Chapter 1
Polymers in Microlithography An
Overview
Downloaded by UNIV OF NORTH CAROLINA on May 20, 2013 | http://pubs.acs.org Publication Date: October 31, 1989 | doi: 10.1021/bk-1989-0412.ch001
Elsa Reichmanis and Larry F. Thompson AT&T Bell Laboratories, 600 Mountain Avenue, Murray Hill, NJ 07974
The evolution in microelectronics technology has progressed at an astonishing rate during the past decade. This is particularly true of microlithographic technology which is the technology used to generate the high resolution circuit elements characteristic of today's integrated circuits. While almost all of these commercial devices are made by photolithographic techniques that utilize 365-436nm UV radiation, within the next 3-8 years, new lithographic strategies will be required. These technological alternatives, such as deep-UV, e-beam and x-ray lithography, will require new polymeric resist materials and processes. A brief overview of the current trends in microlithography is presented along with an examination of the varied chemistries that can be applied to this technology. The reader is referred to alternate sources for detailed reviews of the field.
The evolution i n microelectronics technology, and microlithography in particular, has progressed at an astonishing rate during the past decade. The speed of integrated circuit devices has increased by several orders o f magnitude, while the cost associated per bit has decreased at a still faster rate (Figure 1). These improvements are a direct result of the increase i n the number of components per chip, a trend that has progressed at a rate of Ι Ο - 10 per decade. It is expected that this trend w i l l continue, although perhaps at a slower rate (1). 2
3
This increase i n circuit density has been made possible by decreasing the minimum feature size on the chip. In the m i d 1970's, the state-of-the-art dynamic random access memory ( D R A M ) device was capable of storing 4000 bits of data and had features 5 μ π ι i n size. Today, 4 megabit D R A M ' s are i n production with minimum features i n the 0.8 - 1.0 μ π ι range, while state-of-the-art devices with 0.6 - 0.8 μ π ι features are in pilot production (2). A s shown i n Figure 2 and Table I, contact printing was the dominant lithographic technology used for device 0097-6156/89/0412-0001$07.00/0 ο 1989 American Chemical Society
In Polymers in Microlithography; Reichmanis, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
Downloaded by UNIV OF NORTH CAROLINA on May 20, 2013 | http://pubs.acs.org Publication Date: October 31, 1989 | doi: 10.1021/bk-1989-0412.ch001
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POLYMERS IN MICROLITHOGRAPHY
1950
1960
1970
1980
1990
2000
YEAR
Figure 1:
Plot o f the power delay product vs. year and cost per bit vs. year for commercially available V L S I devices.
1978
1988
1998
YEAR COMMERCIALIZED
Figure 2:
Graphical representation o f the minimum feature size vs. year of commercialization for M O S devices.
In Polymers in Microlithography; Reichmanis, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
Downloaded by UNIV OF NORTH CAROLINA on May 20, 2013 | http://pubs.acs.org Publication Date: October 31, 1989 | doi: 10.1021/bk-1989-0412.ch001
1.
REICHMANIS & THOMPSON
3
An Overview
production well into the 1970's. Although contact printing is ostensibly a high resolution technology, its utility is limited by mask and wafer defects which begin to affect yield and economics at geometries below about 3μπι. Contact printing thus gave way to one-to-one projection printing which obviated the contact induced defects by separating the mask and wafer, and allowed a reduction i n feature size to about 1.5 μηι. A t that time, this was believed to be the resolution limit for photolithography i n a production environment. The development o f reduction step-and-repeat exposure tools allowed further improvement o f the resolution obtainable by optical techniques. H i g h numerical aperture(NA) steppers operating at the conventinal 405 or 436nm ( H or G ) lines o f the H g arc lamp are used to produce today's state-of-the-art devices and it is generally believed that such tools w i l l be capable o f producing chips with features as small as 0.6 μπι. Further reduction i n feature size w i l l require the introduction o f a new lithographic technology the strategy for which is discussed below. Conventional G-line (436 nm) lithography employing 0.4 N A reduction lenses is currently used in manufacturing to produce today's devices with features o f 0.8 μ π ι . Higher N A G line and I-line (365nm) lenses are available that w i l l push the technology to the sub-0.5 μ π ι regime. Concomitantly, deep-UV (230-260nm) systems are becoming available that effectively compete with the I-line technology. W o r k is progressing on development o f higher N A (0.45) deep-UV systems which should be available by mid-to-late 1990 (3). This technology is expected to have a production resolution capability o f at least 0.4 μπι. Ε - b e a m and x-ray lithography w i l l play an increasingly significant role i n the production of devices by the turn of the century when m i n i m u m features are expected to reach the sub-0.25 μ π ι level (4,5). The technological alternatives to conventional photolithography are largely the same as they were a decade ago, v i z . , deep-UV photolithography, scanning electron-beam and x-ray lithography (1,6). The leading candidate for the production of devices with features perhaps as small as 0.3 μ π ι is deep-UV lithography (Figure 2, Table I ) (2,7). Major advances i n this technology i n the past decade relating to improved quartz lenses and high output light sources have
Table I: Photolithographic Trends Lithographic Technology
Years in Use
Minimum Feature
Contact Printing
1960-1973
5 μπι
1:1 Projection
1973-1982
1.5 μ π ι
5:1 Projection Step and Repeat
1982-present
0.8 μ π ι
Deep/Mid U V Step and Repeat
1988-
0.3 μ π ι
X-ray E-beam
1995-
0.1 μ π ι
In Polymers in Microlithography; Reichmanis, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
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POLYMERS IN MICROLITHOGRAPHY
occurred. Several step and repeat 5x and lOx reduction systems that use excimer laser sources have been designed and/or built (8,9). Systems using refractive optics require a very narrow bandwidth light source (less than 0.001 A ) since it is not practical to correct for chromatic aberrations i n quartz lenses. Laser sources provide such narrow bandwidths with enough intensity to accommodate resists with ~50 m J c m ' sensitivities, enabling a rather wide choice of resist chemistries. W o r k is also being done on 4x reduction systems based on a l l reflective optical systems and wide bandwidth H g arc sources i n the 240 to 260 n m region (10). However, since the intensity o f these sources is less than that of laser sources, more sensitive resists (
