BCB

BCBEdit

1.   Deposit:

primer on metallic substrate: AP3000 (special for gold)

spin 5” @ 1000rpm (rise time 10”)
spin ~30” @ 6000rpm (rise time 5”)     ~10 nm layer

The primer layer must be dried (on hot plate 1’ @ ~100°C) and wafer must be cooled down prior to BCB spin.

resist: Bysbenzo Cyclo Butène

dispense 3 mL for 3 inches wafer

Dispense at constant rate in the center of the wafer, and do not empty the container to avoid bubbles. The resist is viscous so any bubble is trapped, and causes irregularity in the layer.

spin 1000 rpm 3” (rise time 10”) to spread the resist over the wafer
spin 30” for a uniform coat (rise time ~2”)

*Depends on development time. Values are given for the procedure explained below for a 3" wafer.

2.  Post spin bake="Soft Bake"  (solvent evaporation)

Hot plate 2’30 - 3’
Temperature range: 75-85°C

If not long enough, the wafer will stick to the mask during exposure.

Temperatures of soft bake and pre-dev bake depend on thickness(ajouter notice)
e.g. for ~5µm Tsoft-bake=75°C, Tpre-devbake=65°C

Wafer must be cooled down before exposure

3.  Exposure:

25 mJ/cm2/µm
(taking into account "after-spin thickness")

Once exposed, the resist can remain undeveloped for days without consequence. The pre-dev bake step ensures reproducibility of the development step.

4.  Pre-dev bake:

Hot plate  30’’ @65ºC (typical)

This step must be done right before development:
jump immediately from the hot plate into the developper

Reproducibility of the development duration depends a lot on the pre-dev bake procedure

5.  Development:

developper: DS3000
Thermally activated development: 31-32ºC
(set the bath to 33-34ºC, control with a thermometer in the DS3000)

First determination of timing on unexposed wafer: observe interference fringes. When fringes are finished ("endpoint"), record the timing and add 100% to develop patterned BCB.
Final thickness depends on development time
One can develop 2 or 3 wafers in 1cm of DS3000 before replacing it. (in a beaker of adequate size for the wafer)

parameters 16/01/2006:
-unexposed wafer 4024 40 spinned at 6000rpm (~4.6µm), pre-dev bake 55°C, immediate immersion in developer at 30°C (thermometer), fringes disappear after 1’35.
-exposed wafer, same parameters for all except exposure 115mJ/cm2 (MJB3), development 3’10. Final thickness 3.5µm before cure.

Rinse: DS3000
to stop development, at Tamb ~1’-2’ with agitation

Rinse: ODI (water tap)
to remove DS3000, ~15’’

6.  Spin dry:

In washing machine: 200’’ (max dry time) @ warm power 7 and N2 2 bar
(set wash time to min, and close the water inlet)

7.  Post-dev bake:

Hot plate  60’’ @75ºC

To stabilize the steps shape (avoid collapsing)

8.  Cure:

vacuum oven or controlled atmosphere (less than 100ppm O2 above 150°C)

60’ @ 200°C : 75% curing. Allows subsequent processing (not soluble in aceton) and adhesion of layers.
60’ @ 250ºC : 98% curing. End of process, to acquire nominal electrical and mechanical properties.

Be careful about temperature rise time: if too fast, BCB might collapse and broaden on the edges.

Preferably avoid vacuum because some bubbles of trapped H2 (from polymerisation) may inflate and create flaws in the dielectric layer.

9.  Descum:

Eliminate BCB residues (~100nm)

RIE (O2 - SF6: 90-10)
O₂:45 cc
SF₆:5 cc
P = 250 µbar 

BCB contains Si which explains the use of SF6.The ratio between O2 and SF6 is thus critical, because it determines etching rates of each species. If there is not enough SF6, one might form a SiO2 layer on the surface.

10.  Recycle

• before cure:

before development: (spoiled spin)
- solvent T1100
- developper AZ400K
- remover 1165

after development:
- AZ400 10’ in warm bath (~30°C)
- RIE (see table)

• after cure:

- RIE (see table)
- Strip-R

• Examples:

wafer spun with 4024-40 one week before, 5700nm
AZ400K (!Al etchan!) @35°C, 15' + 30" US                                                    + Remover 1165 @60°C, 15' + 30" US

11.  Supplementary info

No constraint on ion milling
How to promote adhesion of metal on BCB
General technical info on BCB