Utah-94-721002-System-Manual.pdf - 第150页

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For an ICP 180 or ICP 380 the typical process operating ranges are:

Total gas flows = 10 to 200sccm. The maximum flow depends on type of turbo pump, i.e. its maximum

flow capacity, and the required operating pressure. If you need to use a low pressure, you may have to

limit the flow rate to achieve this.

Pressure = 1 to 60mTorr. Below 5mTorr and above 20mTorr the plasma may not strike easily (or with

sufficient stability) for certain gases and power levels, so you need to check this and adjust the process

accordingly, since operating the system without a plasma either on the substrate electrode or in the ICP

tube could cause damage. This is because it is likely to cause a high reflected power, or dumping of power

into matching unit. It is always essential to check for a plasma in both regions. You can use the ‘low

pressure strike’ feature in the software to allow easier striking for low pressure processes. For certain

flow/pressure combinations, the pressure controller may have difficulty in maintaining a constant

pressure, therefore this may also be a determining factor in the flow/pressure used.

ICP power = approximately 200W to 2500W (or 4000W for ICP 380). The minimum power level will be

dependent on how easily the plasma strikes for certain gases. You will need to check this and adjust

process accordingly, since operating system without a plasma either on the substrate electrode or in the

ICP tube could cause damage.

The maximum ICP power limit is set by the power rating of the RF generator. However, most processes

perform well with only moderate ICP power levels. This also helps to avoid excessive substrate heating.

Substrate electrode RF power = typically 5W to 400W. A plasma may not strike easily for low power

levels for certain gases. You will need to check this and adjust the process accordingly, since operating the

system without a plasma either on the substrate electrode or in the ICP tube could cause damage.

Helium pressure = 0 to 30Torr. Depends on the cooling efficiency required (some processes benefit from

no cooling) and the maximum tolerable helium leakage.

Temperature is limited by the operating range of the electrode or its heater/chiller, depending on type

of electrode or heater/chiller used.

NOTES:

(A) The system base pressure will be of approaching 10

-6

Torr or better when measured using the

Penning gauge. However, the time taken to reach this pressure will depend on whether the

chamber has recently been vented to atmosphere and the cleanliness of the chamber walls. If the

process chamber / electrodes are anodised, the time will increase as the anodised surfaces will take

longer to outgas compared with bare metal surfaces.

(B) Operating with chlorine based processes can cause damage to the electrode unless it is protected

with a dummy wafer.

damage to the matching unit or RF generator. To reduce the high-reflected power, adjust the

process parameters or re-tune the matching unit.

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"Low pressure strike" is a necessary software facility, since for low pressure RIE or ICP processes, the

concentration of free electrons being produced simply isn't high enough to start and maintain the

ionisation 'chain reaction' or avalanche which is required to initiate the plasma. So it is necessary to use

higher pressures during the first few seconds of the process step. The software therefore allows the

operator to raise the pressure briefly at the start of the plasma process to enable the plasma to strike.

This does not cause any problems for the processes involved, because the time taken to strike the plasma

at the higher pressure is very short, and will be a very small percentage of the total process time.

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With any plasma etch system it is important to remember that DC bias readings can be affected by surface

coatings on the lower electrode. These can include:

(a) Quartz cover plate (or any other insulating cover plate material),

(b) Electrode surface anodisation,

(d) Any other insulating coating generated by plasma.

In all of these cases, the DC bias reading will be inaccurate due to the lack of DC contact to the plasma.

Quite often, the measured DC bias will be close to zero if there is a complete insulation of the electrode

or if the process conditions are such that there is minimal contact between the exposed areas of the

electrode and the plasma.

This is not to say that the sheath potential (or ion energy) has actually reduced to zero; in fact it has not

changed at all from the non-insulated case. It is simply that the measurement of this value via DC bias is

no longer possible. It is also quite common in these cases for the DC bias reading to vary sharply

throughout the run.

It is therefore recommended that the DC bias for a particular process condition be measured with a bare

electrode (e.g. prior to loading the quartz cover plate). In load locked single wafer systems, it is necessary

to measure the DC bias without a wafer loaded, as this will expose the central wafer lift pin to provide

accurate DC bias measurement.

This is the only way of obtaining a reliable/stable DC bias measurement.

If the electrode is anodised, it may also be necessary to ensure that the wafer lift pin is exposed and clean

as this will be the only conductive path for the DC bias measurement. If there is no wafer lift pin (e.g. RIE

80 Plus), it may be necessary to use the central locating pin for the DC bias measurement or even to

scratch away a small area of anodisation.

If it is suspected that there has been polymer deposition on the electrode, it may be necessary to clean off

the polymer (with an O

plasma or by mechanical cleaning) to allow an accurate measurement of the DC

bias.

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Strongly electronegative mixtures, such as SF

gas above 10 Pa (70 mTorr), may give close-to-zero DC bias.

This is not a fault, but is due to the formation of negative ions in the plasma. A DC bias exists only

because of the difference in mobility between the negative and positive charges in a normal plasma.

When both charge carriers are heavy ions, the plasma does not rectify and the dc bias collapses.

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The DC bias on the lower electrode can be a strong function of the power in any auxiliary plasma source,

for a fixed lower electrode RF power. At low ICP power, there can be a rise in DC bias reading, because of

the increase in the effective area of the grounded electrode. As the ICP power rises, the DC bias is

reduced, as ions from the source begin to dominate the ion flux at the electrode. This reduction in DC bias

is a good sign of plasma from the source reaching the lower electrode.

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The surface of the RF driven electrode takes a negative bias with respect to the plasma. However, the

literature and the industry tend to refer to DC bias as a positive quantity, and we follow this convention

in our equipment.

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The DC bias value will depend on all the process parameters and several aspects of the machine condition.

When the DC bias value is important to the correct operation of the process, it is often possible to use DC

bias as the recipe parameter, and have the RF bias power as a free parameter. The control mode (power

or bias) is selectable on the PC screen where this feature has been provided. However, if you are planning

to use DC bias control mode, it is worth noting the points raised in the preceding sub-sections about

potential causes of inaccuracies / variability in DC bias readings.

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DC bias is a very sensitive indicator of the state of the plasma tool. While this makes it a useful parameter

to measure and record, it also makes it difficult to ensure that the value is consistent from day-to-day on

the same machine, and between nominally identical systems.

It is occasionally requested that the DC bias reading is adjusted to make the reading the same across

different tools, but we have taken the view that it is better to know the actual value.

The main hardware causes for DC bias changes are:

(a) Electrical conductivity of the cooling medium for the electrode or automatch. Check this

by running

briefly with the cooling fluid removed completely. Shifts in DC bias between

dry and cooled states of up to 5% are common. A shift of more than 10% indicates the

fluid is too conducting.

(b) Inadequate cooling of a fluid-cooled automatch. This is sometimes linked with fluid

conductivity, with an electrochemical reaction depositing material in the stainless steel

bulkhead fitting of the automatch.

changes and the DC bias value is altered

(d) Oxidation of components in the RF delivery path. The connection between the automatch

and the electrode carries several amperes of RF current. The connection must be sound, or

it can become heated, with a progressive addition to the losses

(e) Loss of the ground path. RF current driving the plasma flows in a closed loop circuit. High

resistance or breaks in the ground return path will alter the DC bias – but usually manifest

themselves first as RF interference problems. Pay particular attention to any straps

securing the dark space screen, the RF shielding under the lower electrode, and the

mounting of the automatch.

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