PlasmaTherm SLR770 ICP Minispec

3/11/05
PlasmaTherm SLR770 ICP Minispec
Introduction: The SLR 770 is used to etch deep anisotropic trenches in silicon substrates
using the patented Bosch process. An inductively coupled plasma (ICP) generates a very
dense plasma near the top of the electrode. A 2 nd RIE generator which is capacitively
coupled to the wafer chuck is used to independently bias the substrate. In this way, high
selectivities and high etch rates can be obtained. The chemistry can only etch silicon or
(polysilicon). All samples must be mounted to a 4 inch silicon wafer.
CAUTION: Metals must not be directly exposed in the plasma. Do not put any other
substrates such as glass, compound semiconductors or metal in the chamber. Silicon wafers
thinner than 400 and thicker than 600 microns must be approved by the area engineer.
After long runs or many short runs the chamber may get dirty as evidenced by a poor base
pressure (>3.2 E-6). Do not proceed if the base pressure is too high or the leakback rate is
too high (>.3mT in 3 minutes). Additionally a clean must be run (recipe O2clean) every
100 microns or 1 thru the wafer run.
System Operation:
1 Place system in Standby Mode (if it isn’t already) by clicking on Standby button.
2 Verify from chamber view that chamber pressure is

PlasmaTherm SLR770 ICP Minispec
NOTE 2: Make any thickness or depth measurements before processing if you need
to measure selectivity eg photoresist or oxide thickness on Nanospec etc.
NOTE 3: If throttle valve is closed on chamber view, make sure system is in
Standby and go to ServiceMaintenancePumpSystem
5 To load wafer (make sure wafer is clean and resist is hardbaked to at least 120°C)
UtilitiesLoadlockVent
Go to Overview view
On schematic, “At atmosphere” is displayed and lid pops after a few minutes.
Open lid and place wafer flat against chuck etchside up. (Substrates less than
4 inches in diameter must be “glued” to a standard 4 inch wafer. See
Additional Information section of this document for more detail).
6 Pump down load lock:
Close lid and apply gentle pressure
UtilitiesLoadlockPump
In loadlock view after a few minutes red changes to white and pressure reads
~.5 mTorr
7 Transfer wafer to process chamber:
Go to chamber overview window
ServiceMaintenanceWafer Handling
Press Load button on the bottom left of the screen
An animation shows wafer being transferred.
When wafer is transferred and loadlock valve is closed, press Exit button.
8 Verify chamber is being pumped by going to chamber view.
9 Run Leakcheck again:
Make sure ion gauge is off
UtilitiesIon Gauge Off
ServiceMaintenanceLeak Test
Push Start in Dialog box
Note: System pumps chamber down to base pressure, closes gate valve and after a
minute begins displaying instantaneous leak rate. Note the starting chamber
pressure, and then the pressure after 3 minutes. Subtract and divide by 3 to
calculate leak rate in mT/min. Want

PlasmaTherm SLR770 ICP Minispec
Use most recent data from logsheet or test run for etch rate data. Divide
desired thickness (µm) by etch rate to get time (min). CAUTION: Etch rate is
a function of chamber history, % exposed silicon and many other factors. When
in doubt make a test run with an identical sample.
ProcessChamberOpen
Select recipe from list and OK
Double click on Loop Step to set the time in Step 7.
Use the up down arrows to adjust the number of loops (the dep/etch time is
shown to the right of the loop counter).
Make sure Step 4 is highlighted in the Dialog Box as this determines the loop
starting point.
Click OK
Save Recipe?Yes
Exit
11 Load and run recipe
ProcessChamberLoad
Select recipe to load, Press “OK”
Verify desired recipe is shown in “Process” box at lower right screen
Put machine into Ready mode, press Ready button
Press Run to begin processing
12 Verify in Dialog box that correct recipe is running and parameters are correct. It is
a good idea, if at all possible, to stay and make sure the process runs correctly.
13 After etch is completed, click OK in Dialog box, if the Dialog box is present
(Depends on how program was run.)
14 Put machine in Standby mode by pressing Standby button.
15 Transfer wafer to loadlock:
ServiceMaintenanceWafer Handling
Press Unload at bottom of screen
After wafer is in Loadlock press Exit
16 Vent Loadlock as before:
Go to Loadlock View then,
UtilitiesLoadlockVent
17 After lock vents, remove wafer close lid and pump loadlock down again:
UtilitiesLoadlockPump
18 Leave system in Standby mode and record all data in logbook.

Procedure for gluing silicon wafers together
Silicon pieces (< 4 inch) must be glued to a 4 inch silicon carrier wafer in order for the
loading mechanism to work. Additionally, if a wafer is to be etched all the way through,
the wafer must either have an oxide on the backside or be glued to a 2 nd silicon 4 inch
wafer. This is because backside Helium cooling is used on the wafer chuck and once the
etch breaks through, the helium leaks into the chamber, raises the pressure and shuts
down the machine.
Only silicon carrier wafers are used and must be very clean like the sample to be
etched. The wafer may be either bare silicon or thermally oxidized silicon. A bare silicon
carrier wafer will slow down the etch-rate due to loading effects. For shallow etches, where
very fine control is required, this may be desirable. An oxidized carrier wafer will not load
down the etch and will result in faster etch rates but less control. This may be desirable
for deep etches where speed is required.
The following procedure must be used for etching either wafer pieces or all the way
through the wafer. It is extremely important that no air bubbles be trapped in the
PR as this will cause the piece to flip over during pumpdown or etch. You should
firmly rub the piece onto the 4 inch wafer to remove air and/or use the vacuum bake oven.
Additionally grooves on the underside of your piece will aid in allowing air to escape.
PIECES:
1 Use a Q-tip swab to dab a small drop of AZ5214 resist (or equivalent) on the
backside of your piece.
2 Then carefully center the backside of your piece onto the front-side of the carrier
wafer and press down firmly. Do NOT use too much resist as this will be squeezed
out from the back when you press down and can cause problems.
3 Bake your sample on a hot plate at 120°C for at least 3 minutes. This hardens the
resist. Do not allow the backside of the carrier wafer to become contaminated (eg
from the hot plate).
4 Let your sample cool and bring it to the DRIE machine and follow the loading
procedures as outlined in the SLR 770 ICP spec.
WHOLE WAFERS
1 Use a Q-Tip swab as above but put 2 or 3 small drops on the backside of your 4 inch
wafer near the center spaced about 1/2 inch apart.
2 Carefully center the backside of your wafer over the carrier wafer and press down
firmly. Make sure the flats for both wafers line up before pressing the substrates
together.
3 Bake your sample on a hot plate at 120°C for at least 3 minutes. This hardens the
resist. Do not allow the backside of the carrier wafer to become contaminated (eg
from the hot plate).
4 Let your sample cool and bring it to the DRIE machine and follow the loading
procedures as outlined in the SLR 770 ICP spec.

Additional Information on DRIE
1 For resist thicknesses < 2-3 µm, edge bead removal is probably not necessary if the
resist is baked. Thicker films > 8µm, can outgas and cause clamping problems.
Removal of the edge bead is advised in these cases.
2 Resist profile will affect the etch and sidewall profile. When baking resist, avoid too
high a temperature such that the resist flows (unless you are trying to get a taper).
3 Small silicon pieces can be “glued” to a 4 inch silicon wafer using a dab of PR. If an
oxidized wafer is used, the etch rate will be very fast which may lead to control
problems. A bare silicon wafer will slow the etch down for better control. In any
event, you must use a 4 inch wafer as a carrier.
4 Silicon etch rate is a strong function of exposed silicon (usually expressed as % open
Si). Etching slows down dramatically as the amount of exposed area goes up, so
keeping track of exposed area is important.
5 Some standard Bosch specs for 10-15% exposed area are:
* 1-3 µm/ minute silicon etch rate
* Large open areas etch faster than smaller
* Uniformity of etch and other parameters within 5% across 4 inch wafer
* 75-80:1 selectivity to PR for 12% open Si and 2-100 µm lines
* ~150:1 selectivity to thermal oxide (12% open Si area); PECVD oxide not as good.
* Sidewall angle depends on aspect ratio (Depth to width of trench) and is 88-92
Deg. If process is tuned for 90 Deg at 20 µm, then larger openings will be reentrant
(neg Slope). Conversely if the process is tuned for 90 Deg at larger
openings, a positive taper will result at smaller openings.
6 Run to run uniformity should be about 2 %.
7 For open areas >20%, a bulls-eye pattern can be seen with outer edges etching
faster than the center due to mass loading/gas depletion effects.
8 A seasoning run should be made after opening the process chamber or doing any
substantial maintenance. This involves running the standard Bosch process on a
dummy silicon or oxide wafer (to protect cathode). Depending on total etch time,
some haze may be seen at wafer edges and is considered normal.
9 A cleaning run is not normally needed unless the process chamber is opened and
the fixturing is changed. If so the recipe is:
* 80 sccm O2 + 20 sccm Ar @12 mT

* For 5 “ 500W ICP + 100W RIE
* For 15’ 750W ICP + 10W RIE
* Make sure a blank wafer is on cathode to protect it
10 Materials that should not be used in the deep RIE include any metal masks. Metals
tend to sputter and contaminate the chamber and clamp and shift the process.
Polyimide and TEOS are OK but a de-scum is highly recommended for polyimide.
11 A mixture of SF6 and O2 can be used to de-scum in the DRIE. Experiments need to
be run to characterize this process.
12 For thru wafer etching, a thick enough mask must be used (oxide has better
selectivity and therefore doesn’t require as thick a layer. When the etch completes,
the backside helium cooling fails and requires stopping the process. At this point
short runs of no longer than 5 minutes with He cooling off followed by a 5 minute
cool-down may be run to finish the etch.
A better alternative is to have an oxide on the other side of the wafer which may be
used as an etch-stop and so maintains a seal to keep the He cooling on. The
backside oxide must be thick enough to accommodate whatever overetch after
breakthru is required.
A third alternative is to glue the wafer to be etched to a 2 nd carrier wafer so that He
cooling is maintained.
13 A dielectric substrate (eg quartz) instead of a semiconductor may only be used with
the area engineer’s approval and most likely will require special setup. For
dielectrics, a parasitic capacitance is introduced which interferes with the bias and
therefore the etching of the substrate. Too low a bias indicates the capacitance
problem.
14 PlasmaTherm has done oxide etching in the DRIE without any obvious problems,
but they do not guarantee anything and they are concerned that it may affect the
Bosch process.
15 After running processes in the ICP pressure will degrade with time. Background
pressure in the 10-5 range is normal. Etching is a dirty process especially with the
deposition of polymers which is an essential part of the Bosch process.
16 To see if the chamber is clean enough (or vacuum low enough), run bare Si wafer for
20 ‘ using standard Bosch process. If wafer looks clear to the high (eg no haze), then
system is clean enough.
17 During SEM examination, some charging due to existence of polymer may be seen.
Recommend sputter coating rather than ashing to obtain better image.

18 During the deposition of the standard Bosch process, approximately 1000 Å of
polymer is deposited.
19 PlasmaTherm has measured the temperature of the wafer during a standard 20’
Bosch run as between 40-50 °C. The clamp however can go as high as 80°C.

MISCELLANEOUS PROBLEMS
SYSTEM LOCKUP
Occasionally the system may lock up and not respond to any commands. Try exiting the
Program Sysmon if possible or if that doesn’t work try exiting windows and restarting
(type win after getting the DOS prompt). If this fails, open the lower panel and press the
red reset button on the computer at the bottom. This should reboot all the software and a
dialog box requesting user and password will come up. Enter 3333 in both boxes. If a
wafer is in the process chamber it should still know this. Verify if the turbopump is on by
checking the column of green lights on the back of the machine. If off, turn on the
turbopump from the menu and wait 10-15 minutes for vacuum to recover . If that doesn’t
work, get help.
IDENTIFYING & ACKNOWLEDGING ALARMS
If something went wrong and you are not sure what happened (check info box at the
bottom of screen first) go to:
Display!Alarm History
Date and time appear next to each alarm and can help you identify what went wrong.
Press ALARM SILENCE at the bottom right of screen to silence an alarm or at the end of
a run. If there is a problem, the HOLD button illuminates when the alarm is silenced.
Push the HOLD button to put the system back in Standby or Ready. If the alarm condition
is still present, however, the alarm will continue to go off. In this case the problem must
be fixed first.
UNLOADING PROBLEMS
Wafer transfer problems generally occur because of either poor procedure of attaching a
wafer to a carrier or thick PR at the wafer edge which gets sticky with heat causing the
wafer to stick to the clamp and break in the process chamber. For thicker resist, use edge
bead removal or keep your etches short. For gluing wafers use very small dabs of
photoresist or approved material and bake thoroughly. If a wafer breaks or gets stuck in
process chamber, you must get help. Do not attempt to fix the problem yourself.
CHAMBER CLEANING
As mentioned previously, an oxygen plasma clean (recipe O2Clean) should be run every 8
hours or so of etching for 15 minutes. Load the dummy clean wafer kept at the machine
and load the recipe following standard procedures. Make the entry in the log book that the
O2Clean was run. There is no charge for this.
MORPHING
It is common for the etch rate to slow down as the trench gets very deep (ie the aspect
ratio gets high). Morphing is a software feature which allows a variable to be
automatically incremented as a dep/etch loop is completed. Usually the RIE power is
incremented as the trench gets deeper. The rate of change ie sublinear, logarithmic etc can
be programmed as well. However you must work with a superuser or lab management to
use this feature.

ADDITIONAL RECIPES
An approved recipe list is kept at the machine. You must get approval and help in writing
your own recipe since small changes in a parameter can have a large detrimental effect on
the process and machine. Once the recipe is approved it will be added to the recipe list.
Anyone may run any of the posted recipes but may change only the etch time, nothing
else.
SYSTEM ABORT
The abort button at the bottom of the screen aborts the entire recipe but may cause a
system crash. If possible try using End Step several times first.
EMERGENCY SHUT OFF
The big red button at the front side of the machine will kill all the power and is to be used
only in extreme emergency ie smoke or burning smell coming from the machine, etc. Do
NOT attempt to turn on the machine if it is shut down in this manner.
GAS SUSPECTED IN CHAMBER
This alarm may occur from a system abort, a logic error or a real problem. If the turbopump
is still on (green) and the throttle valve is closed (as seen in Chamber View) try
pumping the system in the Standby mode by selecting:
Service!Maintenance!Pump!System