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Message ID: 2805     Entry time: Mon Aug 30 14:06:00 2021
Author: aaron 
Type: Notes 
Category: Si fab 
Subject: Cantilever fabrication process 

We're starting to make new cantilevers this week. Here is our process, largely drawn from Zach's thesis, based on the process from the Chao group (D1200849) and the standard techniques of hard mask etching.

Wafer stock

Zach started with a 100 mm (4") undoped <100> Si wafer with 500 um thickness. He reports achieving similar mechanical Qs of the final cantilevers when starting with either SSP or DSP polished wafers, though we may want to investigate this further. To avoid spoiling too many large wafers, I'd like to start by processing our 2" x 280 um wafers that are leftover from the cryo Q experiment, and will inventory our larger wafers to determine what we should order. We also have some 3" wafers we can use to fabricate full-length (7 cm) cantilevers in smaller batches than had we used 4" wafer. 

Process

  1. RCA clean, following the steps in elog 2343 but with the HF etch after the RCA2 bath. 
  2. Deposit a 400 nm thick hard mask of SiNx with PECVD
  3. Pattern the nitride mask with photolithogrphy
    1. Apply 1.5 um of AZ 5214E photoresist evenly on one side of the wafer, using a spinner run at 4000 rpm for 1 minute
    2. Bake the wafer with photoresist on a hot plate at 110 C for 50 seconds
    3. Load the wafer and a photolithography mask into the Suss Microtech MA6/BA6 contact aligner, which exposes the photoresist to UV for ~ 10 seconds
    4. Develop the photoresist by bathing in MF CD-26 for 1 minute, then rinse and dry the wafer
  4. Dry etch the nitride with a pseudo Bosch etch, which uses a plasma of sulfur hexafluoride and octafluorocyclobutane. The process produces a vertical etch profile by passivating the exposed surfaces during the etch. 
    1. Etch for 5 minutes to remove the 400 nm SiNx layer in the exposed region
    2. Then, use a remover solution (chemical not mentioned) to remove the remaining photoresist.
  5. Wet etch through the remaining Si in the exposed region
    1. Submerge the wafer in a 30% KOH solution at 80 C. At this temperature, the etch proceeds at 1 um / min, so it takes ~ 8 hours to etch through a 500 um wafer.
    2. 6 hours into the etch (for a 500 um wafer), remove the wafer from the KOH bath and break the remaining wafer into rectangular pieces along the etched lines
    3. Place the rectangular pieces into a fixture and return them to the KOH bath for the remainder of the etch.
    4. Rinse, dry, and finally etch away the remaining SiNx hard mask by submerging briefly in HF
  6. Finally, passivate the surface by depositing a thin (10s nm) layer of SiNx with PECVD

Zach's procedure calls for thinning the central region of the cantilever, which softens the suspension and improves isolation. I expect we'll want to evaluate our cantilevers before thinning, since the procedure to thin the central region is a bit tricky. When we do thin the cantilevers, we will not terminate the process at step 6 but instead continue with the following:

  1. Deposit another SiNx hard mask
    1. deposit 200-300 nm SiNx on the 'bottom' side of the cantilever with PECVD
    2. Also deposit 200-300 nm SiNx on the 'top' side of the cantilever, but physically obstruct the central region of the cantilever with a sacrificial piece of Si. Covering part of the cantilever ensures SiNx is mostly deposited only on the exposed regions.
  2. Another round of wet etching
    1. Etch in KOH for 4 hours, which leaves about 250 um thick region in the center of the cantilever (versus 500 um on the ends). 
  3. HF etch the remaining SiNx, DI rinse, and dry the cantilevers
    • It's unclear to me whether to passivate with SiNx after the final HF etch, or if the final etch is simply terminated before the bare Si surface can be exposed. Will ask Zach.
    • Following the final HF etch, package the cantilevers in a wafer carrier and seal in a plastic bag purged with dry N2 gas.  

Materials

Materials we need to acquire are in bold

  • RCA clean
    • Chemicals provided by KNI
    • Wafer from our stock
    • PVDF reinforced wafer tweezers and baskets for use with the HF bath
  • PECVD
    • Possibly a better wafer holder, but the standard cassettes will do for now
  • photolithography
    • Possibly need to acquire our own developer, does KNI carry MF CD-26?
    • We found what we think is the appropriate transparency mask in Zach's old materials
  • Etching
    • KNI stock materials only

Equipment

Our training status on the equipment is green for 'full user,' orange for 'supervised user,' or red for 'need initial training.

  • Oxford Instruments System 100 PECVD unit
  • Oxford Instruments System 100 ICP 380 etcher
  • Suss Microtech MA6/BA6 contact aligner
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