Report on Recent PlasmaQuest Problem

Jim McVittie mcvittie at snf.stanford.edu
Thu Jun 20 13:03:06 PDT 2002


This is a follow up on Vijit list of PlasmaQuest problem:

1.  Poor RF tuning and lack of DC bias voltage:

>From Vijit measurements after I left on Friday and from Randy lack of
bias after running it at high power, it becoming pretty clear what the
likely RF/bias problem is. The RF after the matching network has to go
through a torturous path in a long 1.5 “ ID pipe to get to the cathode.
At this point the Rf is not in a shielded coaxial cable. Instead, a flat
copper strip with thin plastic insulation is used. To prevent shutting
of the RF to the pipe wall and to the other conductors in the pipe, the
RF conductor is placed in a series of short ceramic sections. Note that
along this path the pipe has at least one 90 degree bend. It appears
some of the ceramic sections have spread apart to the point that the RF
is intermittently shunted to ground. Since the RF is in a power control
mode, the RF voltage on this conductor goes up and down depending
whether RF is making it to the plasma or being shunted in the pipe. This
is why Vijit saw the RF voltage change when he was measuring it near the
entrance to the pipe.

We have seen this problem before. To correct it, the techs will need to
disassemble the whole bottom (cathode) electrode. The work should start
this afternoon. The system should be back up at least the middle of next
week.

2.  Loading Integrity

After Vijit’s suggestion, Len measured the wafer pocket diameter in the
load arm. He found that it is smaller than the largest wafer diameter
allowed by the Semi spec for wafer diameters. This explains the
intermittent loading problem that the system has always had.  While the
tool is down for the RF problem the wafer pocket is being enlarged to
accept all wafers within the Semi diameter spec.

3.  Loading - software problem

Len talked to the vendor about this problem and they suggest that a
wafer sensor may need adjusting. He will do this adjust while the system
is apart.

4.  The load arm sometimes wobbles too much during loading and
unloading.

The vendor suggested that the lead screw may need lubrication. This will
be done once the recommended lubrication has come in.

5.  Noises coming from controller.

The fan will be checked while the system is down.

6. Cl2 MFC problem

Most users are operating at the bottom of the MFC range, the MFC appears
to have an unusually large zero drift and the readout does not show the
true zero reading.

The users will be given a procedure for reading the true zero flow
readout for the MFCs. For the near term, when the zero has drifted
beyond a set value, the techs will readjust the zeros. We are polling
the users to see if we can resize the Cl2 MFC so that we are not running
at the bottom of its range where it is overly sensitive to zero drift.
When we order a new MFC, we will go for one that has a smaller zero
drift.


        Jim McVittie



Vijit Sabnis wrote:

> Hi:
>
> This is an update and follow up to Dr. McVittie's email
> based observations from my work yesterday.
>
> I apologize for the long email, but I hope that current
> users read this carefully and communicate their comments
> and concerns with all of us.
>
> 1.  Poor RF tuning and lack of DC bias voltage:
>
> Problems:  These problems are flaky, but since I have been
> using the machine heavily in the last couple of weeks, I have
> noticed them occurring more and more frequently.
> RF tuning can be difficult to achieve.  Manual
> tuning is required.  Even when the RF power is matched
> appropriately, the DC bias voltage reads 0 or a few volts,
> but at some random point can start reading the correct voltage.
>
> I hooked up the oscilloscope with high voltage probe to
> the RF feed to the cathode.  I was able to confirm that
> when the computer screen and RF power supply display 0 V
> or a very low DC bias that there is indeed no DC bias
> measured on the scope.  This means that there isn't a
> problem with the voltage readout on the computer screen or
> RF power supply display, but that there is a real problem
> with the plasma or RF being supplied to the chamber.  Based on this
> finding we can eliminate any potential problems with the loose
> connector that we found last Friday.  At some point
> during my investigation the DC bias suddenly was displayed
> correctly on the computer screen and RF power supply.  I then
> confirmed the presence of the bias on the oscilloscope.
> I found the same results as last Friday - for a displayed
> voltage of -46 V, the DC bias read on the scope was about
> -80 to -90 V.  The voltage read on the computer screen and
> RF power supply is about a factor of 2 smaller than what
> it should be, indicating a calibration problem or the
> presence of a voltage divider in the readout circuitry.
>
> Some other interesting observations:
> My recipe uses 35W of RF power.  When the DC bias reads 0 V
> (or something close to that), the scope displays a
> sine wave with a 400 V peak-to-peak amplitude.  When
> the DC bias is correct, the sine wave shifts down by
> this bias and the amplitude of the sine wave drops to
> 100 V peak-to-peak.
>
> I intermittently heard strange noises coming out of
> the RF matching box.
>
> When the DC bias reads properly, there is no problem
> with your etching.  I did not have time to investigate
> what happens to the etch rate when the DC bias reads
> close to zero, but based on the above results I think
> you will get poor results.
>
> My guess is that something is wrong with the RF
> matching network and possibly the RF power supply.
> However, someone more qualified should confirm this.
> I think this concern should be taken seriously and
> acted on immediately by the SNF staff.
>
> 2.  Loading Integrity
>
> I have experienced three wafer crashes in the last couple
> of weeks.  This may be due to the new pins that have been
> installed into the machine.  Please also double check that
> your wafers sit properly in the loading arm platen.  I found
> that even certain prime wafers can be slighty too large and
> hence do not sit properly in the load arm.
>
> 3.  Loading - software problem
>
> Sometimes the loading and unloading functions become
> out of phase with proper pin positions, which end
> up confusing the machine about the wafer locations.
> Please pay special attention to this since you
> can easily crash your wafer into the pins or worse
> yet crash the load arm into the clamp.  Should you find
> yourself in this position I have found that cycling the
> load/unload functions along with system reset will
> eventually get the system back into the appropriate state.
> Using the prompt option in your recipe sometimes works.
>
> 4.  The load arm sometimes wobbles too much during loading and
> unloading.
>
> 5.  The module located just below the keyboard (MC-200
> Motor Controller) is making weird noises from time to
> time.  The fan may need to be replaced.
>
> 6.  Many of us use low Cl2 flow rates (ie. < 3 sccm).
> I really don't believe the Cl2 flow rate displayed on
> the screen.  I think this particular flow sensor (better
> yet, all of them) needs to be recalibrated.
>
> My overall conclusion is that the tool is pretty unreliable
> right now and has several issues which need to be resolved.
> YOu can still get your etches to work, but you need to be
> particularly careful about the RF matching/bias and loading.
> I hope that appropriate action will be taken immediately
> by the SNF staff so that I and others do not have to spend
> more time stuggling with the tool.
>
> - vijit
>
> Vijit Sabnis
> vsabnis at snow.stanford.edu
> Stanford University
> Department of Electrical Engineering
> Solid State and Photonics Laboratory
> CISX B113-23
> 650/725-2774 office
>     723-4659 fax




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