Common photolithography problems at SNF
The next few sections explain basic information you need to know about your pattern before you can successfully resolve it in photoresist. Commonly-used terminology is also explained.
The answer to your question, “How long should I expose my resist?” is similar to your parents’ answers to tough questions: “It depends.”
You will need to answer the following questions before arriving at a rough idea of the answer. After finding this rough idea, you must expose several test wafers with your pattern and use a microscope to determine an exposure time that does not under- or overexpose your pattern.
1) How thick is your resist?
2) What is the smallest feature size you need to resolve?
3) How close are your features to one another?
4) What is the substrate your photoresist is on?
5) What type of contact will you use (vacuum, hard contact, soft contact, proximity)?
If your patterns meet the following requirements: 1) your patterns are similar to the weekly staff tests of the exposure machines (a “light field” image of “lines and spaces” on silicon—terminology explained in following sections) and 2) the lamp in the contact aligners is at or near optimal intensity, then the following times are good starting points:
· 1 µm thick 3612: 1.0-1.2 sec., vacuum contact
· 1.6 µm thick 3612: 1.5-1.7 sec., vacuum contact
· 3 µm thick 220-3: 3.0-3.2 sec., vacuum contact
· 7 µm thick 220-7: 3.0 sec. x2 with 10+ sec. wait time, vacuum contact
This section describes the differences between using a vacuum, hard, soft, and proximity contact with the karlsuss or evalign machines.
Vacuum contact means a vacuum is drawn between the mask and the photoresist on your substrate. This provides the “hardest” and closest contact. As a result, your pattern will have the best resolution when using a vacuum contact. The downside is you will have to clean your mask more often and sometimes areas of your photoresist will be removed from your substrate and stuck to your mask.
Hard, soft, and proximity contact types are successively milder on your mask and pattern. You will not have to clean your mask as often, but will not be able to resolve as small a feature size. A 1 or 1.6 µm thick pattern exposed just enough using a vacuum contact will need more time to resolve with a soft or proximity contact.
This section explains the difference between “light field” and “dark field.”
First, you must note that the svgcoat tracks use 5 types of positive resist: 3612,3617m, 220-3, 220-7, and 955-.7. The differences between these are not explained here. Using positive resist means that wherever you are shining light, the resist will get washed away after being developed. A good phrase to remember is that for positive resist, “what shows, goes.”
What we call a “light field” image is one where lots of light reaches your photoresist, or, since we use positive resist, there is little chrome on your mask. This means there is little resist left behind after development. A “dark field” image is one where little light reaches your photoresist, so there is lots of chrome on your mask and lots of resist left behind after development.
Figure 1: "Light field" example (w/ poor 1 µm resolution due to underexposure)
Figure 2: "Dark field" example (w/ poor 2 µm resolution)
Please note that “light” and “dark” are sometimes used differently. If you have a pattern of single lines of photoresist spaced far apart—that is, something referred to as “light field” here if made using positive photoresist, a mask-maker may see your pattern and refer to it as a “data dark” pattern. The mask-maker means he understands you want “dark” chrome where you drew your lines.
“Lines and spaces” simply refers to a test pattern that should produce lines that are as far apart as they are wide. This is an excellent pattern for diagnosing under- or overexposure. For example, one can easily see if lines of resist are fatter than the gaps between them in this pattern—this would be a sign of underexposure in a light-field pattern of lines and spaces. In this same pattern, if the lines are thinner than the spaces between them, this pattern is overexposed.
The lithography staff tests the karlsuss and evalign exposure machines with patterns of lines ranging from 1 µm to 10 µm. Including lines and spaces patterns or arrays of squares or dots at and above your critical feature size is recommended to help you diagnose the proper level of exposure for your process.
Common symptoms of under- and overexposure in a photoresist pattern are described here.
Excess resist where gaps in your pattern of photoresist should be clearly indicate underexposure. A blatant example of this is shown in Figure 3.
Figure 3: Excess resist due to severe underexposure
The image in Figure 3 is an extreme case of underexposure. A more typical image of excess resist left behind due to underexposure is shown in Figure 4.
Figure 4: Excess resist due to underexposure
In Figure 5, two problems of line edge roughness and rounded corners are pointed out.
Figure 5: General problems in a light field image
Line edge roughness can come from several sources. The “WEC” on the contact aligners may not be functioning correctly, you may have particles on your mask or wafer, the wafer and mask may not be coming into contact properly, or you may need a post-exposure bake. The term “WEC” is described in the next paragraph, and the phrase “post-exposure bake” is described in the thereafter.
“WEC” stands for “wedge compensation” and refers to the part of the program when using the contact aligners which automatically levels your wafer and mask to each other. Most of these problems relate to an incorrect gap between the wafer and mask. This can cause standing waves near your patterns during exposure which results in line edge roughness. If you see significant line edge roughness, you should first clean your mask and make sure your resist on your substrate is uniform. You can perform the former at the lithosolv wetbench (see staff for details). The latter is done by viewing your resist under the lamp to the left of the svgcoat tracks. If your problems with line edge roughness persist, please contact lithography staff with your test wafers and the details of your process.
A post-exposure bake is a bake of your wafer after you expose your resist, but before you develop it. In resists called “chemically enhanced photoresists,” there is a “photo-active compound” that needs time and energy to diffuse through and expose your resist. The resists that need a post-exposure bake are described in the svgdev run sheets. In general at SNF, if your resist is thin (≤1.6 µm), it is a resist that needs a post-exposure bake.
Rounded corners can be difficult to interpret. Rounded corners such as those in Figure 5 more likely indicate underexposure. These rounded corners are a little darker and are attached to lines slightly fatter than they should be, meaning there is excess resist around. However, overexposure can create rounded corners as well because the sharper features such as corners in a pattern are the first to be attacked in an overexposed pattern.
An example of pattern “lifting” is shown in Figure 6. “Lifting” refers to when lines or features on your pattern do not seem to “stick” where they are supposed to. The lines circled in Figure 6 have shifted closer together.
Figure 6: “Lifting”
Lifting is a problem when your pattern has adhesion problems to your substrate. Lifting will occur if you did not send your wafers through the Yes oven, which dehydrates your substrate and applies a monolayer of HMDS to promote photoresist adhesion. Lifting may also occur if the surface of your substrate has problems with particulates despite an application of HMDS.
A word of warning: never send a substrate with resist through the Yes oven. Your resist will overheat, bubble, and get all over the walls of the oven. This, of course, this does not help with lifting problems or your co-workers’ high regard of you.
You should apply photoresist to your wafers less than 3 days after you send your wafers through the Yes oven. Waiting less is recommended. The first stage on the svgcoat tracks, which can bake and prime your wafer with HMDS, is not as effective as the Yes oven and we recommend use of the Yes oven for all wafers. If you have lifting problems despite proper use of the Yes oven, you have deeper problems and should contact SNF staff to find a solution.
Assorted tips are collected here.
Large pattern tips
For a pattern with a minimum feature size several microns or larger, purposely overexposing your patterns may save you time in optimizing your exposure time and will not adversely affect your process.
If you are having trouble finding your pattern, start at the lowest magnification and focus on the surface of your wafer. You might need to focus on the edge of your wafer if there are no obvious features in your field of view. Then successively increase your magnification while re-focusing with each objective until you are at the magnification you want.
For thicker resist, you can focus both on the top of the resist and on the surface of the substrate. If you can convince yourself that pattern is thinner on the top of the resist than at the bottom, you are likely seeing overexposure.
Transparency mask tips
When using a transparency mask, 20% overexposure is required to keep the edges of your features sharp. Since the resolution of printers is nowhere near that of the chrome patterns on your contact or stepper mask, you can safely overexpose this sort of pattern without fear of adverse effects to your pattern.