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Some general process trends

This page is supposed to gather some general process trends and good advice for designing IBE recipes. So far these trends has been developed etching Si with resist as masking material and by etching some multilayered films. The work has been done by Kristian Hagsted Rasmussen @ nanolab before 2012

Etch rate

Etch rate	Parameters
increases with	Beam current
increases with	Beam voltage
increases with	Beam current * Beam voltage
not significantly affected by	Stage angle
not significantly affected by	Accelerator voltage

Etch profile

Etch profile (goal 90dg)	Parameters
improves with	Low stage angle (optimum around 5-10 dg)
is effected by	Beam current (low I(B)(400mA) better than high I(B)(600mA))
is effected by	Accelerator voltage * Stage angle
not significantly affected by	Beam voltage

Rotation speed

The rotation is activated to get a good uniformity over the wafer. The minimum number of rotations to get a good uniformity is 100 rotations for the whole etch. the maximum rotation speed is 20 rpm. That means that an etch shorter that 5 min cannot obtain a very good uniformity.

Etch Length [min]	5	6	7	8	9	10	15	20	25	34	50	100
Minimum rotation speed [rpm]	20	17	15	13	12	10	7	5	4	3	2	1

General tips for finding etching parameters (given by Oxford Instruments [1], used with permission)

One thing to remember is that the etch rate varies linearly with beam current while it varies exponentially with beam voltage.

Beam voltage – depends on application, 200-800eV.
Always have neutraliser current 20% extra of the beam current value.
Accelerator voltage between 100V –500V, adjust to get Ia (beam accelerator current) < 10% Ib (bean current) (it helps protect electronics and power supplies).
The real beam current is effectively the beam current minus accelerator current.
Varying Ar flow for source gas as well as beam current will have an effect on the accelerator current.

Arm Tilt affects:

Etch uniformity—the steeper the angle, the better the uniformity. However the etch rate reduces.
Wall angle profile (Argon IBE) theoretical ideal is 11 degrees from vertical. Actual value depends on the design of the grids, i.e. beam divergence etc.

Typical arm angle is 0-12 degrees. The sample is usually rotated to cater for any non-uniformity in the beam profile.

A guide to optimize etch uniformity is as follows:

Set beam energy —a rough guide is, the higher the energy the rougher the surface as seen on an SEM. 500 eV is considered medium energy.
Then starting with beam current of 300mA increase the value until you reach the value where the PR is getting burnt.
Adjust beam current to give optimum etch rate without overheating the sample.
Adjust Accelerator voltage to get uniformity. Note that as this will change beam divergence, the sample temperature will change.

Sample temperature is important because:

Need to keep any resist cool
Temperature can affect chemical processes.