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"Optical lithography has been used for over 30 years as the preferred method of image formation in the manufacture of silicon devices and other semiconductor components.  Its demise as the premier imaging technology was predicted at about 1 Ám feature size by proponents of alternative imaging technologies and others who underestimated the ability of optical tool manufacturers to improve optical and mechanical system performance to the degree necessary to support the production of increasingly complex devices with ever smaller features."

-C.W.T. Knight, The Future of Manufacturing with Optical Microlithography, Optics and Photonics News, Oct. 1990, p.11

CNF maintains a full compliment of optical lithography tools in order to support the needs of our user base.  These tools include simple 1:1contact lithography tools and complex projection step and repeat (stepper) systems.  These tools have a variety of different features.  Together they provide CNF user with a flexible tools set capable of performing exposures with feature sizes less than 500 nm on substrates ranging from 5 mm pieces to full 200 mm wafers.

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Contact Aligner System Comparison

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 ABM Contact Aligner, Suss MJB4 contact Aligner, and Suss MA6-BA6 Contact Aligner mask aligners.


In standard configuration, all three tools use broadband exposure wavelengths (405 ľ 365 nm).  In addition, the ABM has three sets of dichroic mirrors, which allow selection of a desired wavelength range.  The three ranges are:


 365 - 405 nm

 260 - 240 nm

230 - 210 nm


At shorter wavelengths, the combination of relatively poor sensitivity for most resists and low intensity from the ABM 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 MA6, and at least 50 mm for the MJB4 and ABM.


The ABM can expose wafers up to 200 mm in diameter using 9-inch masks, while the MA6 can expose wafers up to 150 mm in diameter using 7-inch masks, and the MJB4 can expose 100mm wafers using 5-inch masks.


The MJB4 and MA6 can also align to patterns on the backside of substrates.  This is accomplished through the use of either a video alignment system or Infra Red cameras.


Advantages of Contact Lithography

  • 1:1 pattern transfer means field size can be large.  The ABM can expose wafers up to 8 inches in diameter using 9-inch masks, while the 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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Step and Repeat (Stepper) Projection System Comparison

At the CNF we have three steppers for projection lithography.  The following is a table of their performance characteristics:




GCA 6300 5X

GCA AutoStep 200

ASML 300C DUV Stepper





Field Size

15 mm

15 mm



436 nm (g-line)

365 nm (i-line)

  248 nm (DUV)




  0.63 - 0.40





0.9 Ám

1.2 Ám


0.45 Ám

0.65 Ám

  0.25 Ám
  0.18 Ám

Depth of Focus

▒ 2.42 Ám

▒ 0.87 Ám

 ▒ 0.25 Ám 





This is not a fixed number, but depends on your pattern, resist thickness, and wafer topography.  For example, it is easier to print an isolated feature than a grating (think about diffraction).  It is also easier to print small features in a 0.5 Ám thick resist layer than in one 3 Ám thick.  Exposing a feature over a step normally results in a linewidth change, so topology is important.



Because focus is so critical, steppers must use an auto-focus mechanism. A stepper uses the reflection of a beam of light off the substrate to determine the wafer surface location.  It then adjusts the lens-to-wafer distance to achieve the proper focus value.  However, there is a limited range over which the GCA systems can move, so there is a requirement that substrates be within ▒ 200 Ám of the nominal thickness.  There are chucks available for various thickness and diameters.  Please check with staff before planning your project.



The alignment scheme on a stepper is very different than the one discussed for contact lithography.  Put simply, it is not a through the lens system - the user cannot look through the mask and lens at the pattern on the substrate.  It is a system in which all masks are aligned to the optical column, and all substrates are aligned to the optical column, but masks and substrates are not aligned directly to each other.


Masks made for the stepper must be made with special mask alignment marks, called fiducials.  These are what allow you to align the mask to the column.  The first level mask must also include special wafer alignment keys, which match marks built into the stepper alignment microscope.  Subsequent levels of lithography do not need alignment marks, unlike the case for contact lithography.  Although this may seem awkward, it actually produces very good alignment, typically ▒ 0.25 Ám on the GCA tools aligning manually, ▒ 0.12 Ám on the AutoStep 200 using automated alignment, and ▒ 0.045 Ám using the ASML.  There are special considerations in placing your alignment marks, so you should attend training before attempting to design masks for the steppers.


Advantages of Stepper Projection Lithography

  • Resolution superior to the contact lithography with no degradation of mask or resist
  • More tolerant of mask errors since mask image is reduced in size on the substrate.  Almost all masks can be made on the PG.
  • Step and repeat means many exposures per wafer, with the flexibility of computer control.
  • Better alignment accuracy, typically ▒ 0.25 Ám for the 5X GCA stepper, ▒ 0.12 Ám for the GCA Autostep, and ▒ 0.045 Ám using the ASML.


  • Focus requirement means that substrate thickness is limited, as well as wedge error.
  • Field size is limited.
  • More complicated to learn than contact lithography.

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