Chapter 2.3
Sink 3
Operation
(sink3)
1.0
Title
Sink 3 Operation
2.0
Purpose
This document specifies information about Sink 3, the isotropic silicon etch sink.
3.0
Scope
Sink3 is assigned for isotropic
silicon etching, which houses TetraMethyl-Amonium Hydroxide (TMAH) and
Potassium Hydroxide (KOH) solutions.
Sink3 consists of two baths, one QDR, a general drain area, a deck hose, and a N2 gun (click here to see an overview picture of the sink).
4.0
Applicable Documents
- H. Seidel, L.
Csepregi, A. Heuberger, and H. Baumgartel, "Anisotropic Etching of Crystalline Silicon in
Alkaline Solutions, I. Orientation Dependence and Behavior of Passivation
Layers," J. Electrochem. Soc., Vol. 137, No. 11, pp. 3612-3626, Nov.
1990.
- H. Seidel, L.
Csepregi, A. Heuberger, and H. Baumgartel, "Anisotropic Etching of Crystalline Silicon in
Alkaline Solutions, II. Influence of Dopants, " J. Electrochem. Soc.,
Vol. 137, No. 11, pp. 3626-3632, Nov. 1990.
- G. T. A. Kovacs,
“Micromachined Transducers Sourcebook,” McGraw-Hill , New York 1998.
- G. Lin, B. Eyre,
K.J.S. Pister, “Bulk Micromachining of Standard Cmos Using Silicic Acid
Added to TMAH for Aluminum Passivation,” UCLA MEMS Technical Report.
Electrical Engineering, UCLA TRN 95-13, 1995.
- Merlos et al., “TMAH/IPA Anisotropic Etching Characteristics,” Sensors and Actuators A 37-38, pp. 737-743, 1993.
5.0 Definitions
& Process Terminology
Quick Dump Rinse (QDR) – DI water fills the sink followed by a quick dump
to get rid of excess acid and/or contaminants.
Plenum L/L High – Early warning alarm indicating sink drain failure.
6.0
Safety
For details, see MSDS
sheets.
EPO red button (Figure 3): Cuts power to the sink in case of emergencies.
Report promptly on FAULTS.
Never touch any surface
while wearing chemical-resistant gloves that other lab members may come into
contact with, such as the table tops, door handles, computer keyboards, face
shields, aprons, etc. If you need to step away from the sink at any time, rinse
off gloves at the glove wash, dry with techni-cloths, and put away in your
drawer until you are ready to resume your work at the sink.
6.1 Tetramethyl-Amonium Hydroxide (TMAH)
Flammable (in 100% solution).
Toxic, causes burns. Poisonous. May be
fatal or cause blindness if swallowed. Vapor Harmful. Target organs: eyes,
kidneys.
Store only in Old Lab chemical cabinet. Bottles of 25% TMAH are stocked in the "bases" cabinet in the Microlab lobby, on the bottom shelf. Open under fume hood in Sink 3 only. Wear chemical gloves, rubber apron, and face shield provided in Sink 3.
6.2
Potassium
Hydroxide (KOH)
POISON! DANGER! CORROSIVE. CAUSES SEVERE BURNS TO SKIN, EYES, RESPIRATORY TRACT, AND GASTROINTESTINAL TRACT. MATERIAL IS EXTREMELY DESTRUCTIVE TO ALL BODY TISSUES. MAY BE FATAL IF SWALLOWED. HARMFUL IF INHALED.
POTENTIAL HEALTH EFFECTS
INHALATION:
RESPIRATORY TRACT IRRITANT, MAY CAUSE SERIOUS BURNS ON ACUTE CONTACT. SEVERE INJURY IS USUALLY AVOIDED BY THE SELF-LIMITING COUGHING ANDSNEEZING SYMPTOMS.
INGESTION:
TOXIC! CORROSIVE TO MUCOUS MEMBRANES AND MAY CAUSE PERFORATION OF THE ESOPHAGUS AND STOMACH. ABDOMINAL PAIN, NAUSEA, VOMITING, GENERAL GASTRO-INTESTINAL UPSET CAN BE EXPECTED.
SKIN CONTACT:
IRRITANT, POSSIBLY CORROSIVE IF CONTACT IS PROLONGED. SORENESS, REDNESS, DESTRUCTION OF SKIN MAY RESULT.
EYE CONTACT:
IRRITANT, POSSIBLY CORROSIVE TO EYE TISSUES. TEARING, REDNESS, PAIN, IMPAIRED VISION ARE SYMPTOMS.
CHRONIC EXPOSURE:
DEVELOPMENT OF A DEFATTING DERMATITIS ON PROLONGED CONTACT WITH POTASSIUM HYDROXIDE HAS BEEN REPORTED. CONTINUED IRRITATION MAY LEAD TO INCREASED SUSCEPTIBILITY TO RESPIRATORY ILLNESS.
AGGRAVATION OF PRE-EXISTING CONDITIONS:
PERSONS WITH PRE-EXISTING SKIN DISORDERS OR EYE PROBLEMS, OR IMPAIRED KIDNEY OR RESPIRATORY FUNCTION MAY BE MORE SUSCEPTIBLE TO THE EFFECTS OF THE SUBSTANCE.
Wear chemical gloves, rubber apron, and face shield provided in Sink 3.
7.0
Statistical/process Data
7.1 Problems and comments section under Equipment section of the Wand.
8.0
Available Processes, Gases, Process Notes
|
Bath |
Chemical |
Temperature |
|
Left-Heated |
KOH |
80ºC |
|
Right-Heated |
TMAH |
80ºC |
Both available silicon etchants etch silicon 100 planes at a relatively high rate to other crystal planes. For the plane (111) etch rate as a reference rate ‘1’, the (100) is 300-400, and (110) is 600 typically. Silicon Nitride is a good mask material for both etchants, and Silicon Dioxide may be used as a mask for TMAH. KOH solutions can produce smoother etch surfaces, yet the free radicals in KOH may affect device performance on wafers with fabricated devices. Solutions can have dramatically reduced etch rates based on doping, and n and p doped regions may be selectively etched with an applied bias during etching. Additives to TMAH can improve the selectivity to metals. See noted references for details.
9.0
Equipment Operation
The sink operation is relatively easy. The main difference
between the old and the new style sinks is: at the new sinks, members invoke
the dump rinse cycle, QDR and heated baths from a keypad mounted on the face of
the station
9.1
Control
Key Description
There are three control/displays at this station
(see Appendix 1). The single UFT-820 (Figure 1) temperature
control panel is for both heated baths. The
UFT-48-8 Dump Rinser (Figure 2) control panel is for the
quick dump rinse (QDR) station. The QDR is currently set up for two dump rinse
cycles. Wafers are initially showered with DI water followed by two DI
fill-dump cycles. These cycles end with wafers submerged in the water for
operator to extract and dry with the N2 gun. See Appendix 1 for more details.
The station performs an automatic self-cleaning, every 60 minutes by one QDR
cycle. The Resistivity Monitor panel is not used at this sink.
9.1.1
Power Control
Key Description for Temperature Controller
PWR
ON/OFF Turns on/off the power
control panel (Figure 1).
HEAT ON/OFF Puts system in HOLD mode. This deactivates the heater
output.
RESET Exit the program mode.
PROG Access to change or step through various setup
parameters.
SAVE Permanently save the system setup parameters.
TIME RUN Use of this button is optional; times the step.
Maximum time is 99 min. 59 sec.
TIMER STOP/RESET Stops
or interrupts a timed etch or the temperature control panel.
ALARM/SIL −
Silences the timer, any alarm conditions.
− Cancels flashing alpha code in the displays.
−
Examines the process set point and the Time Preset.
9.1.2
Control Key
Description (QDR)
START Activates dump rinse cycle /
Reactivates dump rinse cycle.
RESET − Silences alarm.
− Deactivates the dump rinser.
− Automatically reset itself in preparation for another run.
− Exits program mode.
HOLD − Halts rinse cycle.
− Manually dumps the tank.
PROG Parameters to be written in the
EEPROM memory.
9.1.3
Control
Key Functions for Overall Sink Operations
EPO Big red button for emergency stop of
entire sink operation.
All three:
POWER ON Main power on for top control panels (Figure 3).
POWER OFF Main power off for top control panels (Figure 3).
ALARM SILENCE Silences any alarm at this sink.
ALARM RESET Resets alarm system to this sink.
DRAIN ON/OFF Not in use.
ASP ON/OFF Aspirates KOH and TMAH baths.
RESISTIVITY DISP/CHAN Not in use.
HEATED BATH Not in use.
9.2 Quick Dump Rinse Operation (Figure
2)
9.2.1
Place wafers in the
tank; tank initially should be full of DI water.
9.2.2
Press START
button to activate the dump rinse cycle (Figure 2). It will
cycle down from 2 to 1 then show 0 in the display window. At the end of the two
cycles a beeping alarm will sound.
9.2.3
Press RESET to
silence the alarm.
9.2.4
Press RESET to
reset itself in preparation for another run.
9.2.5
Press HOLD to
manually dump the tank.
9.2.6
Upon completion of two
rinse cycles remove wafers dry with N2 gun.
9.3
Etching
Etching instructions are applicable to both TMAH side (right) and KOH side (left) unless noted.
Etch recipes and pertinent information are given in Chapter 1.6.KOH etches silicon dioxide, so it is not a suitable masking material for long etches. Silicon nitride is the preferred masking material.
9.3.1
Rinse
the beaker thoroughly and aspirate to make sure it is clean before processing.
Make sure that the N2 agitation in the bath is on.
9.3.2
(KOH) With the cassette out of the
beaker, add 2 liters of DI water by looking at
the 2-liter mark on the beaker. For Critical runs, the DI water may be measured
using the graduated cylinder located near the sink.
TMAH solution is commonly stocked in a
25 % concentration. Calculate the amount of 25% solution and DI water to make
the desired etchant concentration (often 3%-15%). Under the hood, open TMAH
container, and add desired amounts of solution and DI water to bath. Make
enough solution to reach ½ inch below the cooling coil. This amount should be
sufficient to cover a 6” wafer cassette.
Note: Needs 6 liters of solution to cover 4” cassette with wafers.
Needs 8.5 liters of solution to cover 6”
cassette with wafers.
9.3.3
(KOH)
Add 1 kg of KOH by
simply adding the contents of two 500 mg bottles of KOH. For critical runs,
weigh is on the portable balance inside the sink. Also, other ratios of KOH to
water produce different results, so see references in this document.
9.3.4
Make
sure the Main Power button is on the left front center of the sink.
9.3.5
Press
the power ON/OFF on the Temperature Controller panel at the topside of the
sink.
9.3.6
Locate
the KOH/TMAH switch on the topside panel and set the toggle switch to the
desired etchant.
9.3.7
Press
the HEAT ON/OFF on the Temperature Controller panel at the topside of the sink.
The HEAT LED light will light up when the heater is on.
9.3.8
Typically
the hot plate should be set to 80º. The temperature will reach the set point in
30-60 minutes for the bath. During the temperature stabilization, a visible
interface may form between the hot and cold (KOH and water) liquids. Using the 4” cassette handle as a stirrer, this interface may
be broken to allow the mixture to stabilize faster.
9.3.9
Load
wafers to be etched in the cassette and place in the bath. Take care when
removing the lid because solution condenses on it. Tap the lid a few times to
shake off most of the drops of liquid and place the lid on a techni-cloth.
9.3.10
Replace
the lid and start timing the etch. For critical runs, the wafer can be rotated during the run. The etch
rate is temperature dependant (a factor of 2 to 3 for every 10ºC for KOH), and
temperature significant temperature gradients may develop in the solution. The etch
rate is approximately 1 um per minute for both KOH (2:1) and TMAH (5%) at 80ºC.
9.3.11
When
the etch is completed, remove the lid and cassette. Let the wafers cool briefly
before rinsing by partially dipping the wafers in the QDR (Quick Dump Rinse) to
avoid breaking membranes. Initiate the rinse cycle.
9.3.12
If you
are finished etching, shut off the heater by pressing the heat ON/OFF button on
the temperature controller panel (topside panel). Aspirate the solution and
rinse and aspirate the bath 3 times with DI water. The bottom of each bath has
a perforated screen with a small hole, the size of the aspirator tube diameter:
the aspirator should be pushed through this hole to completely aspirate the
solution from the bath. Disable the sink afterwards.
10.0 Troubleshooting Guidelines
10.1
Status Sidebar
at Temperature Control Panel (Figure 1)
Red LED will light up to
indicate a problem:
HGH TEMP OVER
TEMP DEFFECTIVE
SENSOR SYSTEM
FAULT?
LOW TEMP LOW
LIQUID POWER
FAILURE HEAT
10.2
Sink System
Status Indicators (located on top
sink panel to the immediate left of the KOH/TMAH toggle switch)
LOW
PURGE Red indicator light will come on when there is low
air purge to cool off electronics for the sink.
PLENUM
L/L Red indicator light will come on when the plenum liquid
level is too high.
ASPIRATOR Red indicator light will come on when aspirator is in
use.
DRAIN Not in use.
PLENUM L/L HIGH: If it sounds, push the ALARM SILENCE button on the sink, and notify the process staff and post the problem on Faults, as a full plenum lockout is an early warning indicating sink drain failure. The area under the deck of a wet process station (sink) is known as the plenum. This area receives the water and chemicals when they drain from the tanks and the utility sink. When the level of water in the plenum reaches 3 inches, a float activated switch opens a valve and the waste is removed. Should the water rise too high in the plenum because of a failure of the system, all sources of water to the sink are shut off and the alarm will sound. This is to prevent an overflow. If the user silences the alarm, it will again sound when the problem is solved and the water level has returned to normal.
10.3
If EPO is
pushed, follow instructions:
10.3.1
Pull out EPO button
10.3.2
Open head case pull
down breaker switch and then lift up.
10.3.3
Push button on front of
sink that says on.
10.3.4
Push power button on
heater controller.
10.3.5 Push heat button on controller.
Sink3 Overview
Note: Both
KOH and TMAH must have enough liquid in both baths
to keep the LOW LIQUID alarm silent.
Figure 1 Figure
2
Figure
3
12.0
Appendices
Appendix 1
DECK HOSE Instructions
The de-ionized (DI) water deck hose for the sinks is ALWAYS
available for emergencies; it provides a good safety backup in the event of
exposure to chemicals.
Appendix 2
|
Temperature Controller |
|
Dump Rinser |
||
|
C5 |
99:59 |
|
AF |
|
|
PA |
30 |
|
CY |
2 |
|
P5 |
80.0ºC |
|
rC |
0 |
|
Hi |
85.0 |
|
Fp |
60 |
|
Lo |
16.9 |
|
dp |
3 |
|
dr |
30.0 |
|
SC |
0 |
|
dl |
7:18 |
|
Ad |
0 |
|
Cr |
10 |
|
n2 |
n |
|
Pb |
10.0 |
|
Ac |
0 |
|
rE |
0.5 |
|
PC |
5 |
|
rA |
1.0 |
|
Pn |
1 |
|
CA |
0.0 |
|
rP |
L |
|
AC |
00 |
|
|
|
|
Cd |
dn |
|
|
|
Rev. 00 - 11/02, K. Chan, M. Kushner
Rev. 01 – 10/05, W. Flounders - Suppression of labwide vacuum
drain system to be replaced by new definition of Full Plenum Lockout Alarm.