Spin-on dry ETCH ARC process for submicron lithography

Microelectronic Engineering 6 (1987) 8 5 - 9 0 North-Holland

85

SPIN-ON DRY ETCH ARC PROCESS FOR SUBMICRON LITHOGRAPHY James E. Lamb III, Donna D. Hawely and J. Michael Mori Process Development Group, Brewer Science, Inc. P.O. Box GG Rolla, Missouri USA 65401 1.

INTRODUCTION

With optical lithography's extension to the submicron region techniques for improving linewidth control and eliminating both standing waves and reflective notching phenomena are mandatory. One of the most effective means for solving these problems has been the use of AR (anti-reflective) coatings beneath the resist. Both organic and inorganic AR coatings are evaluated for their performance and ability to control linewidths with varying resist thickness. Results indicate a vast improvement over both resist and dyed resist. Further the work shows progress in organic AR materials that have the best properties of both the inorganic and organic AR coating systems. Currently, the two major technologies for producing AR coatings are dyed spin-on polymers and deposited organic films. Each of the systems have different advantages and disadvantages. For instance, the deposited AR coatings require special deposition equipment and in some cases specialized plasma etching equipment to remove the film after processing. This makes deposited AR coatings very capital intensive. Another cdtical problem is the tight thickness control that must be maintained because most of the reported systems achieve anti-reflective properties by destructive interference. This thickness variation must be held between +/- 50 A. However, once the film is deposited it does not chemically interact with the resist processing step. On the other hand, spinon organic coatings are relatively ease to apply with standard spin coat equipment. With the organic AR coating being etched during the photoresist development, a processes step is eliminated at the expense of adding another processing parameter requiring tight control. Cure temperatures for the films must be controlled tightly and due to the isotropic etching of the film the materials use in the sub micron region is limited. In this paper, results will be obtained from an organic anti-reflective coating* that is applied and removed with simple processing as in previous organic coatings while not chemical interacting with the resist processing as in inorganic coatings. The critical parameter for this new material will be its oxygen plasma etch rate. It will be necessary to either have the organic AR coating have a higher etch rate or remain a very thin film. 2.

DATA AND DISCUSSION

2.1 Linewidth Variations as a Function of Resist Thickness As resist thickness increases, the imaged linewidths increase for a constant exposure dose. In addition when standing waves are present the linewidths vary in a cyclic pattern. These effects make linewidth control increasingly difficult even over small steps. These functions will be evaluated by using Macks Prolith modeling program [1]. Standard resist+, dyed resist^ and both sputtered silicon and organic AR coatings will be compared. For each of the four processes the optimum conditions for producing a one micron line in one micron of resist was determined with Prolith. Then modeling tuns were performed at a set of resist thicknesses that corresponded to minimum and maximum standing wave intensities while all other Prolith parameters were held constant. Resist thicknesses were determined by taking multiples of Equation 1 as reported by R. Coyne and T. Brewer et.al.[2]. exposure waveleneth 4 x index of refraction

= standing wave multiple

Equation 1

An index of refraction of 1.68 was used for the resist with 436 nm (g-line) exposures. The thicknesses used for all process studied were 0.7785, 0.8433, 0.9081, 0.9729, 1.0000, 1.0377, 1.1025, 1.1673 and 1.2321 microns. To use the Prolith modeling program with the AR layers the thickness as well as the real and complex index of refraction was needed for each film. For the organic AR coating the thickness was 3000 A, while for the sputered silicon the optimum thickness had to be determined from Equation 1. Sputtered silicon needs a quarter wavelength in thickness for optimum absorbance. Using Equation 1 with an index of refraction of 3.53 - il.07, reported by K. Polasko and B. Griffing et.al.[3], gave a thickness of 310 A[4]. The refractive index for the organic AR coating was determined by ellipsometry and was

0167-9317/87/$3.50 © 1987, Elsevier Science Publishers B.V. (North-Holland)

,I.E. Lamb III et al. / Spin-on dry etch arc process

86

found to be 1.674 - i0.3036 for g-line exposures. See Figure 1 for description of how values were taken from Prolith plots to be used in a plot of linewidth variation versus resist thickness.

¢-

£

/-

O

Minimum Linewidth

vE oO cO 1.1J Z

.6

0 "I" Il,cO oO 1.1J 13C

.3

.4

~

.2

Base Linewidth

Resist Foot

I

0

0

I

i

200

400

-,

I

i

600

800

LINEWlDTH FROM CENTER (nm)

FIGURE 1 Method for calculation of minimum linewidth, base linewidth and resist foot.

See Figure 2 through Figure 5 for graphs of linewidth variation as a function of resist thickness. The two AR coatings make drastic improvements over the single layer resist systems. With the high absorbance of the organic film, at 436 rim, producing the overall best results.

PROLITH SIMULATION -- PROJECTION PRINTING -SUBSTRATE MATERIAL = ALUMINUM PHOTORESIST = MP1400 RESIST THICKNESS (um) = 1 (b z~

O O

0.80 0.70 "'

~" 0.60 0

L

t,

J

0.50

~O 12111 .......i...,.: :.....i..... ~ : ..........~ i------l--i--- ~ u_ 0.40 "" Y ~ ~ ~ .L-.-0.30 "" ....... i ................. 7 ~< i ! ! i ! -u} 0"200.7 ~ 0.8 0.9 1.0 1,1 1.2 1.3 INITIAL RESISTTHICKNESS(MICRONS) . . . . . . . . . . . .

LINEWlDTH

,5

Base Linewidth

Optimum Distance For 1 Micron Resist MinimumUnewidth

FROM CENTER

FIGURE 2 Single layer resist process: a) profile for one micron resist and b) variations in linewidth due to resist thickness variations.

J.E. Lamb III et aL / Spin-on dry etch arc process

87

PROLITH SIMULATION -- PROJECTION PRINTING -SUBSTRATE MATERIAL = ALUMINUM PHOTORESIST = MP1400D1 RESIST THICKNESS (um) = 1 ~. (b -

~

~

0.80

}

!

0.70 ....... ii...-..~! . .

~

. . . . . . . . . . . . .

~N=

,

~ .......

~

J

''~ /

+ .......

.....'¢-- ,..J

080

T H

/ResistFoot

,---

O. i i i ~ "~ 0 40 .............. "~............. ~"............. i......i ------i............. ~........

I

C K N E S S

i 0 30

~

~

Base Linewidth

I

|

.......

i

='"'"':

0 20 /

"

07

i

..............

!

i

oa

~

i.._ ............

iJ"~.:

.-,'-~ . ~

i

!

09

10

-~F~. Optimum Distance For 1 Micron Resist

i

................

j

I

i

it

~

12

Minimum Linewidth

13

INITIAL RESIST THICKNESS (MICRONS)

o

LINEWIDTH

,5

P

i

FROM CENTER

FIGURE 3 Dyed resist process: a) profile for one micron resist and b) variations in linewidth due to resist thickness variations.

PROLITH SIMULATION -- PROJECTION PRINTING -SUBSTRATE MATERIAL = ALUMINUM LAYER1 THICKNESS (um) = .031 PHOTORESIST = MP1400 RESIST THICKNESS (um) = 1

(a

~.

(b

o

0.80

o_

i

i

~

0.7o

~

0.60

Z a

i

/

/

0.50

...._ ....,..~-.~ _,,~

0.40

~

J

~,,. -""

0.30 0.8

O.g

1.0

1.1

Base Unewidth

4=.-----

Optimum Distance For 1 Micron Resist

, E

Minimum Linewidth

i

0.7

Resist Foot

1.2

\ 1.3

INITIAL RESIST THICKNESS (MICRONS) LINEWIDTH

.5

'

'

FROM CENTER

'1

FIGURE 4 Resist o v e r silicon AR coating: a) profile for one micron resist and b) variations in linewidth due to resist thickness variations.

J.E. Lamb III et al. / Spin-on dry etch arc process

88

PROLITH SIMULATION -- PROJECTION PRINTING -SUBSTRATE MATERIAL = ALUMINUM

1'

(a

R E

LAYER1 T H I C K N E S S (urn)

=

PHOTORESIST RESIST THICKNESS (urn)

= MP1400 = 1

"~

~

(b .

.

.

.

.

.3

.

0.70 ............. ............. .......................... .............i

S T

,,=,

0.60

.

!

~

~

i ;~

.

...

Resist FOOt

!

,.L

Base Linewidth

T

0.50 c K N E s

)

S 0

O

0"40t

o

0.30

-

4'

~ J.TL..~'ilIII~~

"~r"~"OptimumDistance

~

. . . . . . . . . . . . . . .

For, Micron Resist

i

__. 0.201 l ! I ! I l l i l ! , ~ 0.7 0.8 0.9 1.0 1.1 1.2 1.3

~

Minimum Linewidth

ESIST THICKNESS (MICRONS)

LINEWIDTH

•5

FROM C E N T E R

1

FIGURE 5 Resist over organic AR coating: a) profile for one micron resist and b) variations in linewidth due to resist thickness variations.

2.2. Resist Profile Variation as a Function AR Coating Thickness Prolith was used to produce resist profiles with varying thicknesses of the AR coatings studied. For the organic AR coating thickness can vary with step heights and spacing. On 1 to 1.5 micron topography the organic material will vary between 1500 A on top of steps to approximately 4500 A between steps. Thus the thicknesses of organic AR coatings included in profile comparison were 1500 A, 3000 A, and 4500A. For the sputtered silicon case _+50A was used. See Figure 6(a) and 6(b) for resist profile comparisons. A much larger thickness variation can be accepted for the organic coating as well. However, both materials give a large improvement over the single resist and dyed resist processes. 1000

(a ~

800" o3 o3 uJ Zv 600" :

4O0

(b

¢E" 800"

v

l j

31oA

~ 6oo

.....,,..~

260A

~

_.9. I-.-

1.! i ~

360 A

400'

j j ~ 1 1

~"

4500A 3oooA

~

15OOA

20O 0

0

0

200

400

600

LINEWlDTH FROM CENTER (nm)

0

200

400

600

LINEWlDTH FROM CENTER (nm)

FIGURE 6 Profiles of one micron resist process with varying thickness of AR coating: a) silicon, b) organic.

J.E. Lamb 111 et al. / Spin-on dry etch arc process

89

2.3. Plasma Etching of the Organic AR Coating Plasma etch rates were determined for both resist§ and the organic AR coating over a range of power and pressures. The study was conducted in a Branson/IPC 5000 series parallel plate plasma etcher with a matrix of 300, 600, 900 millitorr pressure versus 300, 600, 900 watts power. Etch rates were determined and etch rate ratios calculated by dividing the AR coatings etch rate by that of resist. This etch rate was then used to calculate the maximum amount of resist lost over steps. See Table 1 for the calculated etch rate ratios for the pressure - power matrix. TABLE 1 Plasma etch optimization for achieving favorable organic AR coating etch rates over those for resist. POWER (WATTS)

rc

300

600

300

475 - 0.79 600

750= 0.80 1250

700 1000 = 0.70

600

15.0= 0.60 25o

42.__5= 1.21 350

375 = 1.07 350

900

REDEPOSIT

125 350

625 -325 - - - 1.78

o

900

E

uJ rr 03 co LU rc Q_

-

0.36

Organic ARC Etch Rate (A/min) = Etch Rate Ratio Resist Etch Rate (A/min)

The chart indicates that higher powers and pressures produce the optimum etch rate ratios. Low pressures produce higher resist etch rates while low power at high pressure gave plasma polymerization resulting in redeposition in the plasma process. The optimum etch rate was 1.78. This etch rate ratio was used to determine the amount of resist lost during the oxygen plasma etch. Us ng Equation 2 with 4500 A of AR coating between steps and an etch rate ratio of 1.78 the resist loss was 2528 A However, because 1500 A of AR coating is over the top of the step the actual total thickness loss is equal to 2528 A - 1500 A or 1028 A. AR coatino thickness etch rate ratio 3.

resist thickness etched

Equation 2

SUMMARY

Information generated indicates the potential of AR coatings for dramaticlly improving linewidth control over reflective substrates. With the organic AR coating providing the largest increase in linewidth control. In addition to the properties demonstrated for the organic AR coating it has several additional properties that make it a more adaptable system. These include its ability to be applied by simple spin coat processes and the ability to be stripped in standard wet or dry resist stripping processes. The temperature sensitivity that had to be controlled tightly with other organic AR coatings is not needed with the proposed dry etch ARC material. The major concern with the organic AR coating is the amount of resist loss occurring in the oxygen plasma etch process. However, with the large improvement in linewidth control achieved with the organic AR coating, thicker resist films can be used to compensate for the resist lost in the etch, if it is necessary. It is hoped the reported dry processed organic AR coating will provide a means to obtain the best properties of both organic and inorganic AR coatings.

J.E. Lamb III et al. / Spin-on dry etch arc process

90

ACKNOWLEDGEMENTS We would like to thank Dan Hawley and Russel Pagel who developed the plotting routine used for the Prolith simulations. A special thanks to all those who contributed to the development of the experimental dry etchable organic AR coating.

(FOOT)NOTES AND REFERENCES * + ^ §

Experimental Plasma Etchable Organic AR Coating from Brewer Science Shipley MP1400 Photoresist Shipley MP1400-D1 Dyed Photoresist AZ 1470 Photoresist

[1]

Mack, C., Prolith: Positive Resist Optical Lithography Model, version 1.3 (1987)

[2]

Coyne, R.D. and Brewer, T., Resist Processes on Highly Reflective Surfaces Using AR Coatings, Kodak Microelectronics Seminar, (1983)

[3]

Polasko, K.J. and Griffing B.F., Thin Silicon Films Used as AR Coatings for Metal Coated Substrates, in : Advances in Resist Technology and Processing Ill (1986), SPIE vol. 631

[4]

Harrison, K. and Takemoto, C., The Use of Anti-reflective Coatings For Photoresist Linewidth Control, Kodak Microelectronics Seminar, (1983)