This is the most straightforward method for exposing a substrate using a mask. Light is directed through the mask and onto the resist-coated substrate, which is held in direct contact or close proximity to the mask.
Typically a Hg arc lams is used as the exposure source. The light from the arc lamp must be collimated (made into coherent plane waves) and filtered to pass only the desired wavelength(s). At the CNF, we current operate an HTG System III HR, EVG 620, and Karl Suss MA6 mask aligners.
In standard configuration, all three tools use broadband exposure wavelengths (405 – 365 nm). In addition, the HTG has four sets of dichroic mirrors, which allow selection of a desired wavelength range. The four ranges are:
365 - 405 nm
260 - 315 nm
235 - 260 nm
220 - 254 nm
At shorter wavelengths, the combination of relatively poor sensitivity for most resists and low intensity from the HTG results in long exposure times. For PMMA (a frequently used e-beam resist), exposures can be 10 minutes or more.
The aligners are equipped for split field viewing of two alignment marks simultaneously. The requirement for simultaneous viewing is that the marks have a separation of at least 25 mm for the HTG, and at least 50 mm for the EV 620 and MA6.
The HTG can expose wafers up to 100 mm in diameter using 5-inch masks, while the EV 620 and MA6 can expose wafers up to 150 mm in diameter using 7-inch masks.
The EV 620 can also align patterns to marks on the backside of substrates. This is accomplished through the use of a video alignment system.
Advantages of Contact Lithography
1:1 pattern transfer means field size can be large. The HTG can expose wafers up to 4 inches in diameter using 5-inch masks, while the EV 620 and MA6 can expose wafers up to 6 inches in diameter using 7-inch masks.
Substrates of various sizes and thicknesses can be used because there are no focus problems to consider.
Substrates that have non-parallel front and backsides (wedge error) can be used because chucks on the aligners can tilt to planarize the sample.
Contact lithography is easier to learn than projection.
Good contact is difficult to achieve because of particulates between mask and substrate, and flatness variations.
As a result of particulate contamination, defects are more numerous than in projection lithography.
Alignment can be time consuming and is not very accurate (especially if the scheme for marks has not been well thought out).
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