Solution to the E-beam Gate Resist BlisteringProblem of 0.15 micron PHEMTs

Y.C. Pao, K. Tran, C. Shih, N. HardyFiltronic Solid State, Inc., 3251 Olcott Street, Santa Clara, CA 95054

Copyright © 1999 GaAs Mantech

AbstractIn this paper we report a simple solution to the E-beam

gate resist blistering problem encountered during the

Ti/Pt/Au metallization. This has been one of the major

yield concerns when manufacturing quarter micron or sub

quarter micron PHEMTs. A free space screening method

has been successfully used to eliminate the PMMA based

E-beam resist blisters during the gate metal deposition

process.

Introduction

It is common for PMMA based photoresist, used for E-

beam direct write processing, to blister during the e-beam

metal evaporation of Ti/Pt/Au based 0.15 and 0.25 micron

gate. This problem has been speculated [1] to be due to X-

ray irradiation, substrate heating, or free electron

bombardment. In this study, we will show evidence that the

resist blistering problem is mainly due to the excess

electron charging effect during refractory metal gate

evaporation. This free electron charging affects the PMMA

resist chemistry, causing gas outburst to make the resist

blistered. Experiments were carried out with wafers coated

with PMMA on the bottom and MMA(17.5)MAA EL on

the top. All metal evaporations were done in a commercial

Temescal E-beam evaporator. The gate metal consists of

three layers: titanium, platinum, and gold with a total

thickness ranging from 3500 to 5000 angstroms. From the

test results, it has been demonstrated that the evaporation

of the platinum layer is the source of excess electrons

leading to resist charging, hence causing chemical reaction

in the resist and evolving chemically reacted gas. The gas

trapped under the titanium layer caused bubbles and

eventually blistered the metal and resist layers.

A simple screening apparatus has been developed and

implemented to eliminate the electron charging effect

during the E-beam gate metallization. Details of the

experiment and the construction of the screening apparatus

will be discussed in this paper.

Experiments and results

The photoresist blistering experiment was carried out

in two different Temescal models of E-beam evaporators

with standard gate metal composition of Ti/Pt/Au. Both

systems are equipped with the same 10 KV e-guns. All

experiments were conducted with fixed metal deposition

rates of 3 angstroms/sec for titanium, 2 angstroms/sec for

platinum, and 10 angstroms/sec for gold.

The wafers are first coated with PMMA on the bottom

and MMA(17.5)MAA EL on the top. After E-beam direct

write exposure to form the gate patterns, the resist was

then developed and baked to form the desired cross

sectional profile. After the selective gate recess etch, the

wafer was loaded into the e-beam evaporation system.

Typical operation pressure is below 1E-6 torr. After the

Ti/Pt/Au metal evaporation, resist peeling at different

levels of scale has been observed. Figure 1 shows a 25X

microscopic photo of severe resist peeling after gate metal

deposition.

Figure 1. Microscopy picture (25X) of PMMA based E-beam resist peeling after evaporation ofTi/Pt/Au metallization.

It is speculated that the resist peeling may be caused

by thermal, electron or x-ray radiation [1]. Due to the fact

that the metal evaporation is performed with a high voltage

e-gun, it is most likely the radiation does come from the

free electrons inside the e-beam evaporation chamber.

A simple experiment of installing a free electron

probe, half way between the e-gun and the wafer, inside the

e-beam evaporator was made to monitor the free electron

signal during the metal evaporation. Figure 2(a) and 2(b)

show the free electron activities during the stages of

platinum and gold (the same as titanium) heat-up by a

9KV e-gun source, respectively. It is clear that the

evaporation of platinum generates an order of magnitude

higher free electron signal (i.e., free electron current) than

other two metals used in the gate metallization scheme. At

the same time, an observation was made that for any metal

deposition, the resist did not blister or peel if platinum was

absent.

Figure 2. Free electron signal (i.e., current) generatedinside the chamber during platinum (a) andgold (b) evaporation, respectively.

In order to verify the electron radiation effect, an

experiment of coating three wafers with a gold finger to

ground the metal film deposited on the photoresist was

carried out. Figure 3 shows the experimental arrangement.

The gold finger made electrical contact to the grounded

wafer holding ring, hence providing a ground path to the

metal films deposited on the wafer so the electron radiation

does not accumulate charges on the insulating photoresist.

These wafers were then deposited with Ti/Pt/Au metal

films with known parameters which otherwise cause resist

blistering or peeling. None of the gold finger grounded

wafers showed the problem.

Figure 3. Wafer coated with small gold finger toprovide electrical grounding of e-beamevaporated metal films.

From these test results, it is clear that the evaporation of

platinum layer is the main source of excessive electrons for

resist charging and blistering. In order to find a permanent

solution to solve this problem, a simple apparatus of free

electron retarding screen was constructed to repel free

electrons, generated during platinum evaporation, from

reaching the photoresist coated wafers. Figure 4 shows the

schematic diagram of the improved e-beam evaporation

setup. The curtain like screen is biased at negative voltage

(from –30 to –70 Volts were used) to reduce the number of

free electrons reaching to the wafers. As the result of

applying this improved evaporation arrangement, no

wafers have been lost due to the resist blistering or peeling

Coated Au fingeron photoresist

Wafer

Grounded wafer holding ring

problems during the Ti/Pt/Au based 0.15 and 0.25 micron

PHEMT gate metallization.

Figure 4. Schematic diagram of the free electronretarding screen used during the e-beamevaporation of Ti/Pt/Au based metallization.

Conclusion

The problem of PMMA based E-beam resist blisters

during the e-beam gate metallization has been studied. The

problem is mainly due to excess electron charging effect

during refractory metal (or to be more specific the

platinum) gate evaporation. The free electron charging

effect during the platinum deposition affects the PMMA

resist chemistry, causing reacted gas to outburst under

vacuum. Since the gas is trapped under the titanium and

platinum layers, this causes bubbles and eventually breaks

through the metal and resist layers to create resist peeling,

which is shown in Figure 1.

From experimental results, it is further demonstrated

that there are two possible ways to prevent the E-beam

photoresist blistering and peeling problem; one is to

ground the coated wafer surface, the other is to repel free

electrons before they reach the processed wafers. A simple

screening apparatus has been developed and implemented

to eliminate the electron charging effect during the E-beam

gate metallization. This method has been proven to be very

effective to solve the E-beam photoresist blistering and

peeling problem.

Acknowledgments

The authors wish to thank J. Franklin, S. Ratcliff, and

T. Mirra for useful technical discussions.

References

[1] D. Kuhn, K. Prutt, A. Kashiwagi, J. Peterson, K. Yamamoto,L. Studebaker, S. Lafrancois, N. Caldwell, “E-beam MetalEvaporation: Photoresist Critical Dimensions & SubstrateAdhesion Dependence on Throw Distance, Deposition Rate,Radiation, and Film Stress”, Proceeding of 1998 MANTECHConference, p.213, 1998.

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