durusmail: mems-talk: Double layer SU-8 fabrication problem
Double layer SU-8 fabrication problem
2013-01-12
Bill Chow
2013-01-13
Gareth Jenkins
2013-01-14
Bill Chow
2013-01-15
Gareth Jenkins
2013-01-15
Daniel Figura
2013-01-16
Jocelyn Ng
2013-01-16
Bill Moffat
2013-01-15
Black, Robert
2013-01-17
Gareth Jenkins
2013-01-23
Tinka Li
2013-01-23
Bill Chow
Double layer SU-8 fabrication problem
Gareth Jenkins
2013-01-15
I have only experienced such wrinkles in single layers but I think the
problem is still the same. You do not see wrinkles during the first
softbake because no exposure has happened to induce the crosslinking
reaction.
If the first layer is not fully crosslinked after exposre (either due to
low exposure dose or insufficient PEB), it may continue to crosslink during
the softbake of the second layer and mess things up. Since SU-8 is
negative, it crosslinks from the top of the film down - hence the top
surface of the first layer maybe more crosslinked than the resist
underneath (giving the appearance that it is inbetween the layers).
Another possibility is if you are using an unfiltered UV source for
exposure. <350nm will also preferentially expose the top layer causing
problems. Increasing the dose can help but for best resolution you need to
add an i-line filter.

On 13 January 2013 23:20, Bill Chow  wrote:

> Thanks for your suggestion. I will try it tomorrow.
>
> But the wrinkles exist in between the top and bottom layer. Also, It
> occurs when I soft bake the second layer. That is before the exposure of
> the second layer. I doubt why it happens at that time, but not when I soft
> bake the first layer?
>
> Bill
>
> -----原始郵件----- From: Gareth Jenkins
> Sent: Sunday, January 13, 2013 1:50 PM
> To: General MEMS discussion
> Subject: Re: [mems-talk] Double layer SU-8 fabrication problem
>
>
> By "dry skin" effect, I assume you mean wrinkles. Since they only occur in
> the exposed area, this indicates insufficient crosslinking of the exposed
> resist.
> Either the post-exposure bake is not enough or your exposure dose is
> insufficient (probably the exposure dose is the issue in my experience).
> As for expiry date - 1 month is nothing. As long as it is stored OK it
> should last well beyond the stated date.
>
>
> On 12 January 2013 22:36, Bill Chow  wrote:
>
>  Hi,
>>
>> I am working on a double layer structure with bottom layer 5um and top
>> layer at 15um. I have used SU-8 2002 and SU-8 2010, however they have
>> passed the expiry date for about a month (Would it be a problem?). In the
>> last month, I was successed to make a beautiful sample out without any
>> problem. However, Last week I tried to fabricate one more sample (due to
>> misalignment of the structure in the first sample). I found there is a
>> terrible problem arise. After I fabricated the first layer and spinned the
>> 2nd layer, I put the sample on the 65C hot plate for soft bake. However,
>> the sample shows a “dry skin” like structure between the interface of the
>> first and second layer of SU-8 just after a few seconds on the hotplate. I
>> don’t know why a previously worked process could failed after a few
>> weeks.... I have try to use the same bottle of SU-8 2010 and let the
>> sample
>> to cool down to room temperature before the second layer softbake, however
>> the problem remains. A very interest point is that, only the exposed area
>> have the “dry skin” appearance, for the unexposed area it don’t have any
>> problem. I hope someone could help me in this tough situation. Thanks.
>>
>> First layer 5um:
>> Spin 3” Si wafer with SU-8 2002 at 500rpm for 5s, then 1000rpm for 30s.
>> Softbake the sample  mailto:1min@65C then 3 min@95C (hotplate)
>> Expose it without waiting for 25s with a total of 125mJ/cm^-2
>> Postbake it  without waiting for 1min@65C and 1min@95C (hotplate)
>>
>> Second layer 15um:
>> Spin the second layer without waiting with SU-8 2010 at 500rpm for 5s,
>> then 2000rpm for 30s.
>> Softbake the sample 2min@65C then 5min@95C (hotplate)
>> Expose it without waiting for 45s with a total of 225mJ/cm^-2
>> Postbake it without waiting for 1min@65C and 2mailto:2min@95C (hotplate)
>>
>> Regards,
>> Bill
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> provider of MEMS and Nanotechnology design and fabrication services.
> Visit us at http://www.mems-exchange.org
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