Thinning Options

[GARY HUGHES]

There is more than one way to thin a wafer.

For many years Sarnoff has been using a CCD thinning process originally
developed in the late '70s for Si vidicons that does not require
pre-lapping or pre-thinning. While the details of this process are
proprietary, it has several advantages over the thinning techniques used
for most astronomical grade CCDs:

* An electrically stable back surface is created that has excellent blue
and UV response. No flash gate or lumogen coating is required.
* It is a whole-wafer process (currently 4", scalable to 6").
* No etch-stop layer is required. Silicon thickness of 10-12 micron is
achieved routinely over the whole wafer.
* Bonding the thined whole wafer to a transparent substrate allows dicing
and packaging without exotic silicon bond-pad vias.
* Has reasonably low dark current without MPP (typically 200-500 pA/cm^2 at
25C).
* Provides high MTF (low point spread) with thick Si substrates (for high
red/near-IR Q.E.) while still being compatible with MPP operation for low
dark current.
* No warping of completed chips.

At 04:50 AM 11/5/97 -0500, you wrote:
>Mail*Link® SMTP               RE>CCD Question
>
>In a message dated 97-11-04 18:15:35 EST, ninkov at cis.rit.edu writes:
><< 
>
> As I understand it when CCDs are thinned for backside illumination
> purposes the wafers are initially lapped to some intermediate thickness
>
>*****
>Yes, sometimes mechanically and sometimes chemically
>*****
>
> and then etched to the final desired thickness, which depends on the
> wavelength of operation. 
>
>******
>Ideally very near the frontside depletion edge but no more than a couple
>microns from it.
>******
>
> Normally the etch stops (or at least slows
> down significantly) at the interface between the epitaxial and substrate
> silicon. 
>
>*****
>By about a factor of 100 
>*****
>
> After this etching is concluded (and before backside damage
> control is done) 
>
>****
>you mean accumulation  (i.e., surface passivation) right?
>*********
>
>what is the rms surface roughness of the back surface?
>
>****
>This is sometimes called the orange peel (surface under micro-scope looks
>like an orange skin). Less than a micron. . .
>*******
>
> Is that figure larger if you stop at the epi-bulk boundary or if you
> continue even thinner into the epi ?
>
>**** 
>good question . . you can't go much beyond the auto substrate doping region
>(which is several microns thick typcially) or the global flatness degrades
>which is a bigger concern (regions will thin faster than others because of
>eddies). 
>
>As far as the orange peel variance I would think it would be the same.
>However, this question should be directed to Pauline at SITe who has about 15
>years thinning CCDs. Pauline. . .  what do you think?
>
>Jim Janesick
>
>PS. why do you ask this question ..  . are you thinning CCDs?
>
>******
> Thanks ...
> 
> Zoran >>

Regards,
Gary Hughes
Sarnoff Corporation
609/734-3056
609/734-2565 (fax)