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 punchthrough of the n+ metal damaging the glass has been seen. inputs to APV inputs to APV Sensor edge

What is happening ? Not seen in ARCS or LT tests. Why in RODs ??? Where does the breakdown occur ? Could that be just HV transient on CEAN PS ? (not confirmed by DVM monitor) We don’t know for sure what is causing that discharge

Height of wire bonding TOB bond height over n+ region (not loop height) 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 tentatively 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 This has been the running condition of RODs at FNAL since last four weeks and no further accidents reported. Rod production at UCSB suspended - more studies being done to understand the problem with the modules Rod production at FNAL continues with modules treated as given in Action Plan (previous slide)

Under Close Watch (I) Dots (Affolder Dots) under bias wires on otherwise non-problematic (“good”) UCSB modules

Under Close Watch (II) Lines of metal deposit in the area between the guard ring and n+ metal implant directly under the wire bond (damage seen with good UCSB modules after 20 min of ARCS test !!) n+ metal Guard Ring

Under Close Watch (III) “Passivation / discoloration” at guard ring, bias ring, DC pad and bond pad observed under UCSB channel bonds again with “good” modules Channel bond location

Under Close Watch (IV) Pocked bias return wire bonds in region above n+ metal implant (with good UCSB modules)

Conclusion Serious issue with the TOB modules that we just managed to discover (thanks to the ROD failures) Investigations underway to understand the damage under the wire bonds (simulation study may give us some insight - will be started soon) Bond height could be just one issue with several non-understood issues (?) Feedback from other sub-systems in the Tracker community may help us pin down the problem