Utah-94-721002-System-Manual.pdf - 第46页

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The helium-assisted lower electrode, shown in Fig 3.3, is fabricated from aluminium. The

electrode is fitted with an integral dark space shield.

The lower electrode is heated by an embedded 1250W element and cooled by liquid nitrogen

flowing through embedded tubing.

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The wafer is lifted clear of the table (15mm) for transferring into a load lock or transfer

chamber by the wafer lift assembly. Compressed air flowing into the air cylinder forces its

piston and plunger upwards. The plunger contacts the base of the bellows which is connected

to a push rod. The 3-pin wafer support, mounted on top of the push rod, rises lifting the

wafer clear of the table. The push rod is lowered by the force exerted by the return spring.

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The wafer clamp comprises a clamping ring attached to a lifting mechanism, and a clamping

plate. The clamping plate, attached to the clamping ring via three pillars and screws,

comprises an aluminium annulus with a quartz circular insert.

The wafer clamp is raised and lowered by two air cylinders, attached to the outside of the

process chamber; one located at each side of the wafer clamp. The piston of each air cylinder

is attached to a push rod, which passes through the base of the process chamber. Within the

process chamber, a circular plate mounted on the top of the push rod, is attached to the

wafer clamp by three M5 setscrews and compression springs. Rotating the setscrews changes

the compression of the springs and consequently the clamping force exerted on the wafer.

See Section 6 for the clamping force adjustment procedure.

Note that clamping plates are available with inserts for various sized wafers. Before loading a

wafer into the process chamber, ensure that the correct clamping plate is fitted. See Section 6

for the clamping plate changeover procedure.

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In some systems, the table top plate is fixed to the table by a ring of cap-head bolts. If your

system has a table of this type, refer to the following text and note.

The bolt heads are concealed behind screw covers, which require a special tool (supplied with

the system) for removal. (Tool part number: MD91D21726.)

NOTES:

1) When re-fixing the table top, do not over-tighten the bolts which will cause

the table top to bow. Check with a straight edge after tightening: if the table

top is not flat, release the bolt tensions until it is.

2) The screw covers (MD91D21723) are aluminium. When removing the covers,

it is recommended to use a little iso-propyl alcohol (IPA) to prevent the

thread from jamming.

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The purpose of helium backing is to set the temperature of the wafer close to that of the

temperature-controlled table by heat transfer. Helium is fed from a number of small holes in

the table underneath the wafer (which is clamped to the table) from where it flows radially

Description

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mä~ëã~ä~Ä póëíÉã=NMM lñÑçêÇ=fåëíêìãÉåíë=mä~ëã~=qÉÅÜåçäçÖó== System Manual

to the periphery of the wafer. Helium is the preferred gas, because it has a very good heat

transfer ability. The use of other gases is possible, preferably inert gases.

The supply of helium is fed by a pressure control device, which receives an analogue setpoint

from the machine’s control system. The pressure control device adjusts the gas flow through

itself to control the pressure at its output side. The pressure is controlled within the range 0

to 50 Torr. A pressure of greater than 20 Torr could damage very thin substrates.

If the wafer is clamped down successfully the chamber pressure will show a slight rise of a few

milliTorr when the helium is producing a pressure of 10 Torr on the wafer.

If there is a massive pressure rise and the Turbo Controller display shows a high load, then the

wafer is insufficiently clamped and in order to achieve the set pressure the controller is using

an excessive gas flow.

The helium pressure is released into the process chamber at the end of a process (using a

normally-open valve). This prevents the wafer moving when it is unclamped.

Tip: Finish a process with a ten-second pumping step without helium. This will

reduce wafer mishandling.

A flow meter in the helium supply also reads the gas flow necessary to maintain the pressure.

A typical process uses 5 - 20 sccm to maintain 10 -15 Torr behind the wafer.

Tip: Some wafers mate very well with the electrode top surface and use less than 2

sccm to maintain 10 Torr. This can give a control problem, with the helium

feeding in pulses. Roughening the aluminium electrode with an abrasive pad

can increase the helium flow by a few sccm and allow proper control.

Do not turn on the helium unless the wafer is clamped.

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THERMOCOUPLE

CHAMBER

BASE

WAFER

CLAMP

TABLE

DARK SPACE

SHIELD

TABLE SUPPORT

TUBE

O RING

PUMPDOWN

PIPE FLANGE

FEEDTHROUGH

COMPONENTS

STAINLESS

STEEL

BELLOWS

SPRING

COMPRESSED

AIR CYLINDER

COMPRESSED

AIR CYLINDER

(ONE EACH SIDE

OF CLAMPING

PLATE)

WAFER

LIFT

ASSEMBLY

(FITS INSIDE

TABLE SUPPORT

TUBE)

Clamping

plate

Clamping

ring

Setscrew &

compression

spring (3-off

on each side

of clamping

ring)

3-pin wafer

support

Quartz

insert

SAFETY RELIEF

VALVE

CONTROL VALVE

LIQUID NITROGEN

OUT

HELIUM IN

Fig 3.3: 94-100-5-12A Cryo / heated -150 / 400C He lower electrode

Description

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