Image reversal

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About this Process
Process Details
Equipment	Image reversal
Technology	Lithography
Chemicals Used	Photoresist
Gases Used	NH3

Image reversal is a process to reverse the tone of positive photoresists. Similar to a negative photoresist, areas that are exposed become "protected", while the unexposed areas will be developed away. After exposure, the sample is exposed to ammonia, which reacts with the exposed areas making them impervious to the developer solution. The sample is then flood exposed, which allows the areas that were not exposed initially (and therefore not affected by the ammonia) to be developed. This process is commonly used to create clear field masks but can also be used with most standard photoresists on any sample.

Procedure

The image reversal procedure includes the standard lithography steps with the addition of the ammonia exposure and flood exposure. Please see the section below for specifics with certain photoresists.

1. Photoresist application and softbake
2. Sample exposure
3. Ammonia treatment
4. Flood exposure
5. Photoresist development

Ammonia treatment

The sample is heated and exposed to ammonia using the Image Reversal Oven. Details about how the ammonia works

Flood exposure

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Common applications

Some sort of intro

Mask reversal

1. Standard exposure based on the Heidelberg µPG 501 Mask Maker
2. Run Recipe 2 on the Image Reversal Oven
3. Flood expose
   1. Use MA6/BA6
   2. Flood Exposure setting. Make sure you set it up before loading you wafer
   3. 3s exposure
   4. You do not need to load a mask plate or holder
   5. Load mask onto stage chrome and photoresist side up and hit load, it will cycle through and expose substrate
4. Develop shorter time than normal, typically half the normal time
5. Inspect

SPR 220

Steps:

1. Normal cleaning and HMDS
2. Spin normally for the thickness you want
3. Standard softbake (for 3-5um 90s @ 115C)
4. Standard exposure based on thickness of the resist, substrate reflectivity, and the tool you are using
5. Standard Post Exposure Bake (for 3-5um 90s @ 115C)
6. Run Recipe 2 on the Image Reversal Oven
7. Flood expose
   1. Use MA6/BA6
   2. Flood Exposure setting. Make sure you set it up before loading you wafer
   3. Twice normal exposure for thickness of the resist, substrate reflectivity
   4. You do not need to load a mask plate or holder
   5. Load wafer onto stage and hit load, it will cycle through and expose substrate
8. Standard Post Exposure Bake (for 3-5um 90s @ 115C)
9. Develop shorter time than normal, typically half the normal time
10. Inspect

See also

• Image Reversal Oven
• Optical lithography
• Lithography processing