Jan.7, 2004, Poelker

Loading a photocathode into a gun;

1)  Cleave 15 mm squares wafers from large 3" diameter wafer.

2)  Anodize the edge of the wafer;

    a.  Prepare glassware; clean beakers with "micro" using ultrasonic
        cleaner, followed by acetone and hot water. Only water from the
        Millipore clean-water system is used throughout anodizing
        process.

    b.  Degrease the wafer in trichloroethane, acetone, methanol using
        ultrasonic cleaner.

    c.  "glue" wafer to microscope slide using acetone- soluble wax to
        prevent anodizing the back of the wafer

    d.  Attach wafer to vacuum pump with o-ring wand. This keeps the
        center portion of the wafer free of anodization.

    e.  Immerse wafer in dilute phosphoric acid solution. Anodize edge
        of wafer with low-voltage battery bias.

    f.  Rinse wafer in water, then methanol, to remove phosphoric acid
        residue.

    g.  Soak wafer and microscope slide in acetone until glue/wax
        dissolves, replacing acetone in 3 minute cycles, using
        ultrasonic cleaner.

    h.  Once the wafer has been liberated from the microscope slide,
        rinse the wafer in fresh acetone (3x). Change glassware and
        repeat acetone rinse (3x) followed by methanol rinse (3x).

    i.  Blow dry with N2 boil-off.

3)  Load wafer onto stalk;

    a.  Apply thin sheet of indium to stalk (Indium provides good
        thermal contact between wafer and stalk. It also adds mechanical
        "cushion" to prevent accidental breaking of wafer).

    b.  Place wafer on stalk and affix tantalum cup.

4)  Use atomic hydrogen to clean wafer surface.

    a.  Use N2-purged glove bag to load stalk into the portable atomic
        hydrogen cleaning vacuum chamber.

    b.  Allow time to pump out vacuum chamber, usually overnight.

    c.  Begin purging stalk transport tube with hot, dry N2, usually
        overnight.

    d.  Heat wafer to 500 C for 15 minutes to liberate water from
        stalk/wafer (Temperature values are measured at external
        thermocouple. Wafer temperature is lower than this value).

    e.  Cool wafer to 300 C and apply atomic hydrogen for set time
        period. Record dissociator parameters (RF frequency, absorbed RF
        power, vacuum pressure at dissociator and near wafer). Verify
        consistent, expected dissociator performance.

    f.  Cool to RT. \~ 2 hours.

    g.  Remove stalk from vacuum chamber and load into stalk "transport
        tube" using N2 glove bag. The transport tube has been under
        continuous dry nitrogen purge for many hours. The stalk and
        wafer are kept under positive N2 pressure while en route to the
        gun.

5)  Load a new wafer into tunnel gun

    a.  Prepare for venting; attach glove bag to the "top" of the gun.
        Purge with dry N2 boil-off.

    b.  Insert the stalk transport tube into the glove bag.

    c.  "pump and backfill" the vent beampipe of the gun at least three
        times using dry N2 boil-off.

    d.  Close the gun gatevalve and remove epics control.

    e.  Turn OFF gun ion pumps and vent with dry N2.

    f.  Swap stalks; remove old wafer and install the new wafer.

    g.  Pump out the gun, turn ON ion pumps and monitor ion pump
        current. Close vent port valve when ion pump current is less
        than 1 mA. Pump current should fall below 100 uA within 10
        minutes of pump down.

    h.  Pump out the NF3 reservoir.

    i.  Leak check and bake the gun; 30 hours at 250 C followed by soak
        at 120 C for 6 hours to prevent leaks due to thermal shock.

6)  Activate the new wafer;

    a.  Leak check gun following bakeout.

    b.  Degas the cesiator for a few minutes.

    c.  Re-load the NF3 (pump and backfill the NF3 reservoir with N2
        (3x) and then NF3 (3x)).

    d.  Heat wafer to 675 C for 2 hours. Record ion pump current during
        heat cycle.

    e.  Cool for \> 6 hours.

    f.  Apply cesium and NF3 in succession (yo-yo). Usually, \~ 10 yo-yo
        cycles are enough to achieve good QE.

7)  High voltage condition the gun

    a.  QE scan the wafer following activation. Ideally, scan with two
        wavelengths 770 nm and 850 nm.

    b.  Retract stalk. Apply high voltage using normal HV condition
        cycle; 20 minute ramp to 105 kV. Record HV power supply current.
        Note vacuum activity while monitoring the sensitive nanoA ion
        pump supplies.

    c.  Insert stalk and repeat QE scan.

    d.  Repeat the HV condition cycle; 20 minute ramp to 105 kV. Measure
        QE.

What steps have we eliminated, modified over the years?

1)  We no longer use acids/bases to clean photocathodes using wet
    chemical etching techniques.

2)  We no longer vent our guns to air. We vent with dry nitrogen
    boil-off through clean rough pumps.

3)  Whenever possible, we use N2-filled glove bags when we make gun
    vacuum changes. QE has doubled as a result. Probably because vacuum
    pressure remains \< 10\^-8 Torr throughout the bakeout.

4)  We no longer cut our wafers into circular discs. No more cutting
    with abrasive diamond paste and circular drill bits. We don't glue
    the front and back of wafer to microscope slides

What is new?

1)  Hydrogen cleaning. Known to improve QE on bulk GaAs and strained
    layer GaAs. However, we've determined that hydrogen cleaning is
    unnecessary when wafers are kept clean, during wafer handling,
    preparation and gun bakeout.

2)  Wafers are now indium soldered to the stalk. Good thermal contact
    between wafer and stalk. Added mechanical "cushion" to prevent
    accidental breaking of wafer.

3)  Anodization of the edge of the photocathode. Proven to enhance
    operating lifetime of the gun. Many-step process, more chances to
    dirty the wafer.

What steps will change in the future?

1)  We hope to eliminate the anodization step by using a mask to limit
    the application of cesium to the wafer. Mainz has demonstrated this
    method effectively limits photocathode QE.

2)  Load locked gun to speed up wafer swaps, etc. Failed activations not
    so problematic.

Why have we suffered problems with photocathodes lately?

1)  During August and September, we suffered 3 failed photocathode
    activations because we did not effectively remove wax residue from
    the surface of the wafer following anodization.

2)  The SVT wafers appear to be profoundly adversely effected by
    exposure to atomic hydrogen.

3)  What else? Tests in progress at EEL building using SVT wafers and
    test chambers.