Module Failures on RODs Asish Satpathy For the US CMS tracker group

Failure Modes UCSB: 6 failures FNAL: 5 failures 1 module had a saturated channel on ROD during production exercise Normal pinhole 1 ROD had three modules with saturated channels and one module with high current 1 ROD had a module with a saturated channel FNAL: 5 failures 1 module w/saturated channels on a non-production 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

Investigation of UCSB Modules (by Tony Affolder et. al) In all, visible damage seen under bias return wire bond on 2nd sensor at the edge of metal outside of guard ring (this is point of closest approach between ground and outside metal) Sensor damage always occurs next to blown APV ch. UCSB Module Damage location

Damage to sensor bias wire bond Module # 5850 Module # 6264 UCSB Module No visible damage to APV APV ch# 1 dead Bias current not changed No visible damage to APV APV ch# 768 dead Bias current not changed

Damage to Saturated Ch. APV UCSB Module

Module with high Current Burn mark found on the corner of the second sensor on the metal where one see the numbering Directly under the bias return wire bond Guard Ring Outside Edge Metal UCSB Module

Investigation of FNAL Modules (by Anatoly Ronzhin et.al) Most failed modules have (multiple grouping of) visible blown APV channels (seen around the APV wire bonds of channel) Increase in bias current at 400 V In some cases visible damage on the first or second sensor No visible evidence for damage under bias return wire bond inputs to APV inputs to APV Sensor edge

Height of wire bonds UCSB: 60-90 um height of bias bond above surface metal on n+ implant & ~200 um for channel bond FNAL: 100-200 um height of bias bonds above surface metal on n+ implant and 100-150 um for channel bonds (this values vary a lot over time)

What is happening ? Not seen in ARCS or LT tests. Why in RODs ??? Large transients causing discharge from surface of the sensors over n+ implant to most susceptible wire bond Breakdown of the sensor oxide due to large charge build up We don’t know for sure what is causing that discharge

Height of wire bonding UCSB: 60-90 um height of bias bond above surface metal on n+ implant & ~200 um for channel bond FNAL: 100-200 um height of bias bonds above surface metal on n+ implant and 100-150 um for channel bonds (this values vary a lot over time)

Action Plan for Module Production The US Tracker group agreed to follow these steps in the module production: Continue production of new modules with modified bond parameters to achieve maximum clearance above n+ region UCSB would pull all sensor-to-sensor bias bonds and remake them longer and much higher over n+ region (This yielded a surprise as seen in the next transparencies) FNAL would inspect sensor-to-sensor bias bonds and pull the low bond if less than 200 mm above the n+ region

Action Plan for ROD production All rod test stand HV lines would be equipped with crowbars and current limits would be reduced. Rod production would restart with modules treated as given in the action plan We will monitor for “transients” as witnessed by blown crowbars or damage to modules. If latter occurs, we halt and analyze the issue further. In parallel with rod production, we have studied the rod test stands, and HV supplies in particular for transients (used scope and DVM on bare rod) and saw nothing – although the HV supply did occasionally register spurious very high (e.g. 8kV readings.)

Under close watch Dots (Affolder Dots) under bias wires on otherwise non-problematic (“good”) modules

Further Studies Integrate substantial testing time with new production module from UCSB and FNAL and then pull the bias bonds to look for “Affolder dots”. This could tell us if the discharge is cured or simply moves to whatever bond has lowest clearance We could also study modules with encapsulation on the sensor-to-sensor bonds. If we took modules with old, low clearance bias bonds and encapsulated them, we could determine how much higher the breakdown voltage goes. We may conclude that maximum bond clearance is not enough. In that case, we would need to consider encapsulant or other protective coatings etc. which in any case represents an additional operation.