Method for forming a semiconductor device in which an anti reflective layer ...

1 2

METHOD FOR FORMING A After patterning of the resist mask by such

SEMICONDUCTOR DEVICE IN WHICH AN photolithography, the anti-reflective layer is naturally etched

ANTI REFLECTIVE LAYER IS FORMED BY during the next dry etching process.

VARYING THE COMPOSITION THEREOF However, with the SiON based material, the composition

BACKGROUND OF THE INVENTION 5 *s intermediate between Si and SiOx (silicon oxide), as may

be demonstrated by the fact that Si accounts for approxi

This invention relates to a method for producing a semi- mately 50% of the composition. Thus the SiON based

conductor device. More particularly, it relates to a method material has etching characteristics intermediate between

for forming a fine pattern by utilizing an anti-reflective layer those of Si and SiO*, such that it is not necessarily easy to

which may be formed easily by a simplified process and optimize the etching conditions for such material. The

which is excellent in etching characteristics while exhibiting problems raised during the etching will be discussed by

a sufficient anti-reflection effect. referring to FIGS. 10 to 12.

In keeping up with an accelerated tendency towards high In these figures, the process of etching the SiON antiintegration of semiconductor devices, the minimum working reflective layer coated on a W-polycide film 25 is shown. A dimension is being reduced rapidly. For example, the mini- 5 sample wafer is formed by sequentially depositing a mum working dimension of the 16 MDRAM of the current W-polycide film 25 and a SiON anti-reflective layer 26 on a generation, which is being transferred to a mass-production Si substrate 21 via a gate oxide film 22 and by forming a line, is approximately 0.5 urn, while the minimum working resist mask 27 patterned to a pre-set shape, as shown in FIG. dimension of the 64 MDRAM of the next generation and 10- ^ W-polycide film 25 is made up of an impuritythat of the 256 MDRAM of the next-to-next generation are „ containrng poly siMcon film 23 and a tungsten sihcide (WSix) estimated to be 0.35 urn or less and 0.25 urn or less, 20 Aim 24, debited in tms order from me lower side, respectively ^ now tne SiON anb-reflective layer 26 is etched using a j- .... ^. , Jx, ^ ^ fluorocarbon-based etching gas under the etching conditions The degree of size dimmution depends to a large extent on fQr siQ me etch rate h si mcantl Jcause si0N

me resolution of the hmopphic process producmg a mask .g more si.rich ftan siQ and hence the carbon.based

pattern. For working on the order of 0.35 to 0.25 urn (deep 25 polymer is dep0Sited in excess.

sub-micron class), a far ultra-violet light, such as a KrF ^ on me Qther hand the etchin conditions for Si are

excimer laser beam with a wavelength of 248 urn, is appHed by employing the chlorine-based gas, the edge of the

necessitated. However, since the process employing such resist 27 fs receded gradually because SiOn is more

monochromatic light suffers from the lowering in resolution 0-rich than Si and hence the resist mask 27 is eroded under

or contrast due to halation or the standing M-wave effect, 30 the action of oxygen radicals (O*) yielded during etching,

use of an anti-reflective layer for weakening the intensity of m any cas6) me etdhed SiON anti-reflective layer 26f (the

light reflected from the underlying film is thought to be sufflx t herein indicating "being tapered") has its edge

indispensable. tapered and protruded more outwardly than the edge of the

As the materials making up the anti-reflective layer, resist mask 27.

amorphous silicon, polysilicon, SiOx, TiN or TiON have so 35 If the W-polycide film 25 is etched in such state using e.g.

far been employed. Not only the monolayer anti-reflective a Clj/Oj mixed gas, the oxygen radicals (O*) are yielded

layer, but also the multi-layer film, formed by lamination of from the edge of the ion-irradiated SiON anti-reflective layer

two or more films of different materials, have been known 26f, such that the carbon-based sidewall protection film is

and utilized as the anti-reflective layer. removed in the form of COx. The result is that a gate

In JP Patent Kokai Publication JP-A-63-79322 (1988). 40 electrode 25e (me suffix e herein indicating "being eroded")

there is disclosed a method including sequentially laminat- \s undercut significantly, as shown in FIG. 12 Such undercut

ing a first anti-reflective layer of SiO* having a small 18 f°rf outstanding with the WSix layer 24 than with a

refractive index n (n=1.5) and a second anti-reflective layer P^con ^.2^ecause J m *e ^"

c , ... ,: / „ .. . . , H removed as WC10Y thus rapidly raising the etch rate. Such

of polvsiucon having a larger refractive index (n=1.64) on , ,A ^ ,. * j- J •

^ * . , z. . . • . phenomenon becomes most outstanding during over

the surface of a substrate and carrying out g-ray lithography 45 gtcnm2

on the resist film of a large refractive index n (n=1.64) ff me sh of ^ .g deteriorated m Ms

thereon to realize high resolution. manner? not Qnly thg ... of ^ ^^^^ deviates

It has recently been shown that SiON (silicon nitride fr0m me design value but also it becomes difficult to form

oxide) exhibits excellent optical constants n and k which a sidewall for achieving the LDD structure,

represent the real number part and the imaginary number 50 jf me mum-iayer anti-reflective layer, described in IP

part of the complex refractive index, respectively, in a Patent Kokai Publication IP-A-63-79322 (1988), is

far-ultraviolet region shorter in wavelength than the g-ray, employed, and the etching conditions are changed over in

and may be conveniently applied to excimer laser lithogra- tne course 0f me etching, the edge of the anti-reflective layer

phy even in the form of a monolayer film. This technique is js not tapered so that the underlying film may be etched

disclosed in U.S. patent applications Ser. Nos. 07/988743 55 anisotropically. However, two films of different film

and 08/175299 for Method of Deterrrmiing Conditions for materials, namely the SiOx and polysilicon films, need to be

Anti-Reflective Layer, a Method of Forming an Anti- formed by different process steps. In addition, if the film

Reflective Layer and a Method of Forming a Resist Pattern having a high refractive index of n=5, is laminated, the

by Using Novel Anti-Reflective Layer, each of which has wavelength of the standing wave generated in the anti

been assigned to the same assignee as that for the present 60 refleCtive layer becomes excessively small with the future

application. The disclosure of two noted references is hereby photolithographic technique employing a short wavelength

incorporated herein. As shown in these references, the ught source sucjj mat it becomes difficult to perform film

optical constant of the SiON based material can be changed thickness control for achieving anti-reflection effects, extensively by controlling the gas composition at the time of

the film formation, and hence permits of high design free- 65 SUMMARY OF THE INVENTION

dom degree. The approximate elemental composition of the It is therefore a principal object of the present invention

SiON based material is Si:0:N=2:l:l. to provide a method for preparing a semiconductor device 3 4

wherein an extremely fine pattern can be formed, by utilizing Si compounds are SiOx, SiNx and SiON. If the Si ratio

an anti-reflective layer which can be formed by a simplified variation is afforded to the film of the Si compounds, high

process and which is superior in etching characteristics precision etching may be performed under the dry etching

while displaying sufficient anti-reflection effects. • conditions for the Si-rich region.

According to the present invention, there is provided a 5 It was previously found in the previous application

method for producing a semiconductor device including the assigned to the same assignee as that of the present appli

steps of providing an anti-reflective layer presenting varia- cation ^at ^ refractive index tends to be raised with

tion in the composition of a constituent element along the increase TMthe Si rati°' insofar af °* Si0*> S^ and SiON

film thickness over a semiconductor substrate, forming a ^Zl'T^t \Pj**"?*, T S°

patterned resist on the anti-reflective layer, and dry-etchmg io «?.to * ^ upper layer side than at the lower layer

K ^ a ^ , j * c sl"e of the anti-reflective layer, it is possible to increase the

the anti-reflective layer under etching conditions conform- refractive index at ^ uppellayer s4. Such constitution is

ing to the variation in the composition, using the patterned advantageous m improving the resolution in photolithogra

resist as a mask. pjjy_

Thus, in a majority of cases, the composition of the Although the anti-reflective layer is employed in general

anti-reflective layer formed in accordance with the present 15 for reducing the reflectance of a directly underlying film of

invention is not the stoichiometric composition. the high refractive index material, it may also be employed

If the anti-reflective layer is formed by a vapor phase for decreasing the reflectance from a film of the high deposition method, the variation in the composition may be refractive index material underlying the film directly underafforded by suitably controlling the film-forming atmo- lying the anti-reflective layer. If a via-hole is to be formed sphere. If, in the CVD method or reactive sputtering method, 20 in me transparent SiO* insulating film for having contact the chemical composition or the flow rate of the starting gas witn an underlying Al-based metallization film, it is possible or the sputtering gas is modified with lapse of the film- t0 form an Si0N based anti-reflective layer on the SiO* forming time, the ratio of the constituent element in the mterlayer insulatuig film in order to suppress reflection from composition of the anti-reflective layer is varied in a pro-set underlying Al-based metallization film, manner along the film thickness. 25 According to the present mvention, the ratio of the

„ , T , . . . , , ... constituent element in the anti-reflective layer is varied

If, alternatively, ion implantation is used, the variation in ^ ^ fiJm Mckness for achieving not only the desired the composition may be realized without regard to the anti-reflective effect on me whole but also me optimized dry method of forming the anti-reflective layer. That is, by etching pr0perties. The ratio distribution may be finely suitably selecting the implanted ion species or the ion 30 controlled on the nanometer order by controlling the filmaccelerating energy, the ratio of a specified element may be forming atmosphere in the vapor phase deposition method or elevated at a desired depth or within a desired range from the by ion implantation. Since the elementary composition surface. The ion implantation has an ancillary effect of remains the same throughout the anti-reflective layer, there destroying the anti-reflective layer for providing an amor- is no necessity of effecting film formation by a separate phous structure and raising the etch rate. The dosage nec- 35 process as in the case of a conventional multi-layer antiessary to cause such destruction and to provide an amor- reflective layer made up of, for example, SiOx and phous structure is roughly on the order of 1015/cm2, polysilicon, thereby significantly simplifying the filmdepending on the mass of ion species employed. forming process.

In any case, it is necessary with the present invention to Above all, if the film of Si compounds, such as SiOx,

change over the etching conditions during etching of the 40 SiNx or SiON, is used as an anti-reflective layer, and its Si

anti-reflective layer. The changeover timing can be deter- ratio is varied, the Si-rich area may be anisotropically etched

mined by monitoring the emission spectrum or by timing under the etching conditions for single-crystal silicon or

controlling based on the previously measured etch rate. polysilicon. Thus it becomes possible to prevent the film

Meanwhile, the above variation in the composition may edge portions from becoming tapered to prohibit deteriora

be accompanied by fluctuations in therefractive index. If the 45 tion in the shape anisotropy of the underlying film,

anti-reflective effect is taken into account, the high refractive Since the film of these Si compounds has a composition

index area is preferably provided as an upper layer region region the refractive index of which is increased with

rather than as a lower layer region. If the refractive index of increase in the Si ratio, a high refractive index film may be

the high refractive index region is higher than that of the disposed at the upper layer side of the anti-reflective layer,

overlying photoresist coating layer, the high refractive index 50 thereby exhibiting an excellent standing wave suppressing

region is sandwiched between the low refractive index effect.

regions, that is the substrate side low refractive index region . „TMTM„„T^T

and the usual photoresist region, such that multiple reflec- BRIEF DESCRIPTION OF THE DRAWINGS

tion is effectively absorbed within the high refractive index FIG.l is a schematic cross-sectional view showing the

region. 55 state in which, in an illustrative process of working a

The anti-reflective layer worked by the method of the polycide gate electrode according to the present invention, a

present invention may be any type of film if it exhibits SiOx fUm or a SiON film having its Si ratio varied along the

improved etching performance by varying the composition film thickness is formed on a W-polycide film,

of the constituent element along the firm thickness and also FIG. 2 is a schematic cross-sectional view showing the

if it exhibits satisfactory anti-reflection effect in the wave- 60 state in which a resist pattern has been formed on the

length range in the wavelength area employed in photoli- anti-reflective layer.

thography. FIG. 3 is a schematic cross-sectional view showing the

However, a film of Si compounds is most preferred in state in which a Si-rich upper layer portion of the SiOx

consideration that the Si compounds exhibit optical proper- anti-reflective layer has been selectively etched,

ties that may be easily employed in the far ultra-violet range 65 FIG. 4 is a schematic cross-sectional view showing the

and also exhibit outstanding changes in the etching perfor- state in which a lower layer portion of the SiOx anti

mance conforming to the Si ratio. Typical of the films of the reflective layer of FIG. 3 has been selectively etched.

7 8

was transported into a plasma CVD device and the SiON and a TiON layer 13 about 70 nm in thickness, laminated

film as the anti-reflective layer 6 was formed by two stages sequentially looking from the lower layer side,

of the film forming conditions, namely the first stage of the The wafer was transported into a plasma CVD unit and a

SiH4 flow rate of 30 SCCM, the N20 flow rate of 70 SCCM, SiN^ film as an anti-reflective layer 18 was formed under the

a gas pressure of 400 Pa, the RF power of 500 W (13.56 5 illustrative two stages of the film forming conditions,

MHz), the film-forming temperature of 360° C. and the namely the first stage of the SiH4 flow rate of 180 SCCM,

film-forming time of 40 seconds, and the second stage of the the NH3 flow rate of 500 SCCM, the N2 flow rate of 720

SiH4 flow rate of 70 SCCM, the N20 flow rate of 30 SCCM, SCCM, a gas pressure of 400 Pa, the RF power of 500 W

the gas pressure of 400 Pa, the RF power of 500 W (13.56 (13.58 MHz), the film-forming temperature of 250° C. and

MHz), the film-forming temperature of 360° C, and the 10 the film-forming time of 40 seconds, and the second stage of

film-forming time of 5 seconds. the SiN4 flow rate of 500 SCCM, NH3 flow rate of 100

By the above first and second stages, the SiON film about SCCM, the NH3 flow rate of 100 SCCM, the N2 flow rate of

45 nm in thickness and the Si-rich SiON film about 5 nm in 700 SCCM, a gas pressure of 400 Pa, the RF power of 500

thickness were formed as the lower anti-reflective layer 6L W (13.58 MHz), the film-forming temperature of 250° C.

and as the upper anti-reflective layer 6V, respectively. 15 and the film-forming time of 5 seconds.

On such anti-reflective layer 6, the resist pattern 7 having By the above first and second stages, the SiN film about

a satisfactory anisotropic shape could be formed by excimer 45 nm in thickness and the Si-rich SiON film about 5 nm in

laser photolithography. thickness were formed as the lower anti-reflective layer 16L

The upper anti-reflective layer 6V and the lower anti- ,„ and as the uPPer anti-reflective layer respectively. The

reflective layer 6L were subsequently dry etched in the same refractive index of the upper anti-reflective layer 16,, was

way as in Example 1, whereby an anti-reflective layer higher than that of the lower anti-reflective layer 16^

pattern 6a having satisfactory anisotropic shape could be The photolithographic process was carried out as in

formed. Thus the subsequent etching of the W-polycide film Example 1 to form a resist pattern 17 about 0.35 urn in

5 could be carried out with high accuracy. width. With this photolithographic process, the reflected

light from the Al-1 % Si film IS was efficiently attenuated by

Example 3 the anti-reflective layer 16 composed of the SiNx film, such

m, , . , ,. that a fine resist pattern 17 was formed with excellent

The present Example is directed to formation of a SiOx resolution

anti-reflective layer 6 similar to the anti-reflective layer of ," „ . ..

„ , , . J.. . .. .. „ .. , / „„ The above wafer was set on a magnetic micro-wave

Example 1, m which the upper anti-reflective layer 6r/was 30 . . ,. , . ... &.. „ .. .

„ r. , . . , ,. ^ u i_ • plasma etching device and the upper anti-reflective layer

formed by ion implantation, rather than by changing the , , , , .. „ .. , ,, . , ,, .

j-..- * j j TMrr%Tu. % 16rr and the lower anb-reflective layer 16, were etched m

conditions for reduced-pressure CVD. The process of the u J ^

this order under the etching conditions for Si and for SiOx,

present Example is explained by referring to FIGS. 6 and 7. , _ . „ 0 . , . .... , A

^ * j t> respectively. Basically, the etching conditions for SiOx can

FIG. 6 shows the state in which a SiO* film about 50 nm be appUed t0 ^ etching of siN;f The same etching condi.

in thickness was formed a the anti-reflective layer 6 on the 35 ... as of Example l were used herein. As a result, the

W-polycide film 5. The film forming conditions are the same upper-layer anti-reflective layer 16^ and the lower anti

as those of the first stage o Example 1 except that the reflective layer 16^ exhibiting good shape anisotropy as

film-forming time was set to 2 minutes 30 seconds. shown m mG 9 we formed, such that an anti-reflective

Into this wafer were implanted Si+ ions under the illus- layer pattern 16a having the anisotropic shape on the whole

trative conditions of the ion acceleration energy of 10 keV 40 could be produced.

and a dosage of lxl017/cm2. As a result, the upper layer side Subsequently, the Al-1% Si film 15 and the barrier metal

of the anti-reflective layer 6 was converted into an upper 14 were collectively etched under the conditions of the BC13

anti-reflective layer 6V composed of a Si-rich SiOx film, flow rate of 40 SCCM, the Cl2 flow rate of 80 SCCM, the

with the lower layer side thereof being left as the lower layer gas pressure of 1.3 Pa, the micro-wave power of 950 W

side anti-reflective layer 6L which has retained its original (2.45 GHz), the RF bias power of 50 W (2 MHz) and the

composition. The anti-reflective layer 6, thus formed, waler setting electrode temperature of 20° C. (by cooling

showed excellent standing wave suppressing effects. with water).

The subsequent photolithographic and dry etching pro- During the present process, etching of the Al-1% Si film

cesses were carried out in the same way as in Example 1. In 5Q 15 and the barrier metal 14 proceeded anisotropically, as

the present Example, satisfactory anisotropic etching could shown in FIG. 9, in which the anisotropically etched layers

be achieved. are indicated by the original numerals for the corresponding

non-etched layers with appended suffixes a. There was no

Example 4 undercut produced as a result of the tapered anti-reflective

The present Example is directed to working of an 55 layer pattern as was produced with the conventional prac

Al-based metallization, in which the CVD film-forming tice

conditions for the anti-reflective layer formed by a SiNx film The present invention is not limited to the aboveon the Al-1% Si film were changed during the working to described four illustrative Examples, provide a Si-rich upper-layer portion, and etching was For example, while only the variations in the Si ratio by carried out under application sequentially of the etching 60 the CVD process have been explained in connection with the conditions for Si and those for SiNx. The process for the SiN^ or SiON anti-reflective layers, these variations may present Example is explained by referring to FIGS. 8 and 9. also be achieved by ion implantation.

A wafer made up of an interlayer insulating film 11, a In addition, the CVD conditions, ion implantation

barrier metal layer 14 about 100 nm in thickness formed conditions, types of the dry etching devices, dry etching

thereon and an Al-1% Si film 15 about 300 nm in thickness 65 conditions, light exposure wavelengths in photolithography

formed on the barrier metal 14, was prepared. The barrier or the constitution of the wafer as an etching sample, may be

metal 14 is made up of a Ti layer 12 about 30 nm in thickness selected or modified in a desired manner.