Chapter 8
Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on April 15, 2018 | https://pubs.acs.org Publication Date: September 1, 1998 | doi: 10.1021/bk-1998-0706.ch008
Chemistry of Ketal Resist System and Its Lithographic Performance 1
2
3
1
1
Wu-Song Huang , Kim Y. Lee , Rao Bantu , Ranee Kwong , Ahmad Katnani , Mahmoud Khojasteh , William Brunsvold , Steven Holmes , Ronald Nunes , Tsuyoshi Shibata , George Orsula , James Cameron , Dominic Yang , and Roger Sinta 1
1,4
1
1
5
5
1
5
5
1
I B M Microelectronics, Hopewell Junction, NY 12533 ETEC System Incorporated, Hayward, CA 94545 Olin Microelectronic Materials, Providence, RI 02914 Toshiba Corporation, Alliance Partner Shipley Company, 455 Forset Street, Marlboro, MA 01752 2
3
4
5
Since the introduction of chemically amplified resist systems to DUV technology, the environmental stability and bake latitudes have been the major concern of this type of chemistry. Ketal resist systems have been very robust towards these issues. The methoxypropene protected polyhydroxystyrene resist is ourfirstinitial work on ketal system. This resist, after optimization, has 0 nm /°C PEB sensitivity; no environmental concern; minimal slimming; 0.263N TMAH developer compatible and 5-6 months storage shelf life at room temperature. This paper will discuss its chemistry and its lithographic performance. Recently, a significant shift from I-line to D e e p - U V lithography has occurred i n the semiconductor industries due to the requirement o f printing 250 n m images or below. This migration has spurred many resist companies to develop a high performance chemically amplified D U V resist system within a very short time, i n order to grip some market share i n its early stage. In this highly competitive resist business, many different chemistry platforms have been investigated and various resist products are developed. Although majority o f the resist compositions have an acid labile protecting group on a polymer backbone, the activation energy o f the protecting group seems to dictate the resist property and lithography process. In general, l o w activation energy systems have better environmental stability and bake latitudes, but also have the tendency o f line width slimming. O n the other hand, high activation energy systems usually have poor environmental stability and bake latitudes, but very few reported literature has shown that there is line width slirnrning. T o achieve the environmental stability i n high activation energy system, an annealing concept has been introduced to a hydroxystyrene-t-butyl acrylate copolymer based resist (6-10). This system requires higher post apply bake ( P A B ) and post exposure bake (PEB) temperatures. Ketal resist system ( K R S ) has an © 1 9 9 8 American Chemical Society Ito et al.; Micro- and Nanopatterning Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1998.
95
96
extremely l o w activation energy, which w i l l deprotect during exposure, therefore is extremely robust towards base contamination (1-5). W e w i l l discuss the chemistry and lithographic performance o f K R S , which contains a base resin o f polyhydroxystyrene protected with methoxypropanyl ( M O P ) i n this chapter.
Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on April 15, 2018 | https://pubs.acs.org Publication Date: September 1, 1998 | doi: 10.1021/bk-1998-0706.ch008
Result and Discussion Synthesis of Ketal Protected Polymer. A l o w cost process to synthesize ketal protected polyvinylphenols (PVP) has been developed. A s shown i n Scheme I, the corresponding alkoxyalkenes were added to a solution o f P V P with catalytic amount of acids that would then produce the corresponding ketal protected P V P . The extent of protection is controlled by the alkoxyalkene loadings. The blocking level may deviate from the loading depending on the moisture content o f the polymer and the reaction solvent. One mole of water w i l l consume two moles o f alkoxyalkene. The amount o f protection can be determined by C13 N M R as shown i n Figure 1. The mole percent protection is calculated by dividing the area integration of peak at 121 ppm to the combination of peaks at 121 ppm and 115 ppm. The protection level can also be qualitatively identified by the two peaks at 1065 cm' and 1130 cm' i n I R spectrum (Figure 2). 1
1
The acid can catalyze the deprotection of the M O P group as shown i n Scheme II. The water reacts with the deprotected cation to form acetone and methanol. Both are very volatile species and w i l l not condense on the lenses o f the stepper. However, the triflic acid generated in most o f our formulations can be a concern and should be replaced for the large production environment despite the minuscule amount o f acid detected in the lab experiments. With insufficient amount of water, the methanol w i l l react with the deprotected cation to form dimethoxypropane as intermediate. In the absence o f water, during thermal deprotection, the deprotected cation regenerates the methoxypropene group which has been proven by trapping and analyzing the gas generated during deprotection by N M R . Manufacture of K R S Resist. Figure 3 shows the typical dissolution rate of the M O P protected P V P ' s which were synthesized using p-toluenesulfonic acid as catalyst. This figure shows a sharp change i n dissolution rate from 0% to 10% and starts to level off around 20% protection. This suggests that to achieve a good lithographic performance, it is desirable to have at least 20% protection. The resist sensitivity for partially T B O C protected P V P system is sensitive to the blocking level. For K R S , the resist is very insensitive to the blocking level o f the polymer(/0). W i t h blocking level of M O P between 10% to 45%, the lithographic dose is essentially the same using 0.14 N tetramethylammonium hydroxide ( T M A H ) developer. When the protecting level becomes too high, wetting the resist surface becomes difficult, and the dose drifts slightly higher. Table I shows the dose to clear (E ) vs. the protection level. The Eo changes with the thickness as shown i n the swing curve (Figure 4) but not with the protection level. Only when it reaches 50% protection, the Eo starts to drift to a higher value. W i t h excess methoxypropene i n the reaction, the highest protection level achieved on P V P i n the lab is 77%, but there was no effort spent to find the highest protection level on P V P . When the protection 0
Ito et al.; Micro- and Nanopatterning Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1998.
Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on April 15, 2018 | https://pubs.acs.org Publication Date: September 1, 1998 | doi: 10.1021/bk-1998-0706.ch008
Alkoxyalkene
acid OH
