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====== Fine tuning the photolithography process ======
===== Learning about photoresist chemistry =====
The best information is on the manufacturer's datasheet (see T:\Physics\Minot Group\Group documents\Manuals\Device Making) and in textbooks. Note that Shipley 1813 is a positive DQN resist with no chemical amplification.

Ethan knows two decent textbooks on the subject:
  *"Fundamentals of Microfabrication" 2nd Ed. by Madou (excellent level of detail)
  *"The science and engineering of microelectronic fabrication" 2nd Ed. by Campbell (good overview)

Jaeger also has a well-known textbook "Introduction to microelectronic fabrication" where all he says "carefully follow the manufacturer's recommendations". All three books are available at the OSU library.



===== Establishing values for bake - expose - develop =====

A standard development time for Shipley 1813 should be about 30 seconds (according to Jack Ross at Microchem). If development happens faster - the process is "out of control". Fast development is either a result excessive exposure time or aggressive developer. We have two developers CD 26 (0.26 normal TMAH) and 351. The CD 26 developer should be used for the double layer photoresit process since the underlayer is not soluble in the 351. The 351 developer is the less aggressive of the two and should be used for a single layer process. The 'normality' of a developer is a measure of how fast it dissolves a standard resist (normality depends on active ingredient concentration and surfactants).

  *"A good normality for developing 1813 resist is 0.26. At higher normality, for example 0.31, development is too fast (normality and develop time are not linear). If you already have a bottle of 0.31 normality, it can be adjusted by diluting with 16% extra DI water (normality scales linear with dilution)."*

The standard bake temperature recommended by Microchem is 115°C.

Matt has tried to find the optimal exposure to go with 115°C bake and 30 second develop using CD 26. Theses tables only apply for Si/SiO2 wafers, the recipe for quartz is different! CD 26 is not recommended for the singe layer process since a 30 sec process generally makes the resist fall apart. 

|115°C bake^10 s in CD 26 develop^30 s develop in CD 26|
^2 s exposure|?|Does not fall apart, but trench is only 1-2u wide at the bottom|
^3 s exposure|Does not fall apart|edges fall apart|
^5 s exposure|Does not fall apart-good trench width|edges fall apart|
//for S1813 on top of Si/SiO2 wafers using the Owen Hall contact aligner//

---

|135°C bake^10 s in CD 26 develop^30 s develop in CD 26|
^2 s exposure|will not develop|Will almost certainly not develop|
^3 s exposure|not fully developed|Just barely fully developed|
^5 s exposure|not fully developed|May appear good, but may be under exposed|

//for S1813 on top of Si/SiO2 wafers using the Owen Hall contact aligner//

We tried higher exposure times on quartz all with 30 sec develop 
^6 s exposure|not fully exposed|
^8 s exposure|trench has top width of 26u and bottom width of 12.03|
^10 s exposure|trench has top width of 33.9 and a bottom width of 14.21|



  *My recommendation for lithography on silicon

Clean the wafer first with acetone, IPA, water

Bake at 115 for 5min to remove any water on the wafer

Spin on S1813 at 4000 rpm

115 bake for 2min

expose 5 sec

Develop for 10 sec with gentle agitation using CD 26

Develop for 40sec 4:1 dilution of water:351- (recommended)

  *My recommendation for double layer lithography on silicon

Clean the wafer first with acetone, IPA, water

Bake at 115 for 5min to remove any water on the wafer

Spin on LOR3B at 2500 rpm (should be able to watch the film change color as it spins, if not something is wrong)

190 bake for 2min

Spin on S1813 at 4000rpm

Bake at 115 for 2min

expose 5 sec

Develop for 20 sec with gentle agitation using CD 26

  *Clean rooms recipe fo HF Etching

Clean the wafer first with acetone, IPA, water

Bake at 115 for 5min to remove any water on the wafer

Spin on S1813 at 4000rpm

Bake at 85 for 2min

expose 4 sec

2nd bake at 115 for 3min ( I do not understand the function of the second bake)

Develop for 10 sec with gentle agitation using CD 26

Develop for 40sec 4:1 dilution of DI:351- (recommended)

Bake at 115 for 5min

Etch using 20:1 buffer (Etch rate ????)





===== Undercuts for good liftoff =====


Microchem sells a range of [[http://www.microchem.com/products/pmgi.htm|PMGI materials]] (polymethylglutarimide) for making sacrificial underlayers (creating an undercut so that metalization is clean). You can choose:
  -Different viscosities (so the polymer spins to different thickness).
  -Different developing rates (varying levels of undercut)

PMGI develops fairly slow, but development becomes faster after exposing to 290 nm light (our contact aligner doesn't produce this wavelength). Faster development is also acheived by adding filler to the PMGI. PMGI + filler is called LOR (lift-off resist)*. LOR-A is about 5 times faster than slow-series PMGI. LOR-B is about 25 times faster. The development rate of LOR can be adjusted from 100% to ~60% by increasing the bake temperature from 170 C to 190 C. 

The Microchem technical sheet for PMGI & LOR suggests
  *LOR-A: 0.35 - 1 micron undercut (0.26 or 0.24 Normality)
  *LOR-B: 0.5 - >1 micron undercut (0.24 Normality)

We purchased LOR-B ($391 per bottle). The microscope cross-sectional image below shows 1300 nm of S1813 on top of 350 nm of LOR-B on top of Si/SiO2 wafer. The undercut came out about right. We were aiming for between 0.5 and 1 microns. The bake 190C for 2min with a 2nd bake of S1813 on top at 115 for 2min. The develop time is 20 sec in CD 26

|{{:lor_undercut_500pix.jpg|}}|Microscope image (electron microscope?) showing a cross-section of photoresist on top of a silicon wafer. Image by Matt Leyden and Tal Sharf. First, LOR was spun onto the chip. Then S1813 was spun on top. Then a stripe pattern was printed via photolithography and development. Then the wafer was cleaved in half so we could see the cross-section. The chip was first imaged using an inspection microscope in the OSU clean room - this gave us a good idea of the undercut. For a higher resolution image, we used the electron microscope.|


*//Info from the the technical support at Microchem.//