Hi Kevin,

Well, as far as I know it is fact: cracks propagate under tension -
period. If you are dealing with ductile materials, of which SU-8 is
not, then plastic deformation in the 'wake' of a crack front can
promote progression under compressive loading, as part of varying
stress cycle. In SU-8, treated as a brittle material, residual
stresses cannot be 'relieved' by material deformation/flow, so
instead, when the failure stress (equal to the yield stress in brittle
materials) of the material is exceeded, it cracks.

When dealing with SU-8, one cannot really talk of the "exposure dose"
or the "baking conditions" in isolation. The two are necessary
components of the curing (or cross-linking) process, and reductions in
one component can, to some extent, be offset by increases in another.
Any advice on SU-8 processing needs to consider both baking and
exposure components as a pair; key ingredients to the initiation and
growth of x-links.

If cracks are observed in SU-8 that is apparently 'under'-exposed, the
cracks have still formed under tension. The reason being that the
layer has not achieved maximum/optimal x-linking density to provide
sufficient cohesive strength to resist stress-induced cracking. With
higher x-linking density, the material can achieve higher strength,
but the tensile stresses that promote cracking are also higher. The
trick is to achieve an optimal balance between residual film stress
and cohesive strength to achieve a mechanically sound and crack-free
layer. As Gareth suggests, blanking off regions of a mask that do not
form part of the intended structure is a good way to reduce film
stress. For thick layers, you could end up with a wafer that resembles
a cereal bowl, making subsequent lithographic-driven processes very
frustrating. If the wafer survives the mask aligner, you still have to
deal with the problem of lousy lithographic resolution.

Thanks,

Michael

> ---------- Forwarded message ----------
> Subject: [mems-talk] su8 cracking
>
> Hi, everybody
> I tried to coat a 20um su8 mask on glass. Su8-50 is used in this process.
> After developing, cracks are observed everywhere on su8. These cracks are
> run through the entire su8 layer. It is really wired since I used the same
> protocol and get succeed last year.
>
> Here is the procedure I've followed:
> Plasma cleaning―bakes 20mins @ 200c
> Spin coating
> Soft bake 3mins @65c, 15mins @95c, cool down to RT in 45mins
> Expose 250mj/cm2
> Post expose bake 1mins @65c, 4mins @95c, cool down to RT in 45mins
> Develop
>
> Also I noticed that the su8 which I used was expired last July. Is the
> expiration the explanation for cracking? Or the incorrect exposure dose? Or
> something else?
>
> Thanks
>
> --sonic
>
>
> ---------- Forwarded message ----------
> From: Kevin Paul Nichols 
> To: General MEMS discussion 
> Date: Wed, 20 Feb 2008 09:23:45 +0100
> Subject: Re: [mems-talk] SU-8 Cracking
> Regarding Michael's point below, that cracking in SU-8 is due to
> over-exposure:
>
> While intuitively this makes sense, in practice it's not what I've seen. In
> my experience, a wide exposure matrix, starting well below the optimal dose,
> always shows obvious, macroscopic cracking when *under*exposed. These
> disappear entirely when overexposed, even when highly overexposed (though of
> course, eventually your features eventually disappear as well...)
>
> Anyone else care to weigh in on this?
>
> - Kevin
>
> On 2/19/08 11:43 PM, "Michael Larsson"  wrote:
>
> > Hi Sonic,
> >
> > The cracking indicates your layer is under high tensile stress. I would
> > guess that the through-thickness cracking is the combined effect of
> > over-exposure and/or over-baking (i.e. excess dehydration).
>
>
> *********************************
> Kevin Paul Nichols
> MESA+ Institute for Nanotechnology
> Mesoscale Chemical Systems
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