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On Wafer Ion Flux Sensors

SFR WorkshopMay 24, 2001Berkeley, CA

Tae Won Kim, Saurabh Ullal, and Eray S AydilUniversity of California Santa Barbara

2001 GOAL: Build and demonstrate Langmuir probe based on wafer ion flux probe array using external electronics by 9/30/2001.

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Motivation and Goals• Variation of ion bombardment flux and its spatial

distribution with plasma conditions is critical to plasma etching.

• Ion flux uniformity at the wafer determines the uniformity of etching and etching profile evolution.

• There have been almost no measurements of the ion flux or ion flux distribution across the wafer as a function of both r and in realistic etching chemistry.

• Design, build, and demonstrate an on-wafer ion flux analyzer with external electronics capable of mapping J+ (r,) on a wafer.

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On Wafer Ion Flux Array

10 probes on 3” wafer Evaporated metal on PECVD SiO2 on Si wafer.

Lines insulated by PECVD SiO2

External Electronics based on National Instruments SCXI platform Lab View Interface

The array is scanned at a rate of 1000 Samples/sec (100 Samples/probe/sec)

Application to monitoring of plasma instabilities and spatiotemporal variation, J+(r,,t), of ion flux in an inductively coupled Ar plasma is shown on the left

t = 0 s t = 1.5 s

t = 3.2 s t = 4.5 s

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On Wafer Ion Flux Measurements in a Cl2 Discharge in a Lam TCP 9400 Reactor

• Goal: extend the measurements to a commercial reactor and realistic chemistry.

Heavily Doped Si wafer (Reference)

Measurement Probe (Biased @ -75V with respect to reference)

Challenging because of lack of ground reference and presence of rf bias.

Probe mounted on 8” Si wafer. External electronics. Ion current determined by measuring the voltage

drop across a known resistance. Both reference and measurement probe are

isolated from ground (using a floating power supply.)

Shown on the left is the ion flux in Cl2 plasma The ion flux increases as a function of exposure

time to Cl2 plasma until it finally saturates. Changes in chamber wall conditions is likely to

be responsible for the drift. Exposure to SF6 plasma resets the chamber back

to reproducible condition. Ion flux behavior as a function of time is

reproducible.0 500 1000 1500 2000 2500

3.0

3.5

4.0

4.5

5.010mTorr/100Cl

2/5Ar/800TCP/0BP

I sat (

mA

/cm

2 )

Time (sec)

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Relation Between the Ion Flux, Gas Phase Composition and Wall Deposits

0 60 120 180

3.0

3.5

4.0

4.510mTorr/100Cl

2/5Ar/800TCP/0BP

I sat (

mA

/cm

2 )

Time (sec)

0 50 100 150 200

0

2

4

6

8

10

12

Inte

grat

ed S

i-O

ab

sorb

ance

Time (sec)

0 20 40 60 80 100 120 140 160

0

5000

10000

SF6 SF

6+O

2

SiCl SiCl Cl

Raw

Em

issi

on I

nte

nsi

ty

Time (sec)

Ion Flux monitored using ion flux probe. SiClx and Cl concentrations monitored using optical emission. Wall deposition monitored using the MTIR-FTIR probe. Oxygen plasma oxidizes the surface of the wafer and probe. Cl2 plasma (no bias power) etches the oxide layer slowly compared to the Si. Drift in Ion Flux is due to changing wall conditions and plasma composition.

Ion Flux Cl & SiClx SiO2 on the Walls

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Scale the single probe idea to an array of sensors on 8” wafers: 200 mm on-wafer ion flux sensors

21 pads + 1 reference (heavily doped Si-wafer) on 200 mm wafer. Printed circuit board (PCB) manufacturing methods were used to

pattern the pads and the leads on the machined polyimid board. Pads are nickel plated to reduce reaction with Cl2 plasma. The leads from the pads are covered by a kapton mask. The wafer, polyimide and the kapton stack is bonded together using

vacuum epoxy Heavily doped silicon region is exposed to the plasma and is etched.

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200 mm on wafer ion flux sensors

Polyimid Circuit BoardKapton Insulation Mask

Assembled Probe Array

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Ion Flux Uniformity as a Function of Power in a Lam TCP Reactor

• Ion flux uniformity was measured in a LAM TCP 9400 reactor to demonstrate the probe operation.

• Operating condition; 50 sccm He, 30 mTorr.

100 W 200 W 300 W

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Summary

2002 and 2003 Milestones

Build and demonstrate 8” on-wafer ion flux probe array in industrial plasma etcher with external electronics by 9/30/2002.

Integration of Si-based IC with sensor arrays. Characterize and test integrated MEMS ion sensor array. 9/30/2003.

Designed and build 3” and 8” on-wafer ion flux probe arrays with external electronics and demonstrated their operation.

Completed preliminary experiments in a commercial reactor and demonstrated the effect of plasma composition and wall conditions on the ion flux.

Specific 2002 and 2003 Goals Modify data acquisition to be used with rf bias on the electrostatic chuck. Use the probe array to study the factors that affect the plasma and etching

uniformity in Cl2/O2 etching of Si.

Specifically, the goal will be to understand the role of etching products, wall conditions, and the feed gas composition on ion flux uniformity.