C bilayer resist system for submicron lithography

Vacuum/volume Printed in Great

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An As&/C lithography

1123

to

1126/l 993

bilayer

0042-207X/93$6.00+.00 @ 1993 Pergamon Press Ltd

resist system

G Danev and E Spassova, Bulgarian

Academy

Central Laboratory of Photographic of Sciences, 1040 Sofia, Bulgaria

for submicron

Processes-G.

Bontchev

str, bl 709,

and B Spangenberg,

institute

of Electronics,

Blvd Trakia 72, Bulgarian

Academy

of Sciences,

Georg-August

Universitet,

7784 Sofia, Bulgaria

and J lngwersen 3400

and R Hilkenbach,

Gtittingen,

lnstitut

fiir Roentgenphysik,

Geiststrasse

17,

Germany

A new bilayer inorganic resist system consisting of As2S3 thermally evaporated onto carbon layers is introduced. The amorphous As2S3 layers are investigated as image layers for uv, e beam andX-ray exposure. The wet chemically developed micron and submicron structures of the top layer are transferred onto the carbon bottom layer by oxygen-reactive ion etching (O,-RIE). The etching rates and the selectivities of different etching gases (0, CF,, CF4-0, SF,, CBrF,) for the bilayer system and for Si, Ge and polyimide (PI) substrates are given.

1. Introduction VLSI technology requires resolution of structures below one micron and imposes growing demands on the photolithographic process and particularly on the resist used. Several basic contradictions require the adoption of compromises : (i) the increase of the resolution is related to smaller thicknesses of the resist layer, hence to a decrease in the protection of the resist mask; (ii) the complex topology of the substrate changes the thickness of the resist coating and causes deviations in the accuracy of the image obtained ; and (iii) the wide application of halogen (chlorine) containing gases in the RIE processes imposes the creation of new resist masks which remain unaffected by these gases. Multilayer resist systems (MRS) have been developed in order to overcome these problems by separating the imaging, masking and planarizing functions in different layers’. A new trend in microfabrication is the application as image layers of inorganic photo- or electronsensitive materials such as As,S, : Ag ; Ge,Se, : Ag; Ge,S,.: Ag; AgHal etc., often deposited by vacuum techniques. As,SX has been widely investigated as an inorganic photoresist* 6. It can resolve 30 nm structures when sensitized with a 10 nm layer of Ag,S and exposed to an electron beam4. In another system, when the exposed As,S, without a silver sensitizer is processed with a special developer, resolution of 60 nm is achieved by a holographic Ar laser while exposure with electron beam leads to resolution of 20-25 nm. In contrast to organic photoresists, the As,S, mask is stable to 02-RIE4,7,“. This is a very important feature since As,S, can serve as an effective mask during dry etching of polymers (photoresists, polyimide) or carbon sublayers. Carbon layers (E-C, #diamond-like carbon, i-C) are of substantial technological interlest because of their unique properties : high chemical resistance, very low sputtering yield, high temperature resistance, very fine iamorphous structure, absence of swelling in solutions etc.%13

These properties have been applied to microlithographic fabrication by Kakushi et al 14, Gozdz and Lini5 and Pang and Horn ’ 6 using different image layers. A brief summary of some general properties of the various forms of carbon and As2S3 (bulk) is given in Table 1”. This paper presents the results from an investigation of the formation of patterns with micron and submicron dimensions in a new bilayer resist system. The system combines an evaporated amorphous semiconductor-As,S,-as the top image layer and sputtered carbon as the bottom masking layer. Special attention is paid to the RIE processing of the system in different etching gases. 2. Experimental Carbon layers 150 nm thick were deposited on 3 in. silicon (100) and germanium (100) wafers by argon ion beam sputtering from E-graphite glass target using a Kaufmann ion source. The angle of incidence of the Ar ion beam was 45”, the accelerating voltage was 600 V and the gas pressure was 3 x 10m4 torr. The deposition rate under these conditions was 1.8 nm min-‘. Alternatively, the carbon layers can also be deposited using an ion plating

Table 1

Graphite Evap. carbon Plasma a-C Ion beam a-C Diamond film As,S, (bulk)

Conductivity (a-’ cm-‘)

Optical gap (cv)

Density (g cm-‘)

Refractive index-n

2 x lo4 1x10-2 1 X lo-‘-lo-‘6 1x 10-2-10-‘6 3 1O- ’ * 1 x 1o-‘2

0.04 0.4 1.2-2.2 0.4-3.0 S5.0 2.2-2.4

2.26 1.5-1.8 1.51.8 1.7-2.8 x3.5 3.54

x3.0 1.8-2.2 2.1-2.4 2.38 2.4 1123

G Danev et al: As&/C

bilayer resist system

technique Ix in C&H, (benzene) and Ar 1 : 80 at a pressure of 7.5 x 10 4 torr, flow rate 15 ccm min-’ and substrate temperature of 350400°C. Deposition rates of up to 20 nm min-’ were achieved. A thin As& film (40-120 nm) was evaporated at a pressure of 7.5 x 10 4 torr from MO boats of a special design ’ 9. These ASS, layers exhibit photostructural changes as a result of photon, electron or X-ray irradiation”’ ‘2. The exposed samples were developed in an alkaline developerz3 containing also 50 ml iso-propanol per 1000 ml. The pattern formed in the upper ASS, layer was transferred onto the carbon layer by RIE in an oxygen atmosphere. The RIE experiments were performed in a parallel plate reactor designed by Eislere and working at an operating frequency of 13.56 MHz and a maximum relative power of 0.5 W cm--‘. The reactor and the rf cathode (diameter of 80 mm) were stainless steel. The electrode distance was fixed at 40 mm. The mass flow of 5-15 ccm depending on the etching gases such as Ar, O,, CF4, CBrF, and SF, (for etching the silicon or germanium substrates) was established by MKS flow controllers. High purity etching gases were supplied by Linde and MesserGriesheim, Germany. The gas pressure was varied from I x 10 ’ to I x 10mJ torr. The etching rates were determined by measuring the depth of the etched steps using a Taylor-Hobson Talystep profilometer. 3. Results and discussion The photosensitivity (s) of vacuum evaporated 80 nm thick ASS, layers deposited on carbon/Cr, carbon/Si and carbon/Ge substrates was estimated at different wavelengths as : 550-600 mJ cm ~’ for Hg lamp-light band (380450 nm) ; ’ for Ar laser (257 nm), see Figure I ; 1.6x lo-” C cm ’ for electron beam at 40 keV, Figure 2 ; and 500 mJ cm-’ for X-rays (band 2-7 nm)“.

360 mJ cm

The sensitivity of A&S? does not depend strongly on the underlayer type. However, at a thickness of 100 nm the development time of ASS, layers on carbon substrates was twice as long as that of ASS, layers evaporated on chromium bottom layers. According to Singh et al 4,8 photodoped A$., : Ag layers cannot be etched in an oxygen plasma unless a very high negative bias voltage of -800 V or more is applied. In our experiments

Figure 2. Scanning electron micrographs of electron exposed As2SI layer, 85 nm thick;

(40 keV)

30 nm space/line.

of O,-RIE of the top layer, the bias voltage was -450 V. As seen in Figure 3, the As,S3 evaporated layers exhibit good stability. The etching rates are considerably lower than those observed for instance with polyimide layers (see Figure 6). An increase of the rf power density of 0.5 W cm ~’ had no influence on the behaviour of the etching curve. The O,-RIE process is dominated by the partial pressure: with increased pressure the etching rate also increases. We assume that this is due to pure physical etching of the passive resist surface formed by the oxidation. The studies of the surface of As,S, layers subjected to RIE show relatively good quality ; microdamage and ruptures are not observed. Comparison with organic resists (polyimide layers in our case) suggests that the carbon layers are highly resistant to O2 ions. This is clearly demonstrated by our experimental etching curves obtained with polyimide and carbon layers subjected to O,-RIE (Figure 6). Thus, thick polyimide layers, used as planarizing layers for profiled surfaces, can be patterned. High aspect ratios are obtainable using a carbon mask. The use of a graphite cathode in the RIE system reduces the effect of redeposition and leads to more homogeneous etching of the surface at this stage of the process’4. The degree of anisotropy in oxygen RIE of polymers and carbon is strongly influenced by the gas pressure. The rf

a-

E

.F 1al ;; Lr r

/

,

A+S>

.'

-

/

--

-

RJE

100%

0,

P=0,4w/cm' P=O.Swlcm'

2.

I 10

20

30

LO

50

60

70 Pressure Lm Torrl

Figure 1. Scanning electron micrographs of micro zone plate (MZP) obtained for As,S,/Cr system, 60 nm space/line. 1124

Figure 3. Dependence pressure.

of RIE rate of evaporated

As,S,

layer on oxygen

G Danev et al: As,S,/C

bilayer resist system

I

Etch

140.

160

120.

120.

Rate

Lnmlminl O2

R IPJIC

40.

A% 5,

1’12

+_-+/‘l

P = 0.4 w/cm’ ~~60

Plasma

10

30

20

16mTorr

f.0

50

Pressure

Im Torrl

Figure 6. Etching rate of polyimide (PI) and carbon layers vs oxygen

pressure. CFI I 0

20

LO

60

80

‘OO

in 0,

I %I

Figure 4. Etching rate of As,S, layer as a function of the relative con-

centration of CF, in the CF,/O, gas mixture.

power to oxygen partial pressure ratio has been introduced by Unger et al 25 as an important parameter in such investigations. Anisotropic profiles have been observed at oxygen pressures ranging from 1.87 x 10P3 to 5.26 x IO-’ torr and power densities from 0.16 to 0.64 W cmm2. The corresponding optimal power/ pressure ratio varies from 0.3 to 0.5 W torr-’ (ref 26). Lower values lead to underetching while line width reduction is observed at higher ratios2’. RIE in gas mixtures of CF, and 0, with varying compositions is widely used in microelectronics for processing in planar technology. In order to assess the quality of the As,S,/C bilayer mask, we studied the behaviour of both materials under these conditions using Si substrates. In Figure 4 the etching rate of .4s,S, is plotted as a function of CF, relative concentration in I.he gas mixture. The total pressure was fixed at 60 mtorr. It is seen that the etching rate of the As,S3 inorganic resist increases rapidly with CF, concentration, i.e. with the rise in fluorine concentration. Since amorphous As,S, layers are more stable to oxygen plasmas than carbon ones (Figure 5) the upper pattern can be transferred onto the bottom carbon layer by O,-RIE. The etching r,ates of carbon are IO-1 5 times lower than those of organic resists (Figure 6). Figure 5 shows that oxygen RIE of the As,S,/carbon system is not particularly selective. The maximum selectivity is achieved at a pressure of 40 mtorr and it is sufficient to form a carbon mask with the required thickness.

The stability of the carbon mask in the etching processes was checked using reactive gases such as SF, and CBrF, for etching silicon and germanium. Figure 7 shows the etching rate of a carbon mask on a Si wafer (n+, (100)). Using a carbon mask, silicon can be well structured. The carbon mask is very stable to SF, plasma. The etching rate of carbon decreases strongly with the rise of the gas pressure. Although weaker, the same trend was also observed in CBrF, plasma (Figure 8). The selectivity of the SijC and Ge/C etched systems increases abruptly at etching gas pressures of above 50 mtorr (Figure 9). The observed selectivity values are higher than those reported in the literature for polymer masks.

Etch

Rate

[nmlminl

SelectiVlty

R fC/As,S,I

Selecttvity

lnmlmlnl

RWICI

600 .6

400.

-*-

51 IlOO

‘---we__ 01

30

20

10

I 50°

LO Pressure

Im Torrl

Figure 7. Etching rate of Si(100) and carbon layers and selectivity as a function of the SF, pressure.

Etch Etch

Rote

Rote

lnmlminl

160r C Br F, - Plasma +?*’

120

Powerdenslty -*-

80

:./

s 30 Pressure

[mTorr 1

Figure 5. Etching rate of ASS, and carbon layers and etching selectivity

(I?) vs the oxygen pressure.

0.5w/cm2

SI I1001

-.-

Oe

-*-

c

. LO Pressure

50

I mTorrl

Figure 8. RIE rate of Si(lO0) and Ge(lOO) wafers CBrF3 pressure.

and carbon

layer vs

1125

et al: As&/C

G Danev

bilayer

resist system

of Electronics, for their valuable support during this study. In part this work was funded by the German Federal Ministry for Research and Technology (BMFT) under Contract No 05320 DAB.

Selectivity 20,

1 -+-

SiIC

-.-

Ge/C

References

’ P H Lamey, SPIE, 333,59 (I 982).

01

10

I 20

30

LO

50

Pressure

t m Torrl

Figure 9. Dependence of the RIE selectivity CBrF, pressure.

of SijC and Ge/C

on the

4. Conclusion A bilayer photolithographic resist system based on vacuum deposited carbon and As,S, inorganic photoresist was studied. The image is formed on the upper imaging layer by exposure to photon-uv, duv and X-ray or e beam, and processing in an alkaline developer. The image is then transferred by O,-RIE onto the lower masking carbon layer. The behaviour of the system under RIE conditions is studied using various reactive gases. Since the carbon layer is highly resistant, the system is suitable for photolithographic purposes and both liquid etchants (acids and alkaline solutions) and reactive gases can be applied. In our opinion, the system is particularly suitable for solving some nonconventional lithographic problems requiring the use of powerful etching methods. Acknowledgements The authors are grateful to Prof G Schmahl, Forschungseinrichtung Rbntgenphysik, Georg-August Universitaet, Gottingen, to Prof J Malinowski and the colleagues from the Central Laboratory of Photographic Processes and Prof V Orlinov. lnstitutc

1126

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