MicroChemicals TI 35 ES

MicroChemicals TI 35ES technical data sheet – revised 10/2003
p. 1 of 11
TI 35ES
image reversal resist
Technical Data Sheet
revised 10/2003
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www.microchemicals.com
General Information
The TI 35ES resist is specially designed for the application in the so called “image
reversal technology” for:
•
•
•
subsequent lift-off of deposited layers with a thickness up to 5 µm
plasma etching (very low resist erosion)
Wet chemical etching in HF-solutions (for this application, however, we
recommend TI 35E)
The viscosity of the resist leads to a thickness range depending on the spin-speed
from 2.5-3.5 µm. The typical aspect ratio of the structured features achievable is in
the range of 1.6 .. 2.0.
This technical data sheet intends to give you a guide-line for process parameters
for various applications. However, the optimum values for e.g. spin profile,
exposure dose, or development depend on the individual equipment and need to be
adjusted on each individual demand.
For general information and trouble shooting on image reversal resists, consult
or ask for our latest work sheet "Processing of image reversal resists"
Zur generellen Prozessierung von Umkehrlacken und dabei häufig
auftauchenden Fragestellungen konsultieren Sie bitte die aktuelle Version unseres
Datenblattes "Prozessierung von Umkehrlacken" bzw. fordern Sie dieses ggfalls. an
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MicroChemicals TI 35ES technical data sheet – revised 10/2003
p. 2 of 11
'Image Reversal' – A Short Introduction
What 'image reversal' generally means
soluble
inert
resist
substrate
(1) Exposure using an
inverted mask (the exposed
areas finally remain)
(2) The resist now would
behaves like an exposed
positive resist.
(3) The reversal bake
cross-links the exposed area,
while the unexposed area
remains photo-active
soluble
(4) The flood exposure
(without mask) ...
... (5) makes the resists,
which was not exposed in
the first step, soluble in
developer
(6) After development, the
areas exposed in the first
step now remain
... and for what image reversal is good for:
→
Adjustable undercut for lift-off of thin and thick sputtered, CVD, and
evaporated films like metals, a-Si:H, a-SiN:H etc.
→
→
High thermal- and chemical stability allow the inverted TI 35ES as mask for
wet-chemical and plasma etching under harsh conditions
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MicroChemicals TI 35ES technical data sheet – revised 10/2003
p. 3 of 11
TI 35ES – Fields of Application
Lift-off of PECVD layers
• Amorphous Si layer deposited in device quality at 110°C
• Amorphous SiNx
Lift-off of sputtered layers
• Due to non-directional deposition, lower thickness achievable (approx. 1 µm)
• Possibly ultra sonic treatment necessary
• Lift ability dependent on operating pressure
Etching of SiNx in buffered HF
• SiNx deposited at 400 °C with PECVD
• Hardbake at 125°C (Hotplate) for two minutes
• No visible degradation of the resist after 10 min etching time
• Enables structuring of SiN up to at least 100 nm (tested)
• Use ammonia during SiNX deposition to increase adhesion of the resist on the
surface
(For harsh chemical attack prefer TI 35 E)
Etching of thick thermal SiO2 with 20 % HF
• No visible degradation of resist after 10 min
• Possible etching in HF conc. (50%) for several minutes
(For harsh chemical attack prefer TI 35 E)
Direct mesa grooving
• TI series resists allow direct Si etching without SiO2 masking
• Hardbake at 120-145°C for 2 min necessary
• Use HNO3 (70%) : HF (50 %) : H2O = 75 : 10 : 25 as etching solution and
TI 35ES resist in reversal mode, groove up to 4 µm in 45s
(For deeper mesa grooving prefer the use of TI 35 E up to 15 µm possible)
Technological Requirements
Since the TI 35ES yields an inverted structure in the image reversal mode, an
inverted exposure mask is needed. Beyond this, compared to standard positive
resist processing, no further upgrade in existing technological infrastructure is
necessary.
Also compared to standard positive resist processing, applying the TI 35ES
implements just two further process steps: The reversal bake and the flood
exposure without mask, both very easy to be performed and explained in the
following.
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MicroChemicals TI 35ES technical data sheet – revised 10/2003
p. 4 of 11
Processing the TI 35ES
(in chronological order)
■
Put the substrate on the hotplate at a minimum temperature of 120 °C for 10
minutes to remove adsorbed water from the substrates surface. Alternatively,
use a furnace at same temperature for 30 min. Standard HMDS procedure (only
from vapor phase at an optimum substrate temperature of 125°C!) or
recommended TI PRIME adhesion promoter is also an adequate preparation.
■
Spin-coat the resist after cooling down the substrates, spin at the final speed
level for 30-40 seconds.
■
Softbake the coated substrate at 100°C for 2 minutes on the hotplate (when
using a furnace, 100°C for 20 min is recommended)
■
Exposure the coated substrate with the mask. This 1st exposure dose adjusts
the negative wall profile (“undercut”). 200 mJ/cm² will be a good choice for
most applications, lower 1st exposure doses increase the undercut. A too low 1st
exposure dose will increase the erosion of the resist not to be cleared (see
appendix). Note: Exposure doses refer to i-line (365nm). A standard mask aligner
with a 350W Hg light source has approx. 6-15 mW/cm2 i-line intensity, while in many
cases 20-30 mW/cm² are measured meeting the total (g-, h- and i-line) intensity!
■
Delay time: Keep the coated substrate at room temperature after the exposure
for at least 10 minutes. In this delay time nitrogen, generated during exposure,
will diffuse out the resist. If square shaped substrates are used, the resist
thickness on the edges is significant thicker than 3-4 µm. In this case the N2
needs more time to diffuse out, in this case double the delay time.
■
After the delay bake the coated substrate at a temperature of 120 .. 130°C on
the hotplate for two minutes (when using furnace try 20 minutes at 115°C120°C. Because this step is very temperature critical furnace baking is not
recommended). This step is the reversal bake where the image is reversed due
to cross-linking of the exposed areas making them insoluble in the developer.
■
Exposure the coated substrate for the 2nd time without a mask (flood
exposure). Use a dose (very uncritical) of ≈ 540 mJ/cm2 (300-800 mJ/cm2 has
no dominant effect). When UV-light is present during a subsequent deposition
(plasma coating, thermal evaporation), especially when the temperature raises
beyond approx. 60°C, use a high flood exposure dose to avoid an ‘exposure’
during deposition with bubbles/foaming as a consequence.
■
Develop in AZ developer such as 826MIF (metal ion free) or potassium-/sodium
based (AZ 400K 1:4) developer. When the structure is through-developed
(cleared), add another 10-30% of the time in the bath of the total development
time to finalize the side wall profile (see appendix).
■
Hardbake the coated substrate only when using the resist as an etching mask
under harsh conditions (For hard etch attack, prefer TI 35E). For mesa grooving
hardbake at 135°C to 145°C for 2 minutes on the hotplate. The side-wall profile
will loose the undercut during this step if the hardbake temperature is higher
than the reversal bake temperature. In this case lift-off processes become more
problematic. If you need a high hardbake temperature and subsequent lift-off or
very high temperatures during deposition, make a UV-curing to harden the
resist after development (or contact us for further information).
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MicroChemicals TI 35ES technical data sheet – revised 10/2003
p. 5 of 11
Processing the TI 35ES - Overview
Substrate preparation
TI PRIME recommended
Resist thickness (µm)
2.6... 3.5
Exposure Broadband or
g, h, i (mJ/cm-2)
100 .. 350, 200 (typically)
Typical Exposure time (sec)*
10-35, 20 (typically)
Delay
5 min .. 20 min
dependent on resist thickness and substrate shape
Reversal Bake Time
(Hot plate Temperature)
2 min
(120-130°C)
Flood exposure (mJ/cm-2)
(without any mask)
540 (typically)
300-800
Developer
Clariant AZ® 826MIF, AZ® 400K 1:4, AZ® 351B
Hardbake 135 ... 145°C
(only for hard etching, avoid it
for lift-off )
2 min
Lift off media
NMP, Acetone, MMP, EEP, DMF, ethyl lactate,
PGMEA
Remover (Stripper)
AZ® 100 remover, acetone
* i-line (365nm) exposure dose. A standard mask aligner with a 350W Hg source has
approx. 6-15 mW/cm2 i-line intensity, in many cases 20-30 mW are measured meeting the
total (g-, h- and i-line) intensity!
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MicroChemicals TI 35ES technical data sheet – revised 10/2003
p. 6 of 11
Lift-off with the TI 35ES
The adjustable undercut allows lift-off of metal films with thicknesses
exceeding the resist thickness
5 µm
Figure 1: Resist with typical undercut on a Si substrate
Firgure 2: Metal layer with 4.5 µm in thickness, evaporated and
structured via lift-off. Aspect ratio more than one.
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MicroChemicals TI 35ES technical data sheet – revised 10/2003
p. 7 of 11
Dry-etching with the TI 35ES
Extraordinary stability of TI 35ES in reversal mode (resist erosion by a factor
of approx. 2 reduced as compared to most positive resists) allows dry
etching of several µm of SiO2
Both REM pictures:
TI 35ES in reversal mode
•
•
No plasma etch residual remover
Etching of 3.2 µm SiO2 with
CHF3 + O2
Resist
MicroChemicals gratefully acknowledges Mesa+ Research
Institute, University of Twente, Netherlands for verifying
TI 35ES results and providing the pictures shown in this
data sheet.
SiO2
MicroChemicals GmbH Ulm – www.microchemicals.com
MicroChemicals TI 35ES technical data sheet – revised 10/2003
p. 8 of 11
Process Parameters and Undercut …
1st exposure dose
A1
B1
A2
B2
C1
C2
A high 1st exposure dose (A1) yields after reversal bake, flood exposure, and
development - to a steep resist profile
with minor undercut (A2), while a small
1st exposure dose (B1) not exposing the
substrate-near resist layer leads to a
strong undercut and sometimes to
peeling of narrow resist structures in the
developer
(B2).
Die
optimum
1st
exposure dose therefore depends on the
desired undercut and the minimum
lateral feature sizes. At the beginning of
new processes, an exposure series is
recommended.
Reversal bake
A1
B1
A3
B3
Before the reversal bake is applied, the
exposed resist needs a certain time –
depending on the resist type and
thickness – to outgas nitrogen (N2)
formed during exposure. This will avoid
bubbling (irregular developed structures
(C2)) and foaming (C1) of the resist by
thermally
activated
N2.
Nitrogen
preferentially accumulates near locations
with inferior resist adhesion to the
substrate, which has to be optimised
Development
The undercut forms in the last stage of development when the structures are
already cleared (see p. 10). We recommend an approx. 30% over-developing.
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MicroChemicals TI 35ES technical data sheet – revised 10/2003
p. 9 of 11
Adjusting the undercut of TI 35ES
TI 35ES (thickness 3.5 µm, reversal bake time 1 min) parameter study I
(varying 1st exposure, reversal bake temperature, development time)
115°C
123°C
130°C
137°C
Reversal Bake Temperature =
115°C
123°C
137°C
123°C
130°C
Very stable
conditions for
undercut
137°C
200 mJ/cm2
Wide process window
allows adjustment
of undercut
300 mJ/cm2
conclusion ↓
Feasible only for very short
develop. time or very high
reversal bake temperature
1st exposure 100 mJ/cm2
AZ 726MIF Development time →
Please note: The gap widths are different for the individual pictures. This series
intends to give a survey of the undercut process window.
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MicroChemicals TI 35ES technical data sheet – revised 10/2003
p. 10 of 11
Adjusting the undercut of TI 35ES
29 s
37 s
46 s
←
3s
4s
5s
7s
25 s
9s
22 s
11 s
1st Exposure 200 mJ/cm2
Reversal bake at 126°C for 1min
2nd exposure 1.200 mJ/cm2
Development in AZ 726MIF
19 s
13 s
•
•
•
•
16 s
2s
←
TI 35ES (thickness 3.5 µm) parameter study II
(varying the development time)
Please note: The gap widths are different for the individual pictures. This series
intends to deliver insight in the evolution of the undercut with development time at
given process parameters.
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MicroChemicals TI 35ES technical data sheet – revised 10/2003
p. 11 of 11
Trouble Shooting
Bubbles after coating (but before exposure)
If the resist has been refilled before coating, a delay time for at least a few hours
may be necessary to outgas air bubbles. If the substrate is textured, a spin profile
starting with approx. 500 rpm for 5 seconds helps to completely coat the substrate
without forming ‘comets’ before the final spin speed determines the desired resist
thickness.
Bubbles after 1st exposure (but before reversal bake)
Nitrogen (N2) formed during the exposure of the resist normally diffuses out of the
film. Bubble formation during exposure may be caused by:
§ A remaining solvent concentration too high with expansion of N2 preferentially in
substrate-near regions as a consequence. In this case, increase the softbake
time (e.g. 100°C per µm resist film thickness) which, however, will increase
development time, but reduce dark erosion.
§ A sub-optimum resist adhesion to the substrate. In this case we recommend the
application of TI PRIME adhesion promoter.
Bubbles during reversal bake
Nitrogen (N2) formed during the 1st exposure needs time to diffuse out of the resist
film, strongly dependant from the film thickness, the temperature, and the first
exposure dose. In this case, increase the delay time between exposure and reversal
bake, and/or perform the delay at a moderately elevated temperature (approx. 3050°C)
Bubbles/holes after development
I)
Either the bubbles already have formed during any previous process step
(coating, exposure, reversal bake) and finally become visible after
development. In this case, consult the previous sections.
II) Or the bubbles represent spatial inhomogeneous dark erosion. This points
towards a 1st exposure dose or/and reversal bake temperature/time too low.
General information on processing of AZ® and TI resists in our work sheets
§
§
§
§
§
Processing of image reversal resists
Reproducible litho-processes
Baking and delays
Exposure and development
Processing of thick resists
available on request.
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