Electron beam exposure of diazoquinone based resists

579

MicroelectronicEngineering 9(1989)579-583 North-Holland

ELECTRON

BEAM EXPOSURE

OF DIAZOQUINONE

WOLFGANG

A. KR;ziUTER, HUMBERT

BASED

RESISTS

M. NOLL

Austria Mikro Systeme International A-8141 Unterpremstgtten, AUSTRIA

(AMS) GmbH

The resist process parameters and the process limits for the electron beam exposure of the three optical resists MP1350, ~~5200 and S1400 and of the electron resist RE5000P are evaluated. The influence of the electron dose, the beam diameter, the and the number of exposure passes on resist thickness, the developer concentration the contrast and the control of the exposure behaviour e.g. the sensitivity, critical dimensions are discussed. An upper dose limit for single pass Ebeam exposure is determined by radiation damaqe which has been found to occur at beam currents above 1600nA. For 5000 w thick resist films a lower exposure dose limit of 7 PC/cm2 is determined. At this dose about 20% of the resist is removed during development. A possibility to decrease the required exposure dose to 3 kc/cm2 by resist thickness optimization is proposed. At doses between 8 and 12 PC/cm' the contrast reaches a peak value of 30. The influence of this high contrast on critical dimension control is discussed in detail.

1.

INTRODUCTION

Optical resists have been used at AMS for the Ebeam generation of 10x and 5x retitles on a production basis. The demands for tighter CD control and better imaqe edge quality on lx masks required a further optimization of the resist process. The main idea of using optical resist instead of electron resist for electron exposure arises from the fact, that the development process of the optical resist is better controllable, is more simple, does not require harmful organic is more time efficient and chemicals, generates less defects (1, 2, 6). The disadvantage using optical resists reflects in a much higher exposure dose, that requires either multiple electron beam passes, or special EBEAM systems, which are provided with a so called 'hiql current option'. This paper compares the electron lithographical characteristics between the three optical resists MPl350, ~Z5200, S1400 and the electron resist RE5OOOP. All these resists are based on novolac resin and can be developed in the same aqueous developer. 2.

EXPERIMENTAL

The resist coatings which were investigated are prebaked at 90 deq C for 30 minutes. The substrate was qlass, coated 0167.9317/89/$3.50 0 1989, Elsevier Science Publishers B.V. (North-Holland)

with

low reflective

chrome.

The exposure was performed on a VARIAN VLS 20, high current option EBEAM system using a 10 keV electron beam at a rate of 20 MHz. With this system beam currents of 3000nA and 200nA can be achieved at beam diameters of lum and 0.5pm, respectively. The development was done at a temperature of 21 deg C by an immersion method using the AZ351 developer, which was diluted with d-i. water for achieving the different normalities. The sensitivity was measured by visual observation of the development using big exposed fields. The contrast was determined by calculation from thickness monitor data and sensitivity curves ( Fig.1 ). For the CD measurements the chrome was etched on an APT914 spray processing system using CR5 chrome etchant supplied by KTI. After resist stripping the CDs were measured on a LEITZ MPV CD microscope. 3. 3.1.

RESULTS Sensitivity

Figure 1 shows the dependence of the sensitivity Eo for resist films at different developer normalities N. The arrows indicate the development times, where 20% of the resist are removed in

W.A. Krci’uter, H.M. NON/ E-beam exposure of diazoquinone

580

the unexposed

3.2.

area.

Note, that the curves for the optical resists approach assymptotically the time to ( to is the time, where the whole 'resist is removed in the unexposed area ). This indicates a low differential solubility DS, which is defined as : developmentrate

DS =

developmentrate

in exposed

area

in unexposed

area

based resists

Contrast

By definition the contrast , is the negative value of the slope of the 'characteristic curve' at the point, where the resist thickness goes to zero The 'characteristic curve' plots the relative resist thickness remaining after the development versus the logarithm of the dose. Therefore -; is given by

The curves of the RESOOOP resist approach assymptotically the threshold exposure dose, indicating a very high DS.

Y

=

az

-~

*_

a(lgEjZ

=

1

(El)

d max

Zd

where E is the exposure dose, zd is the initial resist thickness and d,,, is the remaining resist thickness in the unexposed area.

lo3

lo2 .5N .6N 10'

I

1 AZ 5200

I

In order to calculate y , a relation to the sensitivity Eo(td) and to the solubility has to be found. An exposure dose E causes an increase of the solubility rate of the resist during development. Due to the electron absorption and the solubility change with increasing resist depth z, this solubility becomes a function of E and Z.

In order to determine this function, the thickness monitor measurements during the development have to be considered. After the time t, the resist is developed to the depth z. This behaviour which was found experimentally, can be fitted as follows :

t

= f(E) * zL

(E2)

where f(E) is independent from z and z=O represents the resist surface. The results of the experiments show, that Y is constant within the considered exposure dose range. Q only varies with the resist type and the developer concentration. RE5000P

As f(E) is independent from z, it can be determined at any value of z, for example by the sensitivity curve (Fig.1) using equation (E2)

f(E) Equation

E, 1 PC/cm21 Figure

1

-l/,i _ -

-1 /Il. Zd

*

td

(E2) can be written

z = f(E)

as

-l/r * t+l/Li

(E3)

581

W.A.Krtiuter, H.M. No11 / E-beam exposure of diazoquinone based resists Substituting gives

-td+l Y=

equation

/a *

(E3) into

MP1350 0.5N

.AA . 8-t . . ? . 'r

a(f(E)-"")

a (IgE)

d max

(El)

(E4) Z=Zd

multiple

h A& *

.‘; .

passes A

The contrast resulting from equation (E4) is plotted in figure 2 for a 5000 A thick MP1350 resist film.

0

1

/

I 0

50

t,

100

[secl

c t-

Figure

2

In order to find the nature of the 'radiation damage', the effect of the beam diameter on the sensitivity at 2000nA has been investigated (Fig. 5).

E z

00

0.6 N 0

’ 0

t, Figure 3.3.

1 200

1 100

300

40 -

[secl

l

00

l l

0

l

.*

z z+ 30 -

2

+" Radiation

4

20-

MP 1350

Damage

Figure 3 demonstrates the difference bet. ween multiple passoand single pass exposure for a 5000 A thick film of MPl350 developed in 0.5N AZ351 developer.

2000

10 0

I

I

I

1

1.5

2

BEAMDIAMETER MP1350 0.5N

nA

Figure

[pm]

5

The graph shows, that the effect does not change by defocussing the beam up to 2 pm. In Ref. (5) it is shown, that a 10 mW focussed laser beam absorbed on a metal coated glass substrate causes a temperature distribution which is independent from the focus diameter and can reach several 100 deg C.

E, 1 PC/cm21 Figure

3

At beam currents above 1600nA the single pass curve drops. This results in a very high contrast value of 30 (Fig. 4). The S1400 and AZ5200 show the same behaviour.

that an electron beam of Considering, 2000nA represents a beam power of 20mW, we have to expect, that the temperatures at the electron exposure reach similar values and cause thermal degradation of the novolac resin, which is known to start above 150 deg C (3). These facts are hints, that the exposure above 2000nA is rather determined by thermal degradation than by the usual photochemical reaction.

W.A. Krtiuter, H.M. Nell / E-beam exposure of diazoquinone

582

3.4.

CD Control

Figure 6 shows the CD variation versus beam defocussing for the MPl350 resist for different contrast values at a constant pixel size of 1 pm. The different contrast values are achieved by different exposures and development processes.

based resists

MPl340 resist. The sensitivities, where 4000 A of the resist is remaining in unexposed areas are 1 PC/cm' and 4 KC/cmz for the 10000 fi and 5000 d ( indicated by thick film, respectively the arrows in Figure 7 ). Therefore it is possible to reduce the necessary exposure dose by increasing the resist thickness.

_

500

:T

.I.

MP1350

r:4

_

.

.

8

a

MP1350

.

.I r=14

1

BEAMDIAMETER

Figure

6

I

I,,,,,

10

Figure

of using

higher

The differential solubility becomes times are higher. Longer development possible without impact on defect control.

elec2. The influence of backscattered trons decreases. The ratio of primary to back?cattered electrons is 1.3 for a 5000 A thigk resist layer and 2.8 for a 10000 A thick resist layer (4). Figure 7 shows sist thickness

I

4.

7

CONCLUSIONS

The evaluation of the Ebeam exposure of the different photoresists has shown that for each resist an optimum developer concentration can be found, where similar results for sensitivity, contrast and defect control can be achieved. An upper limit of 1600nA beam current independent from the beam diameter has been found. Experiments give hints, that at higher beam currents the exposure effect is dominated by thermal degradation of the novolac resin due to substrate heating by the absorbed beam.

Optimization

There are two advantages resist thicknesses : 1.

I

E, 1 yC/cm21

the roughness of the horiFurthermore, zontal edges increases with Y. When Y exceeds a value of 10, the differences between horizontal and vertical CD bars are not adjustable any more by the beam blanking electronics ( at 1 Urn pixelsize the difference can be as high as 0.6 urn).

Thickness

1

1

[pm1

The graph shows that high contrast values cause a strong CD dependence on the beam diameter. Therefore the CD control at high '{valuesis limited by the ability to control the beam focus and the astigmatism. ( CD control over 1 plate : 3s = 0.4 pm at Y =I4 and 3s = 0.1 urn at y=4 )

3.5. Resist

L

3

2

the influence of the reon the sensitiviy of the

The lower limits of the exposure have been determined by the condition that at least 4000 .& of the resist must be left in the unexposed areas in order to maintain reasonable defect control. The achieved sensitivities are still in the range, which can only be achieved by high current option EBEAM systems or by multiple pass exposure. The increase of the resist thickness yields in a higher differential solubility and indicates the possibility to decrease the necessary exposure dose, although further experiments have to be performed in order to optimize reso-

W.A. Kriiuter, H.M. Noll / E-beam exposure of diazoquinone

lution

and CD control.

J. Shaw et al., 'Polysiloxanes for Optical Lithography' Solid State Tech. Vo1.30/6 June 1987 , pp 83 - 89

(4)

G. Brewer, 'Electron Beam Technology in Microelectronic Fabrication' Academic Press 1980

(5)

F. Petzoldt et al., 'Lateral Growth Rates in Laser CVD of Microstructures' Appl. Phys. Lett. A35 1984 pp 155 - 159

(6)

M. Kaplan and D. Meyerhofer, 'Response of Diazoquinone Resists to optical and Electron Beam Exposure' RCA Review Vol. 40 June 1979 pp 166 - 190

REFERENCES (I)

(2)

Mirghani Widat-alla et al., 'Submicron Ebeam Process Control; PBS and AZ5206' Proceedings Kodak Interface 1987 J.M. Shaw and M. Hatzakis, 'Performance Characteristics of DiazoType Photoresists under Ebeam and Optical Exposure' Proceedings Kodak Interface 1977

583

(3)

ACKNOWLEDGEMENTS The authors would like to thank Michael Strmsek for his support at the experiments. We also whish to thank Mary Lou Meloni from VARIAN SEG and HOYA Corp. for supplying chrome blanks with different coatings.

based resists