Established litho steps

Established litho stepsEdit

step 1: resonators

resonators in Nb

• Oxide layer removal

HF:ODI 1:20, 30"
rinse ODI 30"

• Nb sputtering (SPEC)

Pump at least 3h
vacuum <2.5e-7 mb is good
clean sample: DC plasma 60V, 1' PAr = 1.1 e-2 mb (to check)
clean source (start sputtering without opening shutter): 1' @ 500W (I=, V=)
open shutter: 1.9 - 2nm/s
150nm

• Positive resist :

bake wafer 1' @ 110°C (solvent evaporation)
spin S1813 (batch x, per xx/xx/xxxx) 60" @ 4krpm
bake 2' @ 110°C
Expo 150mJ/cm2 @ 365nm, Vacuum contact
Dev MF319 45" @ 19.0°C, rinse ODI 60" beaker + 30" water tap

• etching of Nb layer

RIE CF4/Ar 20/10, P=50µb, 50W.
for 150nm:
Usually, after 3'15 laser signal begins to drop, @ 4' it is at a minimum
Add 20" (~10%)
NB: should see interferences in SiO2, not on unoxidized wafers

resonators in Al

• Negative resist :

bake wafer 1' @ 110°C (solvent evaporation)
spin AZ5214E (batch x, per xx/xx/xxxx) 60" @ 4krpm
bake 1' @ 110°C
Expo 30mJ/cm2 @ 365nm, Vacuum contact
bake 2'30 @ 120°C (=setpoint) or 2' @ 125°C (not so critical)
Flood expo ~250-300mJ/cm2 @ 365nm
Dev MIF726 50" @ 19.0°C, rinse ODI 60" beaker + 30" water tap

• Residue removal

30" ozone plasma @ 100W, 200mb

• Al evaporation

- Ti pump, 10nm @ 0.2nm/s -> P_sas = 2.4e-8mb
- Al 130nm @ 2nm/s, 10°, planetary 16°/s, P_ev ~ 2.5e-7mb
- Ti 20nm @ 0.5nm/s, 10°, planetary 16°/s, P_ev ~ 2e-8mb
- Au 10nm @ 0.1nm/s, 10°, planetary 16°/s, P_ev ~ 4.6e-8mb

/!\ WARNINGS

- Al is not resisting the subsequent (step 2) lithography step because it is highly etched in opitcal resists developpers ~50nm/min (more specifically, in the PMGI developper, here MIF726). Even 10" exposition spoils the layer, even with 10nm Ag capping.
- Moreover, we need a lower Tc in the ground plane, so that it traps quasiparticles from the box
- Finally, we need to be able to take good contact on the second step

So I found the only way to get things working properly was to used a capped Al layer.
But to lower the Tc by only 200mK, we need a thickness ratio Al/Ag, (or Al/Au or Al/Cu) of 10:1, which tells us the capping layer might not be continuous.
The choice was then to make a trilayer: Al 130nm, Ti 20nm, Au 10nm
Ti protects (and slightly lowers the Tc), Au controls the lowering of Tc and ensures good subsequent contacts.

• Lift-off

warm aceton for as long as needed (typ. 1h) + few seconds US at minimal power 
 to get rid of last small residues

step2: CPB
PMGI / PMMA bilayer

• PMGI/PMMA Bilayer spin

spin TI prime @ 4000rpm 60"
bake 120°C 1'
spin PMGI SF8 (batch #, exp. xx/xx/xxxx) @ 3000rpm 60"
bake hot plate setpoint 175°C, 5', under beaker (measured: 159.6°C, same at different places)
spin PMMA A6 (batch #, exp. xx/xx/xxxx) @ 6000rpm 60"
bake hot plate setpoint 175°C, 15', under beaker
spin UV III (batch #, exp. xx/xx/xxxx) @ 4000rpm 60"
bake hot plate 140°C, 90"

THICKNESS : 613 +/- 15nm for PMGI, 253+/-21nm for PMMA
NB: temperature fluctuations (left hot plate on 'lithographie 2') can be important, excursions down to 156°C have been seen.

NB2: There were PMGI adhesion issues, resulting in very large (few 10th of µm) areas developped in 1', cracks in PMMA mask etc.
These issues are even more drastic on the metla layer, resulting in difference between patterns being only on substrate, and patterns being partly on metal pads.
Using one layer of Ti prime solved statistically the issues on Si, but not on metal pads.
Next time will try 2 layers of Ti prime

• dicing
• remove UVIII in IPA 30"

/!\ PMGI is electrosensitive
PMGI SF8 dev rate in PMGI 101 developper is around 0.7nm/s when baked at 190°C, but increased up to 40nm/s when exposed with e-beam @ 40% of PMMA dose.

/!\ PMGI has adhesion issues on metallic parts.
The symptoms are that the PMGI layer can lift on several 10th of µm in a few seconds (by capillarity of the developer beneath the PMGI layer)
Troubleshooting: bake wafer before spin (5' @ 120°C), add Ti primer before, bake harder (check hot plate temperature, it could be that setpoint differs from actual value).

PMMA / MAA bilayer

• spin

When spin on full wafer:

- spin MAA8.5 EL10 (batch , exp. ) @ 2000rpm for 60"
- bake hot plate setpoint 180°C, 5'
- spin PMMA A6 (batch , exp. ) @ 6000rpm for 60"
- bake hot plate setpoint 180°C, 15'

The thickness was measured with two techniques:

First, we define lines with SEM, develop, cleave, sputter and observe under angle
This gives the following values (statistics on 5 - 6 samples, with 30 lines on each)
Drawback: the resist is modified by the SEM observation, even though it is sputtered, and we make fast images.
THICKNESS :
MAA = 530 +/- 20nm
PMMA = 240 +/- 20nm

Second, we infer the bilayer thickness from double angle displacement (first evaep @ 0°, second evap at 35° for instance). With a statistic over 4 samples, with 4 patterns on each, we obtain:
Drawback: the mask is closing with the first evaporation. The mask might be lifted up or down, faking the displacement.
MAA = 670 +/- 60nm
PMMA =  +/- nm

When spin on single 5x5mm chip:

- spin MAA8.5 EL10 (batch , exp. ) @ 2000rpm for 45" + 10" @ 8000rpm (acc. 4000rpm/s)
- bake hot plate setpoint 180°C, 5'
- spin PMMA A6 (batch , exp. ) @ 5000rpm for 45" + 10" @ 8000rpm (acc. 4000rpm/s)
- bake hot plate setpoint 180°C, 15'

comparable thicknesses, relatively more uniform spin than with standard recipe.
Measured on one sample, at ellipsometer,
MAA = 660nm
PMMA = 320nm @ 4000rpm (no measure @ 5k or 6k)

e-beam exposure of fine patterns
Guidelines

Precise exposure pattern was quite long to optimize.
GUIDELINES:

• Have a sufficient dose for the connections. Standard dose = 300µC/cm² for 30keV

spot 1: dose x 1.4 (in position list)
spot 4: dose x 1.2 (in position list)
spot 7: dose x 1.8 (in position list). This compensates for drift in current along exposure. Do critical patterns first (ie alignement cross, or small arms)

• Have a large overlap between patterns

on x2000 field, overlap is at least 200nm
between x2000 spot 1 and x1000 spot 4, overlap is ~ 1µm
between x1000 spot 4 and x32 spot 7, overlap (tolerance) is 3µm

Note there is significant proximity effect of large connecting areas onto the small patterns (eg the undercuts), over few µms.
There is also sigificant proximity effect when exposing patterns above heavy metal areas (more secondary electrons)

• Expo XL30:

Put BB gold above chip alignement marks
exposition parameters are :

30keV, 300µC/cm2, 1200pC/cm, spot 1, I~ 22pA, WD=17mm, 
field x2000 :  area step size 4nm (dwell time ~ 2.2µs), line step size 1nm  (dwell time ~5.5µs)

bilayer Development

• MAA/PMMA development @19°C:

Development will affect the subsequent resist residues and undercut.
Here undercut is fixed
Although no recipe has been shown to really impede resist residues, we found that rinsing using a mix of ethanol and IPA (1:1) increases the undercut, which might also reduce the residual resist atomic layer on the substrate.

No comparative measurements were done on the undercuts. (TODO)

dev: MIBK - IPA (1:3) 1'
rinse: IPA - ethanol (1:1) 1'
rinse 2: IPA 15"

• PMGI / PMMA development @19°C:

MIBK+IPA (1+3) 1'30
rinse IPA 30"
no dry
rinse ODI 15"
MIF726 30"
rinse ODI 1'
rinse ethanol 15"
very very gently dry N2

Junctions

• junctions evaporation:

Heat Al source 1h @ 160mA during pumping to desorb it
ion mill 500V, 3mA, 2x10" @ +/- 22°
P_chamber for first evap should not be too good (increase the gap of the island). 
One can think of adding O2 during evap.
Al 10nm @ 1nm/s @ +22°
Dynamic oxidation, 150µb (1.1e-1 Torr), 5'
Flash Ti (~75nm @ 0.5nm/s) P_ch < 5e-8 before 2nd evap (decrease the gap of the ground)
Al 60-90nm @ 1nm/s @ -22°

• lift-off:

~ 30' @60°C in remover PG, rinse in ODI (mandatory for PMGI based bilayers)
or 
~ 30' in warm aceton, rinse in IPA (can choose both lift methods for MAA/PMMA)

• Junction post-processing

Junction stabilization