Hasan Sharifi,
Even though the rated film thickness is 5 um, the SU-8 materials can definitely
stretch to greater film thicknesses, especially considering what you are trying
to do. At lower spin speeds (such as 1000 RPM), it is about 7.5 um. At 500 rpm,
this thickness gets even greater.
In addition, since you are filling a gap, a large amount of material will pour
into that gap during the spinning process. By the end of the spin coat process,
if you choose your parameters correctly, the resist can have the gap completely
filled (at least, prior to the soft bake).
Your biggest problem will show up during the soft bake. SU-8 2005 has a fairly
low solids content (I don't know exactly, but it is definitely less than 50%).
So, if your trench is full prior to soft bake, it will be, at best, about half
full after soft bake.
Since your trench width is 150 um, you actually could get away with a thicker
version of SU-8, which will have a higher solids content. The only thing to
bear in mind is that the film thickness above the surface of your wafer will be
higher, as well.
In either case, you need to be aware of the fact that, when spin coating
substrates with trenches as you describe, there's a pretty big risk that your
coat surface will be non-uniform, and not just because of the thickness shrink
during soft bake. Since your trenches will break up the smooth flow of material
across the surface of the substrate, you may end up with "shadows" on the edge-
ward side of the trenches, where the resist layer is thinner.
One final bit of advice. The glass transition temperature of SU-8 is 55 C. The
soft bake is a two step process at 65 C and 95C. If you hold the bake longer at
95C, the material will reflow somewhat, allowing more material to "pour" from
the wafer surface into the trench, especially with thicker layers of SU-8.
Best Regards,
Chad Brubaker
EV Group invent * innovate * implement
Technology - Tel: (602) 437-9492, Fax: (602)437-9435 e-mail:
[email protected], www.EVGroup.com
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-----Original Message-----
From: [email protected] [mailto:mems-talk-
[email protected]] On Behalf Of [email protected]
Sent: Monday, November 24, 2003 10:17 AM
To: [email protected]
Subject: [mems-talk] SU-8 2005
Hi all,
I want to use SU-8 2005 in order to fill a gap inside of my si wafer. The
dimension of the gap is: L=3mm, W=150 um, and Hieght= 550 um. I have a
condition for 5um thickness from microchem but my thickness is abour 550 um. I
think I should use SU-8 2005 b/c the gap is too samll. I appreciate if any body
has the condition (soft bake times, exposure time , ..) for this high thickness.
Best regards,
Hasan Sharifi
Purdue University
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