1. ROX sensor packaging and mounting scheme for use on dilution STM sample plates
Ben MacLeod
Jan

2. RTD directly mounted to in-situ exchangeable sample plate
Leads thermalized at MC plate
Mixing chamber plate
Common lead
Isolated contact
Isolated lead
Sample plate
ROX RTD
38 AWG Phosphor Bronze leads
(could twin these for pseudo-4-wire measurement but we are running short on wires)
contacts in STM head

3. Overview Problem: Want to measure sample temperature down to ~50mK
Need a very compact package for mounting bare-chip ROX RTD’s to STM sample plates
Don’t want to cool thermometer down to MC plate temperature via its leads since experiment will be at a somewhat higher temperature (which we want to measure)
Proposed Solution : small flat OFHC RTD carrier
can be bolted or glued in place
M3 clearance hole
M2 tapped hole
ROX-102A-BR chip
Dimensions in mm

4. The bare chip (Lakeshore RX-102A-BR)
Wrap-around contacts exposed on top and bottom of chip

5. 1 – bottom contacts removed
Contacts sanded down to bare alumina on one side
(alternative : chip glued to thin sapphire plate)

6. 2 – leads added
40 AWG gold wire leads epoxied to contacts with EPOTEK H20E conductive epoxy

7. 3 – chip epoxied to carrier
Bare chip epoxied to carrier with epotek H77 electrically insulating epoxy

8. 4 – mechanical anchoring to carrier
Leads anchored mechanically to carrier with epotek H77 electrically insulating epoxy

9. 5 – thermal anchoring to carrier
Leads wrapped around carrier and epoxied down all around (~32mm run)
40 AWG kapton coated copper leads
Leads joined with Epotek H20E conductive epoxy

10. 6 – carrier glued to sample plate
Carrier glued down to sample plate with conductive epoxy
(RTD ends up mostly shielded inside copper cavity)
Sample
2 x 40 AWG Cu leads
(get joined to 2 x 38 AWG phosphor bronze leads which go to mixing chamber plate)
-could join to 4 leads
Sample plate

11. Summary of sensor package assembly
The bottom contacts on the bare chip RTD (ROX 102-A-BR) are insulated either by:
Epoxying the chip to a thin sapphire wafer OR
bottom wrap around contacts are carefully sanded off
Small uncoated gold wires (~40 AWG) epoxied with electrically conducting UHV epoxy (Epotek H20E) to contacts on bare chip
Bare chip epoxied to OFHC carrier with electrically-insulating UHV epoxy (epotek H77)
The chip leads are thermally and mechanically anchored to the carrier with H77 insulating epoxy (~5mm linear run of wire)
Kapton coated copper wires (~40 AWG) are wrapped around the chip carrier for additional thermal anchoring (~32mm run)
For use on the sample plate, carrier is glued down with H20E conductive epoxy
For use elsewhere, carrier can be bolted down with a single M3 screw
The leads to the thermometer package (carrier + bare chip) from the thermalized plugs on the MC plate will be long runs of 36 AWG kapton coated phosphor bronze to minimize the cooling of the thermometer via it’s leads

12. Questions/Concerns
How bad an idea is the 2-wire measurement (given that we are going to calibrate these sensors against our MC plate sensor before using them and can compensate for offsets due to lead resistance)?
Do we need to filter the lines to the ROX RTD?
If so, how? At room temperature or at 4k or base temperature? Is the filtering from resistive shielded twisted pairs (fridge wiring) perhaps sufficient?
The scheme of epoxying the bare chip to a thick copper plate is definitely not strain free (but we will be calibrating the thermometer including the strain effects). Is this a bad idea?
Lakeshore seems to say it’s okay as long as the strain due to mounting is repeatable…
Is the thermometer sufficiently well thermalized?
The leads are all thermalized at the MC plate but we could thermalize them again closer to the sample (slightly warmer than base temperature and closer to the sample temperature) if neccesary
Is there anything else we have overlooked /should be worried about ?