Utah-94-721002-System-Manual.pdf - 第154页

mä~ëã~ä~Ä = lñÑç êÇ=fåë íêìãÉåí ë=m ä~ë ã~=qÉÅÜåçäçÖó= System Manual PKOKU= rëÉ=çÑ=ÜÉäáìã=Ä~ÅâáåÖ=Ñçê=ÉÑÑÉ ÅíáîÉ=éêçÅÉëë=íÉãéÉê~íìêÉ=Åçåíêçä= PKOKUKN= pÅçéÉ= For all systems with wafer clamping and helium ba cking for wa…

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sputtered photoresist during GaAs ‘via hole etching’ it is common to use a mixed Chlorine/fluorine

chemistry:

RIE chamber:

85 sccm

50 sccm

Pressure 45mT

Power 150W

Temperature 20 C

Quartz carrier plate

ICP chamber:

Step1: 40sccm Cl

, 20sccm SF

, 50mT, 500W ICP, 200W RF, 22C, 0Torr He, 20mins to remove GaAs

and PR residues (may need to be longer after lots of ‘via hole etching’).

Step2: 50sccm O

, 20mT, 2000W ICP, 200W RF, 22C, 0Torr He, 30mins

Step3: 50sccm O

, 60mT, 2000W ICP, 200W RF, 22C, 0Torr He, 30mins

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It is quite a common requirement to process small samples or pieces of wafer. If the process requires

cooling to improve the etch profile or to allow use of resist mask at high power levels, then the small

pieces of wafer must be glued/fixed to a carrier wafer which is clamped and helium cooled. There are

several ways of attaching the small pieces of wafer to the carrier:

(a) Vacuum grease (after etching has been completed the vacuum grease can be removed from back

of wafer using IPA or acetone).

(b) Thermal compound.

the resist is still wet, push sample down well into resist, and then bake resist).

(d) Use a thermally conductive elastometer pad (see

EMI Shielding and Thermal Management Solutions).

With methods (a), (b) and (d) it is important that the sample completely covers the bonding material, so

that no bonding material is exposed to the plasma and therefore cannot be re-deposited on the wafer.

With all these methods it is necessary to also clamp the carrier wafer and apply helium pressure to the

back of the carrier wafer to provide cooling to the sample (there is no cooling effect simply from gluing

the sample to the carrier if there is no cooling of the carrier).

If the process does not need cooling (as with most low power RIE-only processes) then it is not necessary

to bond the sample to carrier. If the sample is liable to slide off the carrier during transfer, it is often

better to glue pieces of Si to the carrier wafer to act as locating pieces to hold the sample in place. This

avoids the need to glue the sample and therefore keeps the sample cleaner.

Process Information (Information contained in this document is confidential)

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For all systems with wafer clamping and helium backing for wafer temperature control. i.e. Plasmalab

System 100 with ICP 65, 180 and 380 sources. Also, occasionally RIE 133 systems and RIE 80 Plus.

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It is important to ensure that the helium is sealed adequately behind the wafer. If the helium is leaking

out past the wafer with a poor seal against the table, the thermal contact to the temperature-controlled

table will be degraded. The wafer will then heat up more than expected and the process results may

suffer. For example, in SiO

etching the profile may become partially isotropic and/or any photoresist

masking used may burn too easily.

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(a) If the wafer is sealing the helium effectively, the measured He flow will be less than that when no

wafer is present.

(b) Set a range of He pressures and note the measured helium flows with no wafer loaded. (Set all

process gases, RF and pressure to zero and work in ‘manual’ mode.)

the He flows for the same range of set He pressures.

(d) Load a typical customer wafer (e.g. with a thick SiO

layer) and note the He flows for the same

range of set He pressures. If a carrier is appropriate for the system, use that.

(e) Fill results in the following table. (If you do not have the capability to measure Helium flow then

measure CM gauge chamber pressure with APC fully open, no other gases flowing).

Set He/Torr He flow/sccm

No wafer

He flow/sccm

Si wafer

He flow/sccm

Customer wafer

(f) The larger the difference between ‘No wafer’ and ‘With wafer’ flows, the better the seal. ‘With

wafer’ values should be less. Pass criteria are still being evaluated but a recent example with

acceptable results is as follows.

(g) Recent acceptable example:

Set He pressure No wafer He flow With wafer He flow*

7Torr 4.2sccm <3.9sccm

10Torr 7.2sccm <6.5sccm

*These were the maximum values observed (usually occurring for wafers with thick SiO

layers)

and cooling was thought to be adequate because profiles were acceptable.

If there is little or no difference between the ‘No wafer’ and ‘with wafer’ flows, then the seal is

ineffective.

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(h) Troubleshooting:

Check the backs of wafers for excessive contamination, scratching or curvature/bowing. Vent

chamber and check electrode for particles, scratches, or erosion. Check wafer clamp integrity and

wafer clamping force i.e. can you move the wafer by finger pressure when clamped?

Compare results with blank Si if possible. If blank Si is OK, there is a problem with the customer

wafers, i.e. they are warped or too flexible or too thin (thin wafers may require reduced He

pressure to avoid flexing of wafers), or the clamp ring does not have sufficient clamping points to

maintain wafer flatness.

Also check that the measured He pressure is correct – if the Helium pressure gauge is faulty, the

actual pressure could be far too high. Typical CM gauge pressure when wafer is clamped and

helium pressure applied (APC fully open and no other gases flowing) is in the range 0.3-2mT for

the range of Helium pressures given above.

For the range of helium pressures given previously: Typical CM gauge pressure when the wafer is

clamped and the helium pressure applied (with the APC fully open and no other gases flowing) is

in the range 0.3mTorr to 2mTorr.

Checks with the system vented:

(1) Ensure that the electrode is very flat and clean (no bumps or grooves eroded into it) and that the

back of the wafer is clean and smooth (no resist or glue or anything else adhering to the back),

and is mechanically strong so that it does not buckle or bow.

(2) Check that the wafer lift star (or pins) retracts fully below the surface of the electrode. This can be

checked with a flat edge placed on top of the star - if it wobbles then the star is too high. If the

star sticks up above the electrode, the helium will escape and the cooling efficiency will be

severely reduced (also, because of the increased gap between wafer and electrode surface, which

needs to be a few tens of microns for best cooling).

(3) Check that the clamping ring is actually clamping the wafer to give maximum clamp force. Often,

there can be a discrepancy between the wafer clamp recess height and the wafer thickness,

meaning that the wafer is not clamped and 'rattles' about inside the clamp ring. The clamping

force should therefore be adjusted as described in Section 6. of the system manual.

You should try to move the wafer with your finger, if you can move it then it is not clamped

properly and you may need to temporarily modify the ring by adding strips of PTFE or aluminium

foil to make it press down on the wafer.

(4) The wafer should be placed centrally in the clamp ring.

(5) Press down on the wafer in various places and see if it moves. This will indicate that the wafer is

not sitting down flat on the electrode. Try polishing away any bumps.

Checks with the system under vacuum:

(1) A good test of whether the wafer is being clamped properly is to measure the helium pressure in

the chamber (measured on CM gauge) both with and without a wafer in place for a variety of

helium pressure setpoints.

There should be a clear difference between helium pressure with and without wafer. If there is no

difference then it indicates that there is a helium leak caused by incorrect clamping.

Process Information (Information contained in this document is confidential)

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