D ISCUSSION

Originally the measured dimension error of trench pattern is the largest due to large area exposure. From ADI CD comparison chart of trench patterns, Fig 4-7,Fig 4-15,Fig 4-23,Fig 4-31 and Fig 4-39, the deviation induced by proximity effect is greatly reduced after proximity correction. All of the proposed proximity effect parameters improve the performance of E-beam trench print capability.

For isolated lines, the features are the most important for MOS gates pattern. From ADI CD comparison chart, Fig 4-8,Fig 4-16 ,Fig 4-24 , Fig 4-32 and Fig 4-40, CD error induced by

proximity effect is also reduced after proximity correction. Sub-100 nm features were printed for all proximity correction exposed wafers except copper film. The poor photo-resist adhesion of copper film makes small pattern feature lifting seriously. The lifting of 260 nm isolated line of copper process was shown at Fig 4-47.

Fig 4-47 PR lifting of isolated line of 260 nm on copper film.

For dense lines, a little improvement was seen. But the proximity effect correction is not good enough. This is probably due to the fact that the exposure distribution does not accurately fit a double Gaussian distribution [12]. To better correct the proximity effect, Triple Gaussian proximity correction should be developed [12]. And lower backscattered coefficient could be obtained by using higher accelerating voltage, such as 100KV. And the backscattering range will be large. Thus the proximity effect could be alleviated by averaging effect. On the other way, low accelerating voltage (1~ 2 KV) systems were also used to do nanolithography. To prevent the proximity effect, a thick buffer layer was used under the photo-resist coating. The advantage of this process is that no proximity effect correction is needed [33].

For isolated dots patterns, the measured CDs of small dots (<150 nm) are smaller than the designed CD. This is due to the acid diffusion effect of the photoresist. The photo-resist, NEB, is a chemically amplified resist. The acid will diffuse during post exposure bake. The diffuse phenomenon effect could be check by cross link density [32]. The cross link density of NEB-A4 of 100 nm isolated line of the process is shown at Fig 4-48. The X coordinate is the print bias of the exposed pattern, and the cross link density is normalized to 1 for zero bias. For Fig 4-48, we could see that cross link density of 50 nm bias is about 0.6, so the exposure is the reciprocal of 0.6 and it is about 1.67 times of zero bias exposure. The diffusion phenomena could be check by the slope of cross link density of isolated lines.

-4 0 -2 0 0 2 0 4 0 6 0 8 0 0 .5

0 .6 0 .7 0 .8 0 .9 1 .0 1 .1 1 .2

cross Link density

P rin t b ia s (n m )

C ro s s L in k d e n s ity

Fig 4-48 cross link density chart of 100 nm isolated line of NEB-A4 photoresist.

For dense dots, the proximity is not so serious. 100 nm dots could be printed both for corrected or non-corrected exposure methods. This is due to the counteraction result of diffusion effect and backscattering effect. The diffuse effect tends to reduce the exposure intensity of the pattern while the backscattering effect tends to increase the exposure intensity from adjacent patterns. 100 nm dense dots patterns could be obtained both for corrected and uncorrected exposure.

Chapter 5.

Conclusion and future work

Experimental fitting methods and Monte Carlo simulation method were developed to derive the proximity effects parameters. The parameters derived here can be combined with design patterns to obtain modified exposure data by PROXECCO. The proximity effects will be alleviated.

For Isolated-lines, 80 nm features were successfully exposed at Leica We-Print 200 after the proximity correction for all processes except copper’s. For small features of copper process, the poor photo-resist adhesion made photo-resist patterns lift.

From the result of proximity effect parameters, the forward and backward scattering ranges and the energy ratio between backward and forward scattering from experiment fittings are larger than those from Monte Carlo method. This is due to diffusion effect of chemically amplified photo-resist.

The Monte Carlo simulation can make a quick solution for proximity correction. The result shows comparable with experiment fitting. It can serve for most experiment need.

In the future, wafers with different material film, such as high k, will be developed and tested.

And wafers of multiple layer films, like ONO (Oxide-Nitride-Oxide) structure, will be developed and tested. The resist profile simulator and post exposure effect of chemically amplified resist will be developed.

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Vita

Chen-Hsiang Fang was born in Tainan on July 27, 1964. He received the B.S.

degree in Mechanical Engineering in Chiao Tung University (NCTU) in 1986. He entered the College of Electrical and Computer Engineering of NCTU in February 2005. He received the M.S. degree from NCTU in August 2007.