1. Unexpected Failures of Modules on Rods
Claudio Campagnari – UCSB
for the US TOB group
5 December 2005

2. The problem
FNAL
1 module w/saturated channels on a non-production DS rod
1 rod had three modules w/ saturated channels on a non-production rod
1 rod had a module w/ saturated channels on a non-production rod
Groups (~3) of channels in middle of wafer
Always on MRT after SRT test OK
UCSB
1 rod with three modules with saturated channels and one module with high current
1 rod had a module with a saturated channel
Single channels at the edge of wafers
Always at the very first test (SRT)
During tests of several tens of rods, last ~ 2 months

3. Damage on input to APV
Most saturated channels show damage on APV input
Silicon channel itself seems OK
Works when bonded to different APV channel
UCSB mod 6046
FNAL mod 9103

4. Corresponding Damage on Sensor
Damage Location
UCSB Module
Damage is on n+ region, which is at HV, under bias bond, which is at ground
Bias bond is lower than channel bonds
Saturated channel next to sensor damage under bias bond

5. UCSB: more magnification, bias bond moved over to look under
No visible damage to APV
APV channel 1 dead
Bias current not changed
No visible damage to APV
APV channel 768 dead
Bias current not changed

6. FNAL Signal (Si-to-Si) bonds are lower than bias bonds
See damage similar to UCSB under signal bonds corresponding to broken APV channels
Bond shape asymmetric
Top picture: 2nd sensor
where bond lower
Bottom picture: 1st sensor
where bond higher
perhaps passivation flaw (?)

7. Hypothesis HV breakdown between metal over n+ and bonds
At UCSB, breakdown to bias bond, since it is lowest
Then nearest channel "zapped" through capacitive coupling (?)
At FNAL, breakdown to lowest signal bond
Bonds to PA much higher

8. Obvious questions Why haven't we seen this before?
Could we have missed it?
Can we understand where HV breakdown happens?
How high are these bonds?
Why now?
What to do?

9. Could we have missed it?
We tested 1000's of modules on ARCS and in Vienna box
We have never seen this before
There is no way we missed it there
But this problem showed up on rods
Less rod experience, but we have been testing rods on-off for over a year
Although emphasis had been on DAQ debug, we would have most likely noticed if it had been happening

10. How high are the bonds?
UCSB measurements, including some FNAL modules at UCSB....FNAL measurements ~ agree
TOB bond height over n+ region (not loop height)

11. TEC modules built at UCSB
They appear to be a bit less vulnerable than TOB modules built at UCSB
TEC R5 bias
152 
150 
TEC R6 bias
138 
143 
TEC R7 bias
141 
new bias
247 
289 

12. Where does breakdown occur?
Increase HV up to 1000 V on UCSB TOB module. Ramp as fast as possible.
No breakdown
Plug and unplug HV wire at 1000 V
Breakdown (but only on 5th try!)
Intentionally lower the bias bond, ramp HV
Breakdown at 40 height, HV=800 V

13. HV transients? Suspect that breakdown caused by HV transient on rod
PS mixture of CAEN A132 and A332
These supplies go to 6kV
All "accidents" happened on A132
This supply does not have a HV-max limiting screw
Have tried to catch HV spike on PS
No real success
Have seen HV go from 0 to 80V when 1st establishing SW communication
Have seen occasional crazy readings, up to 8kV on front panel and in SW readings
In at least one case we were looking at HV when crazy reading occurred, and the DVM-monitor did not confirm

14. What to do? Plan agreed last week, may be revised this week
Continue module production with modified bond parameters for maximum clearance above n+ region
UCSB pulls all sensor-to-sensor bias bonds and remake them longer and much higher over n+ region.
Rod production at UCSB suspended
FNAL inspects sensor-to-sensor bias bonds and pulls if less than 200 m above the n+ region
All rod test stand HV lines would be equipped with crowbars and current limits would be reduced.
This has been the situation at FNAL for ~ 4 weeks, with no further accidents
Rod production at FNAL continues

15. More Worries No sparks at 400 V  good
Higher bond clearance gives more headroom  good
Worry that electric field could still locally be high enough to cause ionization (corona, partial discharge) even if no sparks.
What happens then over long period?
We are seeing localized damage even in modules that did not spark

16. Damage under UCSB bias bonds
After pulling bias bonds damage seen on all modules on second sensor
At minimum, “dots” seen at the edge of the guard ring and n+ metal directly under bias bond wire
At maximum, lines of metal (?) deposited in the area between the guard ring and n+ metal directly under wire
Beginning of damage seen after 20 min on ARCS test
Guard Ring
Guard Ring
n+ metal
Guard Ring
n+ metal

17. Damage under UCSB signal bonds
Under all channel bonds, passitivation discolored at guard ring, bias ring, DC pad, and bond pad
Channel bond location
Channel bond
location
Guard Ring

18. Damage to Bias Wire Bonds
Found bias return wire bonds pocked in region which would lie above outside edge of bias ring to halfway between guard ring and n+ metal