of different test data, we may draw: to increase OP time and OX buff time together plays positive role in reducing W plug recess amount. When OP time and OX buff time decreased together to a certain level, the amount of W plug recess increased rapidly. This is because to ensure no Tungsten residue, P2 OP is necessary. On this premise, polish rate for Tungsten is higher than for dielectric material because of the characteristic of used CMP Slurry. When P2 process finished, there must be W plug recess. However, too short P3 OX buff time cannot finish the adjustment for W plug protrusion, thus W plug cannot protrude from the surface of dielectric material. Therefore, the combination of too short OP time and OX buff time cannot decrease W plug recess amount. As OP time increasing, when P2 process finished, the change trend of W plug recess level slows down. Even if keep increasing OP time, the level of W plug recess would not change any more. The P3 OX buff time relative to OX loss is a linear relation. When P2 process finished and W plug recess level do not increase, while the CMP Slurry used in P3 has higher polish rate for dielectric material than for Tungsten, as P3 OX buff time increasing, W recess after P2 process would gradually protrude from the surface of dielectric material. Therefore, to increase P2 OP time and P3 OX buff time together helps to reduce W plug recess. Next, we tried to decrease P2 OP time and increase P3 OX buff time, and we found it was very effective. Because P2 OP time was decreased, when P2 process finished, W plug recess would not reach the peak, while increase P3 OX buff time directly increased the probability that Tungsten protrude from the surface of dielectric material. Therefore, this experiment condition takes best effect on W plug recess defect improvement.

Fig.6 Range performance

Next, we comparatively analyze the Range data of this experiment. Under different polish time ratio, we measured Full-map THK on polished wafer. Different polish time ratio also effected the Full-map THK range. As P3 OX buff time increasing, the Full-map THK range also shows increasing trend. This is because in this polish platform, the Polishing Pad used in P3 is softer than used in P1 and P2. To use softer Polishing Pad plays a positive role in improving scratch defect. At the same time, the negative effect it brought is the homogeneity is not as good as using harder Polishing Pad. And this trend gradually became larger as polish time increasing.

Fig.7 THK performance

In this experiment, the selectivity of CMP Slurry used in P2 is 1:4 (OX:W). The amount of OX loss under different polish time ratio also can be reflected through the measurement of the overall thickness of wafer. The more polishing time, the more OX loss is.

CONCLUSION

When using Tri-step process, Step 2 and Step 3 are key points affected the generation of W plug recess. In fact, over polishing time of Step 2 is the key point of improving W plug recess. To decrease OP time of Step 2 and increase OX buff time of Step 3 have improving effect on W plug recess. However, too short OP time would cause risk of Tungsten residue, and too much OX buff time would cause the exceeding of dielectric material loss. To summarize, the key point of adjustment and optimization is to find the balance of OP time and OX buff time.

REFERENCES

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