2014-08/09 (test samples)

2014-08/09 (test samples)Edit

Venredi 22/08

Comsol simulations

Idea: Develop a second design with Cg,det=500aF and Crf=100aF and Cg,feedback=20aF. Furthermore Ctot>2fF.

Ideal: Length of island is 5um, gap to all gates is 75nm, single JJ, island thickness=100nm, gates thickness=60nm. Comsol gets the following capacitances:

reality: Small changes in the geometry which should not affect the capacitances too much. The doubel layer evaporation will increase the size of some elements which again was calculated in comsol. The capacitances are finally:

Mechanical stress: As the fabrication for BPC2S6 was mainly irreproducible due to strains in the mask which are likely caused by stress (thermal expansion coefficent might be different for the two resist layers), we tried to simulate the stress in comsol. Basic parameters for comsol: lower PTFE layer with undercut of 500um on each side, 600nm thick, fixed (i.e. does not move), no forces are acting on the bottom resist layer. Top PMMA layer, 150nm thick, forces: boundaries of the cutout which are facing to the east(west) will feel a force of 100N/m^2 to the west(east), the same for boundaries of the cutout which are facing north(south) and will feel a force of 100N/m^2 to the south(north), gravity is accounted for 1/1000 relative to the forces given by the resist. The stress tensor for BPC2S6 looks like this:

Blue means little stress and red means highest stress.

The same simulation was done for the second design with the following result of the stress in the top resist:

Lundi 25/08

BPCT2_31

Idea: First attempt to fabricate the new design.

EBL: Problem zith unstable and too low beam current. Using 30kV and Spot 2 to get around 16 pA. Beam current changes up to 25 percent within a couple of minutes. Using previous settings:
Area step size=40 nm, area dose=300 uC/cm^2 and line step size=10 nm, line dose=1200 pC/cm.

dev: MIBK/IPA=90s, IPA=30s, ODI=15s, MIF726=60s, ODI=60s, Ethanol=15s, N2 dry.

Evap:  Note that direction of double angle evaporation changed. Ar milling (3mA, 505V), 3.0x10^-7 mbar, Al 10nm -22deg, dynamic oxydation 150ubar for 5min, Ti-flash 130nm at 0.5nm/s (after pressure 4.2x10^-8 mbar), Al 60nm +22deg.

Lift-off: 30min in PGRemover, 60 C. Some low energy ultrasonic pulses.

Obs: BPC3_1.zip. The island area did not lift off probably due to resist collapse. Last part of feedback gate is shorted to the other gates.

Mardi 26/08

BPCT2_32

Idea: Remove last part of feedback gate. Try to build island with 1-4 single lines and also increase distane of island to the right hand side gate (i.e. detector gate). Divide detector gate into to parts at the bottom in order to support the freestanding PMMA layer located around the island. Do gold evaporation at two angles.

EBL/dev: Same as for BPC2T_31 above.

Evap:  Ar milling (3mA, 505V), 6.0x10^-7 mbar, Au 30nm -22deg, Au 30nm +22deg.

Lift-off: 30min in PGRemover, 60 C. Some low energy ultrasonic pulses.

Obs: BPC3_2.zip. The best structure was achieved twice and with the right dimension but the island attached to the detector gate:

Mercredi 27/08

BPCT2_33

Idea: Similar to BPCT2_32 but split the detector gate vertically in several parts with a gap of 100-200 nm in order to mechanically support the 5 um long and freestanding PMMA layer around the island.

EBL/dev/Evap/LiftOff: Same as for BPC2T_32 above.

Obs:  The structures on the left hand side of the chip (the "singleJJ"designs in the positionlist) look typically like this:

The other structures(the "squid"/"DC" designs in the positionlist) look typically like this: