Hi,
   I happened across this interesting lecture from Dr. Nguyen of UC Berkley
on thermal circuits and he touches on micro-hotplate design near the end.
I suspect Lecture 5 will also be helpful.
http://www.youtube.com/watch?v=1zZV5PsexKc .

Heres a nice example of a metal oxide sensor that is reaching 480C:

 Detection of SO2 and H2S in CO2 stream by means of WO3-based
micro-hotplate sensors
*Sensors and Actuators B: Chemical*, *Volume 102, Issue 2*, *13 September
2004*, *Pages 219-225*
M. Stankova, X. Vilanova, J. Calderer, E. Llobet, P. Ivanov, I. Gràcia, C.
Cané, X. Correi

I suspect that they could have reached a higher temperature by improving
the thermal isolation of the active area.  Here's a second one that reaches
about 600C using MOSFET heaters fabricated with SOI's:

Novel design and characterisation of SOI CMOS micro-hotplates for high
temperature gas
sensors
*Sensors and Actuators B: Chemical*, *Volume 127, Issue 1*, *20 October 2007
*, *Pages 260-266*
P.K. Guha, S.Z. Ali, C.C.C. Lee, F. Udrea, W.I. Milne, T. Iwaki, J.A.
Covington, J.W. Gardne

In general some important considerations for hot plate engineering include:
1. Thermal isolation of the hotplate area.  Usually this means suspending
the heaters on a dielectric membrane as in the above two papers.
Personally we've found it helpful to put holes in this membrane between the
heater and the support frame of the chip (unlike in the two papers above).
The higher the operating temperature the larger this gap should be.  For
example, isolate a hotplate with dimensions of 500um x 500um by a gap of
1-2mm or more from the supporting chip frame.

2. It is important to consider stress due to thermal expansion of the
materials and inherent film stress as deposited.  We once made beautiful
silicon nitride hotplates from "low stress" LPCVD nitride only to have them
break every time they were released from the residual tensile forces.
Holes in the released membrane also help here and can give the stresses
somewhere to go.

3. Thermal mass.  Make heaters and the active area as thin as possible.

4. Electromigration and peak current.  Many times the peak temperature (and
the heating rate) is governed by the peak current one can put through the
heater which becomes a function of maximum current density.  For very high
temperature hotplates you might consider thick tungsten (>.5um) as a heater
material, this is also convenient since the residual film stress from
sputtered Ti 10%:W 90% can be tuned as a function of deposition
parameters.

5. Design. Watch out for sharp corners in the heater meander, these will
heat more.

Hope this helps,
Michael


On Sat, Jan 26, 2013 at 1:16 PM, hamideh jafarpoor <
hamide.jafarpoor@gmail.com> wrote:

> Dear all,
>
> Hello,
>
>  I am also interested in this topic. I need to fabricate micro heaters as
> hot_spots (low temperature heater) but I could not find a coherent source
> about the design parameters, possible materials and technologies and
> the necessary characterizations. I appreciate your help in finding
> references.
>
> Best,
> Hamideh
>
> On Fri, Jan 18, 2013 at 3:43 PM, Michael Martin <
> michael.martin@themicrowerks.com> wrote:
>
> > Hi Andrea,
> >     Much of your design will depend on the temperatures you would like to
> > reach.  What type of sensor will this be, e.g. metal oxide,
> > chemo-capacitor, chemo-resistor?  Hot plates on the micro-scale can be
> > tricky to properly thermally isolate so that a uniform target temperature
> > is reached with a reasonable power budget and time constant.  For low
> > temperature hotplates (<325 deg C) we have had good luck with low stress
> > polyimides as the heater support material though for higher temperatures
> > you will need a material like stress compensated silicon nitride (or
> > carbide).  I can provide references depending on which variety you are
> > interested in.
> >
> > Cheers,
> >    Michael
> >
> >
> > On Thu, Jan 17, 2013 at 1:39 PM, Andrea Mazzolari  > >wrote:
> >
> > > Good morning,
> > > i'm interested to learn about fabrication processes of micro hot
> plates,
> > > typically used in gas sensing.
> > > Any advice on the most advances silicon micromachining processes and
> > > technology used for micro hot plates fabrication ?
> > >
> > > Best regards,
> > > Andrea
> > >
> > >
> > >
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