e-beam lithography on transparent substrates: challenges

Vistec Electron Beam • www.vistec-semi.com

Vistec Electron Beam

E-beam lithography on transparent

substrates: challenges and

results

Thomas Händel


Outline

• Motivation

• Material properties

• Experimental samples, process and electron-beam lithography system

• Experimental steps and results

• Conclusion

• Acknowledgement

27.10.2011 Beams & More 2011 page 2


Motivation

Suppliers view

• Large variety of transparent substrates for the semiconductor industry

• Among them are Quartz, Boron Nitride (BN), Gallium Nitride (GaN), Sapphire

(Crystal of Al2O3), Silicon Carbide (SiC) and others

• Check and ensure the functionality of the electron-beam lithography system from the

automatic handling until the exposure of the substrates

• Requests from potential and existing customers

27.10.2011 Beams & More 2011 page 3


Motivation

Customers view

• GaN material of choice for applications in high-power and high-frequency as well as

optoelectronic devices

• Increased product development in the commercial market such as wireless

infrastructure, cable television, satellite and power electronics*

• Development of high-performance GaN-on-SiC power transistors at NXP

Semiconductors**

– Excellent thermal properties of SiC Higher operating temperatures

– High electron drift velocity and large electrical field breakdown of GaN

Increase in extended frequency range, efficiency and power density performance

• Development of High Brightness Light-Emitting Diodes (HB LED’s) at Optogan**

– Large band gap and stimulated emission at room-temperature Enhanced

power efficiency and highest light intensity

* Source: CS Europe, Frankfurt/Main, March 2011, ** Source: Compound Semiconductor Magazine, August/September 2011

27.10.2011 Beams & More 2011 page 4


Outline

• Motivation




• Conclusion

• Acknowledgement

27.10.2011 Beams & More 2011 page 5


Material properties

27.10.2011 Beams & More 2011 page 6

Si GaAs GaN SiC

Density1, g/cm3 2.33 5.32 6.15 3.16…3.21

Dielectric constant1 11.7 12.9..13.1 8.9…9.7 9.66…10.03

Refractive index1, 2 3.42

Opaque

3.3..3.6

Opaque

2.3…2.4

Transparent

2.55…2.59

Transparent

Linear thermal expansion, 10-6 1/K 2.6 5.73..5.8 3.1…5.9 2.77

Resistivity1, cm up to 105 up to 108 up to 1010 up to 1010

Thermal conductivity1, W/cmK 1.31 0.46..0.55 1.3 3.6..4.9

Young’s module1, GPa 62…202 86 105…398 52...553

Current wafer size, mm up to 450 up to 150 up to 100 up to 100 1 at 300K 2 infrared refractive index

- Values vary depending on modification and source


Material properties (Cont.)

27.10.2011 Beams & More 2011 page 7

1,E-03

1,E-02

1,E-01

1,E+00

1,E+01

1,E+02

1,E+03

1,E+04

1,E+05

1,E+06

0,01 0,1 1 10

Ab

sorb

ed e

ner

gy,

eV

/µm

³

Radius, µm

Proximity function for 50keV electrons in 170nm PMMA 950k Monte Carlo Simulation

GaN / SiC

GaAs

Si


Material properties (Cont.)

• Back scattering on GaAs and GaN larger than on Si substrates Individual

proximity correction function required for each substrate material

• Adaption of proximity correction is a default procedure in electron-beam

lithography No special requirements on transparent substrates

27.10.2011 Beams & More 2011 page 8


Outline

• Motivation




• Conclusion

• Acknowledgement

27.10.2011 Beams & More 2011 page 9


Experimental samples

Substrate number Material Diameter, mm Total thickness, mm

#1 GaN on SiC 50 325



#4 SiC 100 400

27.10.2011 Beams & More 2011 page 10

GaN

SiC

2mm

363mm

Substrate #2

SiC

400mm

Substrate #4


Process

Process steps

1. Coating: 170nm PMMA 950k, 180°C, 3min

2. Au evaporation (Anti-charging layer)

3. Exposure

4. Au removal

5. Development: MIBK:IPA=1:1

6. Rinse: IPA

27.10.2011 Beams & More 2011 page 11

GaN

SiC

2mm

363mm

Substrate #2

PMMA 950k

Au

170nm

20nm

SiC

400mm

Substrate #4

PMMA 950k

Au

170nm

20nm


Vistec SB250 Series

27.10.2011 Beams & More 2011 page 12

Cost-effective system for mask and wafer

direct writing

Electron beam shape VSB

Acceleration voltage 50kV

Current density up to 20A/cm²

Maximum substrate size

wafers up to 200mm

masks up to 7 inch

Stage travel range 210 x 210mm²

Laser interferometer / 1024

Minimum feature size < 50nm

25nm (HSQ)

Source: Ferdinand-Braun-Institut für Höchstfrequenztechnik,

Berlin, Germany


Outline

• Motivation




• Conclusion

• Acknowledgement

27.10.2011 Beams & More 2011 page 13


Experimental steps

1. Check and ensure the functionality of

a. Sensors to be used for substrate recognition and tracking

b. Prealigner to be used for substrate alignment

c. Fully automatic substrate handling

d. Electrostatic chuck and height measurement

e. Process

f. Exposure

2. Inspection and measurement

27.10.2011 Beams & More 2011 page 14


Substrate alignment

Fully automated prealignment

Alignment results

• Alignment offset ≤ 10µm

• Alignment angle ≤ 0.4mrad

27.10.2011 Beams & More 2011 page 15

1200

1250

1300

1350

1400

1450

1500

1550

1600

125

150

175

110

0112

5115

0117

5120

0122

5125

0127

5130

0132

5135

0137

5140

0142

5145

0147

5150

0152

5155

0157

51

Encoder counts

CC

D s

igna

l

Main flat

Second flat


Fully automated Shaped Beam Systems

27.10.2011 Beams & More 2011 page 16

Cassette-to-cassette

Material handling

for mask and wafer

substrates

Example: SB250 Series – 200mm platform


Electrostatic chuck and height measurement

Planation of wafer surface

Height mapping difference* between un-chucked and chucked wafer surface on

electrostatic chuck

* Each height mapping interpolated by a 4 degree polynomial

27.10.2011 Beams & More 2011 page 17

Substrate #2 Substrate #4


Exposure results

Core and pad of gates

100nm core in 170nm PMMA 950k

27.10.2011 Beams & More 2011 page 18

Substrate #2

101nm 101nm


Exposure results

Direct Write Overlay Accuracy

Pattern for first exposure run

• Cross

• Cross width 1.4mm

• Cross length 24mm

• Cross step 27mm

Pattern for second exposure run

• Four Angles

• Width 1.4mm

• Distance to cross 2.6mm

• No alignment marks used during exposure

27.10.2011 Beams & More 2011 page 19


Exposure results


Substrate #4

27.10.2011 Beams & More 2011 page 20

17nm 18nm

No alignment marks used during exposure


Exposure results


27.10.2011 Beams & More 2011 page 21

76mm GaAs Wafer

15nm 13nm

No alignment marks used during exposure


Exposure results

Contact holes

27.10.2011 Beams & More 2011 page 22

100nm contact holes in 170nm PMMA 950k

Substrate #2

102nm


Exposure results

Resolution

27.10.2011 Beams & More 2011 page 23

Lines 50nm half pitch in 170nm PMMA 950k

Substrate #2


Outline

• Motivation




• Conclusion

• Acknowledgement

27.10.2011 Beams & More 2011 page 24


Conclusion

• We demonstrated successfully the functionality for Vistec Shaped Beam Systems on

transparent substrates

– Sensor adjustment for recognition of opaque and transparent materials

– Fully automated handling including substrate alignment

– Electrostatic chucking and height measurement

– Exposure performance in the same range as on GaAs substrates

27.10.2011 Beams & More 2011 page 25


Acknowledgement

This presentation had never been done without the cooperation and support of my

colleagues at Vistec Electron Beam GmbH.

Special thanks to Monika Böttcher, Dr. Helder Alves and Thomas Keil

27.10.2011 Beams & More 2011 page 26


Thank you for your attention!

27.10.2011 Beams & More 2011 page 27

e-beam lithography on transparent substrates: challenges

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