Fabrication

GaAs

Quantum Point Contact Fabrication (Optical + Electronic litography)

Mesa Etching

Start : Substrate 2DEG HEMT 40nm+40nm

NB: if first use after cliving the sample, use 10% HCl solution to eliminate As element

Important : KEEP AWAY FROM UV !

First step in QPC Fab.

I. Spin-coating of Optical resist/photresist (in clean "yellow"room)

1. Clean the substrate in hot Aceton@50°C during 3-5min ( Ultrasonic Bath ), rinse in IPA during 3min.
2. Pre-bake @150°C 2 min on Hot Plate ( outgazing ) .
3. S1805 PR spin-coat (see Documentation), expected thickness ~450nm. Spin@5500rpm (prg: THIB2/ S1805_5500): ramp 1s up to 500rpm, spin for 10s, ramp to 5500 spin during 30sec, then stop without ramp.NB: on small substrates, an apparently excessive amount of resist may be required to reduce edge effects ( a large drop of resin wets the edges while a small one does not ). S1805 PR is extremely liquid and so is easily vacuum-sucked into the spinner, which could be a problem on the long term.
4. Bake@115°C during 2min ( Hot Plate, check temperature, in our clean room usually 120°C is necessary to reach the target ). Cool down.

II. Optical lithography with MJB3 Karl Suss Photomask Aligner (see Documentation) (in clean "yellow"room)

1. Use Standart/Soft contact mode.
2. Align, and bring to contact. ( be careful with small substrates : do not force contact, it will result in breaking the wafer )
3. Expose 6.5sec. ( with a warmed-up lamp dose : 5mJ/s/cm² )

NB: Use antistatic scotch tape to maintain the sample.

III. Development (in clean "yellow" room)

1. Develop 40s in MF319 solution ( shaking the sample ) (see Documentation)
2. Stop reaction/Rinse in DI water 60s
3. Drying with Nitrogen Flow.

If you want to check the aspect, use UV-filtered optical microscope.

IV. Post-Bake (in clean "yellow"room)

A short post-bake of 2min at 90°C is used to hardened even more the resin layer.

V. Acid Attack/ Mesa-etching (in clean room)

1. Prepare workspace and Piranha solution according to safety instructions in clean room. Use proportion 1/8/100 H2SO4/H2O2/H20
2. Keep in the Piranha solution during 25s. Expected Etched rate : 6 nm/s.
3. Stop/Rinse in DI water. 1min
4. Remove photoresist in Aceton 5min. Rinse with IPA 3min. Blow Dry.
5. Clean the workspace according to safety instructions in clean room.

Result : 130nm.

Ohmic Contacts

Previous : Mesa-etching

Start : Substrate 2DEG HEMT 40nm+40nm, mesa already etched ( step 2/4 )

Important : KEEP AWAY FROM UV !Bilayer resist deposit is necessary to get a correct lift-off. LOR stands for Lift-off resist.

I. Spin-coating of Optical resist/photresist. (in clean "yellow"room)

1. Clean the substrate in hot Aceton@50°C during 3min ( Ultrasonic Bath ), rinse in IPA during 3min.
2. Pre-bake @150°C 2 min on Hot Plate ( outgazing ) .
3. LOR 20B spin-coat (see Documentation). Spin@5500rpm (prg: THIB3/ LOR20B_5500): ramp 1s up to 500rpm, spin for 5s, ramp to 5500rpm, spin during 50sec, then stop without ramp. NB: on small substrates, an apparently excessive amount of resist seems required to reduce edge effects. Also, a manual capillarity-dragging towards edges and corners may be necessary with thick resist like LOR20B. Otherwise, bumps of resist still exist even after high-speed spinning.
4. Clean carefully the resist on the substrate back and edges ( LOR20B is so thick that a lot of resist remains attached to the edges of the sample ). NMP is necessary to remove LOR20B. Slowly rub the resist away. Use a cotton swab, but wring, squeeze or drain it before you attempt to touch the sample : drops of NMP can easily degrade the coating. The same logic holds for any pair Remover/Resist.
5. Bake@150°C during 8min ( Hot Plate, check temperature usually 155/160°C may be nescessary). Cool down.
6. S1805 PR spin-coat (see Documentation), expected thickness ~500nm. Spin@5500rpm (prg: THIB4/ S1805_3000): ramp 1s up to 500rpm, spin for 5s, ramp to 5000rpm, spin during 30sec, then stop without ramp.
7. Bake@115°C during 2min ( Hot Plate, check temperature ). Cool down.

II. Optical lithography with MJB3 Karl Suss Photomask Aligner (in clean "yellow"room).

1. Use Standart/Soft contact mode.
2. Align, and bring to contact.

* Expose 6.5sec. ( with a warmed-up lamp dose : 5mJ/s/cm² ) (Why dose identical to monolayer S1805 ?)

NB: Use antistatic scotch tape to maintain the sample. Use some scrap from the same wafer to create a tripod ( stability ). Make sure the sample and the Cr side of the mask lay face-to-face. Make sure your sample only experience a reasonable mechanical constraint while maintained contacted to the mask. If you untight the mask, it should not be lifted up by the sample plate more than a fraction of a mm. Otherwise, your sample will break.

III. Development (in clean "yellow" room)

1. Develop 35s in MF319 solution ( shaking the sample ) (see Documentation)
2. Stop reaction/Rinse in DI water 60s
3. Drying with Nitrogen Flow ( long, water do not evaporate that easily ).

If you want to check the aspect, use UV-filtered optical microscope.

IV. Metalisation in Joule Evaporator (in room 110)

AuGeNI content = 84/12/4% ( Weigth )

Thermal deposition 120nm ( if possible at stable rate 0.5nm/s, but do not wait too long for this rate to stabilise, or the metal will be all evaporated )

Ohmic Contacts

If source is empty for too long, we have oberved a red color on the surface of the contacts

V. Lift-Off (in clean room)

Use NMP1165 remover @45°C during 20/30min.

VI. Annealing (in "caractérisation" room)

In furnace "Jipelec", under flow of N2/H2 mixture. Check the water and pressurized air supply.

Recipe details ( to be reviewed, but yodchay's way works fine ) : …etc … two cycles at 450°C… etc…

Ohmic Contacts

Optical picture of an ohmic contact after annealing

Final Check : Measure resistance under probe station. At room temperature and cooled down at 77K with liquid nitrogen.

NB : Use polystyrene cup/dish.

Achieved : 48O Ohms, i. e. at 77K. Calculation provides a 2DEG square resitance of : . Hence we can expect a resistance of 150Ohm per contact.

Markers and circuit pattern

PREVIOUS : Ohmic Contacts

Start : Substrate 2DEG HEMT 40nm+40nm ( step 3/4 )

Important : KEEP AWAY FROM UV !Bilayer resist deposit is necessary to get a correct lift-off.

I. Spin-coating of Optical resist/photoresist. (in clean "yellow"room)

NB : identical to Ohmic-contacts deposit.

1. Clean the substrate in hot Aceton@50°C during 3-5min ( Ultrasonic Bath ), rinse in IPA during 3min.
2. Pre-bake @150°C 2 min on Hot Plate ( outgazing ).
3. LOR 20B spin-coat (see Documentation) Spin@5500rpm (prg: THIB3/ LOR20B_5500): ramp 1s up to 500rpm, spin for 5s, ramp to 5500rpm, spin during 50sec, then stop without ramp. NB: on small substrates, an important amount of resist may be required to wet the edges and reduce their effects. Also, a manual capillarity-dragging towards with edges may be necessary, especially with thick resist.
4. Bake@150°C during 8min ( Hot Plate, check temperature ). Cool down.
5. S1805 PR spin-coat (see _S1800_G2_Serie.pdf S1805 PR doc), expected thickness ~400nm . Spin@5500rpm (prg: THIB4/ S1805_3000): ramp 1s up to 500rpm, spin for 5s, ramp to 5000rpm, spin during 30sec, then stop without ramp.
6. Bake@115°C during 2min ( Hot Plate, check temperature ). Cool down.

II. Optical lithography with MJB3 Karl Suss Photomask Aligner (see Documentation) (in clean "yellow"room).

1. Use Standard/Soft contact mode ( = vacuum aspiration during both "aligment" and "exposure" phase ).
2. Align, and bring to contact.

* Expose 6.5sec. ( with a warmed-up lamp dose : 5mJ/s/cm² ). Dose is identical to monolayer S1805, it should be optimised.

NB: Use antistatic scotch tape to maintain the sample, and two extra pieces from scrap wafer to form a tripod with your sample (stability). Ensure your sample does not suffer from excessive mechanical constraints when brought to contact ( it could break ).

III. Development (in clean "yellow" room)

1. Develop 35s in MF319 solution ( shaking the sample ) (see Documentation)
2. Stop reaction/Rinse in DI water 60s
3. Drying with Nitrogen Flow.

If you want to check the aspect, use UV-filtered optical microscope.

IV. Metalisation in Plassys E-gun 550S evaporator (in "small" clean room)

(see Documentation)

* Ti 5nm@ 0.2nm/s
* Au 95nm@ 0.5nm/s

V. Loft-Off (in clean room)

Use NMP1165 remover @50°C during 15/20min.

Optical litography Test

Optical microscope picture of the test lines (magnification *100)

Optical litography Mask

Comparison with Optical microscope picture of the mask (magnification *100)

Optical litography Test

Optical microscope picture of the markers (magnification *100)

Optical litography Test

Comparison with Optical microscope picture of the markers (magnification *100)

Markers/QPC Grid E-beam exposure

PREVIOUS : Markers/circuit pattern deposit

Start : Substrate 2DEG HEMT 40nm ( step 4/4 )

I. Spin-coat of E-Beam resist. [in clean room]

1. Clean the substrate in hot Aceton@50°C during 3min ( Ultrasonic Bath ), rinse in IPA during 3min.
2. Pre-bake @180°C 3-5min on Hot Plate ( outgazing ) .
3. PMMA-A6 spin-coat (see PMMA doc), expected thickness ~400nm (to be checked !!! ) :

a. Spin@5000rpm (prg: TIBO/PMMA-5000): no ramp, spin during 60sec, then stop without ramp.

NB: on small substrates, an apparently excessive amount of PMMA-A6 seems required to reduce edge effects

b. Bake@180°C during 2min ( Hot Plate )

II. E-beam lithography with SEM Philips XL30S. (See documentation)

See also : E-beam Lithography for a step-by-step description.

Markers dose 400µC/cm² ( mag. x32 , spot 7, usually 4nA )

Grid dose 500 µC/cm² ( mag x500, spot 1, usually 0.023nA ) ( under revision )

III. Developpement in diluted MIBK (MIBK:IPA=1:3). (in clean room)

1. Keep in the MIBK solution during 130s
2. Rinse in IPA. 1min

Split Gates

Optical microscope picture of the split gates after development (magnification *100)

IV. Metal deposit/evaporation in MEB 550S e-Gun PLASSYS. (See documentation) (in grey room)

1. Put the substrate into laodlock. Check safety maintaining tips. Pump till pressure <1E-6mBar is reached inside loadlock chamber. If pressure is <2e-8mbar inside High-vacuum chamber, final pressure should be in the deisred range <4e-7mbar.
2. Evaporate 5nm of mn2.0|iT#mn2.0|iT/sec rate (adhesion layer )
3. Evaporate 55nm of mn5.0|uA#mn5.0|uA/sec rate ( to be increased : visibility ? )

For split gates evaporate Ti: 5nm Au: 95nm (Yodachai says that thick split gates could induce hysteresis. He evaporates 5nm+35nm)

(rec: Ti_5nm_Au_55nm) in directory

NB. DO not wait between evaporation and lift-off !!!!

V. Lift-off (in clean room)

1. Keep in warm Aceton>30min. @45°C NB. Use careful enforcement if necessary to end the process.

E-Beam Litography

I. Loading the sample:

1. Check out the absence of resist left on the rear side of the sample
2. Maintain the sample with both metallic wire on the sample holder, to get a focus tuning homogenous on the whole sample.

II. Focus Tuning

1. The focus and astigmatism tunings are realized on the focus area (22/06/11: go first to the cup stage, and then move manually to the focus area, a gold sputter on the left)
2. To tune the astigmatism, the best is to find out a rounded dirt.
3. Focus tunings: Spot 1, 25.0kV
4. Once a dirt is focused, use a smaller window with an Averaged 4 (Filter) and tune the focus and astigmatism. A Brightness around 18 and a Contrast in the range 65-70 are intresting to see both gold dirts and background details. Focus is tuned with the right button of the mouse. For the astigmatism press Shift simulatneously. An idea of the Focus reference is given in the attached pictures below.
5. Move to the cup area and measure the beam current (typically 0.023nA for spot1)
6. Move to the sample

Focus Tuning

Example 1 of a good focus

Focus Tuning

Example 2 of a good focus

II. Sample exposure

1. Open Elphy Quantum (User: thibaut Password: brestnews). Load the .gds file in GDIIDatabase
2. Design mode: choose the edit mode to modify the design. Once modification done, choose the view mode
3. Microscope control mode: choose the writing field. Magnitude 500 for 20nm precision. Magnitude 32 for bigger structures.

Adjustments mode:

1. Move to the right left corner of the sample. On stage control, shift x by a given distance. The rough alignment is realized with "Align X", "Align Y" in the Microscope control software. For a more precise alignment pick "Align Feature". Check alignment by moving the stage from Quantum Elphy.
2. Realize a new focus on one side (be careful to be still on the surface of the resist ! Pick a dirt on the surface of the resist for example) of the sample. The astigmastism does not have to be modified. Press Z<—>FWD. On the Asga surface, a very low Brightness ( under 4 ) and a high contrast migth be necessary to see the defects and perform focus tuning. An idea of the Focus reference on the resist is given in the attached pictures (Dose3.TIF file)
3. Adjust UVW (global). Read XY position as mark 1. Then Enable adjust mark 1 (tick P1). Then Press Adjust. Do the same for P2 and P3. On each marker, adujst once again the focus and press "control s" on the microscope control software.
4. Check that the working area has the same size as the writing field (given in the microscope control Mode)

Exposure mode:

1. Select layer to expose.
2. Check the "area dose" in "exposure parameter calculation".
3. Move to absolute values (U,V,W)=(0,0,0), coordinates which normally corresponds to the center of the pattern. Unblanck beam on the "microscope control" software. Choose "Slow Scan 1" in "Scan".
4. Start exposure. (Press OK on the message "Switch Beam to OFF")
5. Be careful : the spot might be unblanked at the end of the process ! It will results in an overexposure at the center of the working area !

Silicium / Graphene