Method of X-ray lithography

A method of lithography employing an electromagnetic wave having a very short wave length such as X-ray and a resist layer sensitive to that electromagnetic wave is disclosed. The irradiation process is divided into two steps. One step is selectively irradiating the electromagnetic wave, as in the prior art, to the resist layer in a desired pattern. The other steps is non-selective irradiation over the entire area of the resist layer. The latter step may be conducted with a plurality of workpieces such as semiconductor wafers simultaneously.

BACKGROUND OF THE INVENTION 
The present invention relates to a method of lithography using 
electromagnetic wave and resist sensitive to that wave for micro-fine 
pattern of 1 .mu.m or less on a semiconductor substrate, and more 
particularly to X-ray lithography. 
The exposure technique by electromagnetic wave having shorter wave length 
than visible light or ultraviolet light, such as X-ray or electron beam, 
is expected as a reprint technique that has great potential in the future 
because of reliable high resolution for a submicron width pattern and, at 
present, vigorous research and development are being carried out at 
various places. The technique is explained, for example, in a publication 
issued in 1972, "Electronics Letters" Vol. 8, No. 4, p.p. 102.about.104. 
The X-ray or electron beam exposure technique in the prior art, however, 
requires a long time to reprint a pattern in an X-ray or electron beam 
resist layer provided on a substrate such as a semiconductor wafer and, 
therefore, the throughput in the technique is low. Factors determining the 
throughput in the X-ray or electron beam exposure are the intensity of an 
X-ray or electron beam source and the sensitivity of the resist. In the 
case of an X-ray source, the most popularly used source is of a type which 
can emit X-rays from a target by impacting an electron beam on to the 
target. However, the conversion efficiency of the electron beam output to 
X-rays is as extremely low (i.e. .about.10.sup.-4) and it is a diverging 
radiation source. 
Therefore, X-rays obtained by the source are weak. On the other hand, a 
plasma X-ray source which is expected as a high-luminescence X-ray source 
is a pulsed X-ray source, and a conversion efficiency for one pulse is 
.gtoreq.10.sup.-2, but in a practical use, many problems must be resolved 
such as various damage that results from generation of pulses. In 
addition, with regard to a synchrotron orbital radiation source for which 
a trend of research and development has developed recently, it is too 
early to determined its practicality. Accordingly, so long as an X-ray 
source concerns, at present, it can be said that the source excited by the 
electron beam is most suitable for commercial X-ray exposure apparatus. In 
this respect, that is, in the case of employing the source excited by the 
electron beam, particularly a high sensitivity of X-ray resist is 
necessary. In order to obtain a sufficient throughput, development of 
X-ray resist having a high sensitivity of 10 mJ/cm.sup.2 or less is 
necessary. However, X-ray resist . having such a high sensitivity and 
utility is readily not available at present. It is believed that a 
considerable period of development in the future will be necessary until 
X-ray resist having a sensitivity of 10 mJ/cm.sup.2 or less, which is 
applicable to fabrication of practical devices, is available. In 
commercial exposure apparatuses, one of the most important factors is the 
throughput and, as described above, in the X-ray exposure technique also, 
enhancement of the throughput is an important problem. The situation is 
similar for the case where electron beam exposure techniques are used. 
SUMMARY OF THE INVENTION 
One object of the present invention is to provide a novel method of 
lithography in which high-throughput in the exposure process is made 
possible. 
According to the present invention, there is provided a method of 
lithography comprising the steps of preparing a workpiece having a 
substrate such as a semiconductor wafer with a metallic film formed on the 
major surface of the wafer via an insulating layer, coating the surface of 
the substrate with a layer of a resist material sensitive to a 
short-wavelength electromagnetic wave, selectively irradiating the 
mentioned electromagnetic wave to the layer of the resist material to form 
a selectively irradiated pattern, and non-selectively irradiating the 
mentioned electromagnetic wave to the layer of the resist material such 
that the substantial area of the surface of the layer of the resist 
material is entirely and uniformly irradiated. 
According to a preferred embodiment of the present invention, there is 
provided a method of an X-ray lithography comprising steps of preparing a 
workpiece having a substrate such as a semiconductor wafer with a metallic 
film formed on the major surface of the wafer via an insulating layer and 
an X-ray resist layer formed on the substrate, that is, on the metallic 
film of the semiconductor wafer, performing a first steps of irradiating 
X-rays on the X-ray resist layer without interposing any X-ray mask 
between an X-ray source and the X-ray resist layer so that X-rays in the 
first step are uniformly irradiated over the whole parts of the surface of 
said X-ray resist layer, setting an X-ray mask above the X-ray resist 
layer, the X-ray mask having a support member made of, for example, 
silicon, a membrane transmitting X-rays, made of, for example, a thin 
silicon film, a thin silicon oxide film, a thin silicon nitride film or a 
composite film of these thin films and supported by the support member and 
an X-ray absorber pattern selectively provided on the membrane to form an 
X-ray absorber section where the X-ray absorber pattern is present and to 
form an X-ray transmitting section where the X-ray absorber pattern is not 
present, and performing a second step of irradiating X-rays on the X-ray 
resist layer through the X-ray mask. 
The X-ray absorber pattern may be made of gold, platinum, copper, etc. The 
X-ray apparatus in both X-ray irradiating steps are different each other. 
In the selectively irradiating step, a conventional apparatus of the X-ray 
or electron beam lithography is used. In the non-selectively irradiating 
step, neither mask nor pattern-forming means is necessary, and it is 
preferred that a plurality of workpieces, that is, semiconductor wafers 
are simultaneously irradiated to increase the throughput. As a result, a 
special and simple apparatus may be provided. 
In case of X-ray irradiation, an X-ray source has a target, and the soft 
X-rays are emitted by exciting the target with an electron beam. For 
example, if an electron beam of 10 KW collides against the target, 
AlK.alpha. (8.34 .ANG.), SiK.alpha. (7.13 .ANG.), MoL.alpha. (5.41 .ANG.) 
or PdL.alpha. (4.37 .ANG.) is emitted from the target when the target is 
made of aluminum, silicon, molybdenum or palladium, respectively. In any 
case, the strength of the soft X-ray by respective targets is weak because 
the conversion efficiency of the electron beam to X-rays is extremely low 
as mentioned above. Further, the X-ray resist may be a negative resist 
such as JSR MES-X, PGMA, JSR MES-E, CMS-EX or .alpha.M-CMS(S) each having 
a sensitivity value of 60 mJ/cm.sup.2 or more. Also, the X-ray resist may 
be a positive resist such as FBM-120, RE-5000P, RE-5000P, FPM-210 or 
AX-2400 each having a sensitivity value of 45 mJ/cm.sup.2 or more. When 
the negative resist is used, portions of the negative resist layer under 
the transmitting section of the mask must be sufficiently irradiated by 
X-rays above the sensitivity value so that after a development process and 
a rinsing process, the portions of the negative resist layer would remain. 
Also, when the positive resist is used, portions of the positive resist 
layer under the transmitting section of the mask must be sufficiently 
irradiated by X-rays above the sensitivity value so that after a 
development process and a rinsing process, the portions of the positive 
resist layer would disappear. 
In the case of manufacturing a semiconductor device, when a negative X-ray 
resist having 200 mJ(milli joule)/cm.sup.2 sensitivity value is coated on 
a semiconductor wafer of 4 inch diameter, because of a restriction of an 
effective area of an X-ray mask the exposure must be conducted nine times 
by dividing the surface the semiconductor wafer. Namely, after exposure of 
one area of the wafer is completed, the wafer is horizontally moved in X 
and/or Y directions about 3 cm. Then, the exposure of X-rays in the next 
area of the wafer is conducted, and repeated sequentially. The portions of 
the resist layer under the transmitting section of the mask must be 
irradiated about 240 mJ/cm.sup.2 or more, and the portions of the resist 
layer under the X-ray absorber section of the mask must be restricted to 
irradiation of 180 mJ/cm.sup.2 or less. In this case, about 7 minutes 
irradiation of X-rays is necessary for one area of the wafer. Therefore, 
the whole exposure time to conduct the X-ray lithography on one 
semiconductor wafer in one process step becomes about 63 (7.times.9) 
minutes. It is a very long time and canses the low throughput in the X-ray 
lithography. 
According to the method of the present invention, the non-selective 
irradiation may be performed either after or before the selective 
irradiation. Therefore, the whole exposure time to conduct the lighography 
on one semiconductor wafer in one process step becomes short and the high 
throughput can be expected. More particularly, in the above mentioned 
case, without any mask, the semiconductor wafer is wholly irradiated for 
about 5 minutes, whereby the resist on the semiconductor wafer uniformly 
receives an energy of about 170 mJ/cm.sup.2. Before or after the 
non-selective irradiation, the above mentioned conventional selective 
exposure is conducted. However, according to the present invention, the 
selected portions of the resist subjected to irradiation necessitate an 
energy of only 70 mJ/cm.sup.2. Therefore, the exposure time on one area of 
the wafer to conduct patterning is only 2 minutes. Therefore, the total 
exposure time to conduct the lithography on the semiconductor wafer in one 
process step becomes only 23 (2.times.9+5) minutes. Of course, the total 
amount of energy in portions other than the pattern during the two 
irradiation steps can be maintained 180 mJ/cm.sup.2 or less. The uniform 
irradiation without mask of the present invention can be simultaneously 
conducted with a plurality of workpieces, that is, a plurality of 
semiconductor wafers. In this case, the time of the uniform irradiation, 5 
minutes for the above mentioned example, results in greater throughput. 
In case of X-ray exposure, the time of the uniform irradiation is favorably 
determined such that an amount of X-ray irradiated onto the whole surface 
of the negative or positive resist layer during the uniform irradiation of 
X-rays without mask is 10 to 35 mJ(milli joule)/cm.sup.2 below the 
sensitivity value of the resist layer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1, an X-ray mask 33 is positioned above a workpiece 42, 
and an X-ray source 31 is installed on the X-ray mask. The workpiece 42 
comprises a semiconductor body 35 and an X-ray resist layer 36 formed on a 
metallic film (not shown) provided on the major surface of the 
semiconductor body via an insulating layer (not shown). An X-ray mask 33 
comprises a support member 38, a membrane 39 and an X-ray absorbing 
pattern 43, and forms an X-ray transmitting section 34 and a X-ray 
absorber section 37. Soft X-rays 32 radiated from a target 40 of the X-ray 
source 31 by impacting an electron beam 41 to the target pass through the 
X-ray transmitting section 34 of an X-ray mask 33, and irradiate the X-ray 
resist 36. At this moment, the X-rays do not pass through the X-ray 
absorber pattern section 37 formed on the membrane 39 and, thus, section 
37 acts as a mask pattern. In the X-ray lithography of the prior art, the 
mask 33 is always provided between the X-ray source 31 and the workpiece 
42 and, therefore, the throughput thereof is decreased. 
First embodiment: 
Referring to FIG. 2A, a workpiece 10 is prepared. The workpiece 10 
comprises a semiconductor wafer 1 with a metallic film (not shown) on a 
major surface of the semiconductor substrate via an insulating layer (not 
shown), and a negative type X-ray resist layer 2 of 1 .mu.m thickness 
formed on the semiconductor wafer, that is, the resist layer is attached 
to the metallic film. The negative type X-ray sensitivity value is 200 
mJ/cm.sup.2, that is, D in FIGS. 3A and 3B is 200 mJ/cm.sup.2. In the 
preferred embodiment, the sensitivity value of a negative resist layer is 
characterized in that after the development and the rinsing of the 
negative resist layer which has been irradiated by X-rays in an amount 
equal to the sensitivity value, the thickness of the resist layer becomes 
one half of the initial thickness. In this embodiment, the initial 
thickness of the resist layer is 1 .mu.m, and therefore, if X-rays 
irradiate the surface of the resist layer at the dosage of 200 
mJ/cm.sup.2, after the development and rinsings, the thickness of the 
resist layer becomes 0.5 .mu.m. 
Subsequently, as shown in FIG. 2B, the workpiece 10 is installed in one 
X-ray lithography apparatus 100. An X-ray mask 6 is positioned on the 
negative resist layer 2, and an X-ray source 7 is positioned above the 
mask 6. The X-ray mask 6 comprises a ring shape, in the plan view, support 
member 5, a membrane 12 supported by the support member, and an X-ray 
absorbing pattern 4 so as to form an X-ray transmitting section 3 of the 
mask where the X-ray absorbing pattern is not provided and to form an 
X-ray absorber section 14 where the X-ray absorbing pattern is provided. 
Soft X-rays 8 is emitted from a target 10 of the X-ray source 7 by 
impacting an electron beam 11 to the target 10 and irradiated through the 
mask to the surface of the resist layer 2, selectively, about 2 minutes. 
Then, portions of the resist layer 2 under the X-ray transmitting section 
3 of the mask 6 is irradiated X-rays of 70 mJ/cm.sup.2. That is 
represented as T in FIG. 3A. Portions of the resist layer 2 under the 
X-ray absorber section 14 of the mask 6 is irradiated X-rays of not more 
than 10 mJ/cm.sup.2. That is represented as A in FIG. 3B. If the 
semiconductor wafer has the large size with respect to an effective area 
of the mask, the semiconductor wafer is moved X and/or Y direction for 
example, 3 cm intervals as mentioned above. 
Then, as shown in FIG. 2C the workpiece 10 is installed in another 
apparatus 200 with other workpieces 20, 30 which has been completed the 
process step shown in FIG. 2 as the workpiece 10. In the apparatus 200, 
any X-ray mask is not provided between an X-ray source 7' and respective 
workpieces 10, 20, 30. Soft X-rays 8' are emitted from a target 10' by 
impacting an electron beam 11' to the target and irradiated directly and 
uniformly the whole surface of the respective negative resist layers 2 of 
the workpices 10, 20, 30 during 5 minutes. Then, X-rays of 170 mJ/cm.sup.2 
are irradiated in the respective negative resist layers 2 uniformly in 
their entire portions. Consequently, portions of the resist under the 
transmitting section 3 of the mask 6 in the first irradiating step in FIG. 
2B are irradiated the total X-rays of 240 mJ/cm.sup.2 that is indicated as 
T' in FIG. 3A and is above the sensitivity value D.sub.0.5. On the other 
hand, portions of the resist under the absorber section 14 of the mask 6 
in the first irradiating step in FIG. 2B are irradiated the total X-rays 
of 180 mJ/cm.sup.2 or less that is indicated as A' in FIG. 3B and is below 
the sensitivity value D.sub.0.5. Namely, in the second irradiating step in 
FIG. 2C, uniform X-rays of 170 mJ/cm.sup.2 indicated as (T'--T) in FIG. 3A 
and (A'--A) in FIG. 3B are irradiated in the resist layer. FIGS. 3A and 3B 
show a sensitivity characteristic of negative type X-ray resist as a 
relation of a film remaining rate versus an X-ray dose. FIG. 3A indicates 
a sensitivity characteristic of X-ray resist under the X-ray transmitting 
section of the X-ray mask, while FIG. 3B indicates the same characteristic 
of X-ray resist under the X-ray absorber pattern section. It is assumed 
that in the case shown in FIG. 3A (under the X-ray transmitting section) a 
dose of T was given to the negative type X-ray resist by the X-ray 
exposure making use of an X-ray mask shown in FIG. 2B. This T is set at a 
smaller value than a sensitivity D.sub.0.5 (a dose for which a film 
thickness remaining rate after development becomes 0.5) of the negative 
type X-ray resist. At this moment, in the case shown in FIG. 3B (under the 
X-ray absorber pattern section), a considerably smaller dose A than T is 
given to the negative type X-ray resist layer. Subsequently, X-ray overall 
irradiation without the X-ray mask, as shown in FIG. 2C, is carried out in 
another apparatus. It is possible to make setting such that at this time, 
in the case of FIG. 3A, a total X-ray dose T' exceeding the sensitivity 
D.sub.0.5 is given to the negative type X-ray resist, while in the case of 
FIG. 3B a total X-ray dose A' which is smaller than the sensitivity 
D.sub.0.5 but larger than A (a film remaining rate equals zero) is given 
to the same. In other words, in the case of FIG. 3A, an X-ray dose of T is 
given to the negative type X-ray resist by the first exposure and an X-ray 
dose of (T'--T) is given to the resist by the second exposure, while in 
the case of FIG. 3B, an X-ray dose of A is given to the negative type 
X-ray resist by the first exposure and an X-ray dose of (A'--A) is given 
to the same by the second exposure. In this way, it is possible to make 
exposure settings such that a difference between an X-ray dose in the 
X-ray transmitting section and an X-ray dose in the X-ray absorber pattern 
section, applied to the negative type X-ray resist by X-ray exposure that 
uses the X-ray mask shown in FIG. 2B, may provide a negative type reprint 
pattern in which remaining film rates are, for instance, unity and zero, 
as a result of the subsequent X-ray overall irradiation without the X-ray 
mask (FIG. 2C). As described above, it is an essentially necessary 
condition of the present invention to perform X-ray exposure twice, that 
is, to perform X-ray exposure making use of an X-ray mask and subsequent 
X-ray overall irradiation without an X-ray mask. Where X-ray exposure is 
performed twice, and, a plurality of apparatuses are used, parallel 
processing is possible. Especially, with regard to the step of X-ray 
overall irradiation, since it is believed that the exposure time for each 
one of the workpieces painted with the negative type X-ray resist may be 
long, a system in which a large number of workpieces are exposed jointly, 
is possible. If such parallel processing is carried out, after all it 
becomes possible to greatly shorten the exposure time for each one of the 
workpieces, and thereby a high throughput can be realized. Eventually, the 
exposure time is almost restricted by the first X-ray exposure making use 
of the X-ray mask, the throughput would be determined thereby. 
Second embodiment: 
In FIGS. 4A to 4C of the second embodiment, the same functions as those in 
FIGS. 2A to 2C are indicated as the same numerals. In the second 
embodiment, a plurality of workpieces 10, 20, 30 are uniformly irradiated 
by X-rays 8' in the apparatus 200 in the first irradiating step during 5 
minutes without interposing any X-ray mask between each workpiece and the 
X-ray source 7'. Thereafter, each workpiece is installed in a conventional 
X-ray lithography apparatus 100, and performed the second irradiating step 
to irradiate X-rays 8 during 2 minutes from the X-ray source 7 through the 
X-ray lithography mask 6. Portions of the negative X-ray resist layer 2 
under the transmitting section 3 of the mask 6 are irradiated by X-rays 
of, for example, 70 mJ/cm.sup.2 in the second irradiated step in FIG. 4C. 
Whereas, the portions have been already irradiated X-rays of, for example, 
170 mJ/cm.sup.2 in the first uniform irradiating step. Therefore, the 
total amount of dosages of X-rays becomes 240 mJ/cm.sup.2. As seen in FIG. 
5A, the portions are irradiated by X-rays of V (170 mJ/cm.sup.2) in the 
first step, and of (T'-V) (70 mJ/cm.sup.2) in the second step. The total 
irradiating X-rays T' becomes 240 mJ/cm.sup.2 which is beyond the 
sensitivity value D.sub.0.5 of 200 mJ/cm.sup.2. On the other hand, 
portions of negative X-ray resist layer 2 under the absorbor section 41 of 
the mask 6 are irradiated by X-rays of, for example, 7 mJ/cm.sup.2. 
Whereas, the portions have been already irradiated X-rays of, for example, 
170 mJ/cm.sup.2 in the first uniform irradiating step. Therefore, the 
total amount of dosage of X-rays becomes 178 mJ/cm.sup.2. As seen in FIG. 
5B, the portions are irradiated by X-rays of V (170 mJ/cm.sup.2) in the 
first step, and of (A'-V) (7 mJ/cm.sup.2) in the second step. The total 
irradiating X-rays A' becomes 178 mJ/cm.sup.2 which is below the 
sensitivity value D.sub.0.5 of 200 mJ/cm.sup.2. 
Third embodiment: 
The process is the same as the first embodiment shown in FIGS. 2A to 2C. 
However, a positive X-ray resist layer, for example, FBM-120 is used in 
the third embodiment. Therefore, the negative X-ray resist layer 2 in 
FIGS. 2A to 2C is replaced with the positive X-ray resist layer. Referring 
to FIGS. 6A and 6B, the sensitivity value D.sub.0 of a positive resist 
layer is characterized in that after the development and the rinsings of 
the positive resist layer which has been irradiated by X-rays at an amount 
equal to the sensitivity value, the thickness of the resist layer becomes 
just zero. In the first irradiating step through the mask 6, portions of 
the positive resist layer under the transmitting section 3 of the mask are 
irradiated by X-rays 8 of 30 mJ/cm.sup.2, which is represented as W in 
FIG. 6A. On the other hand, portions of the positive resist layer under 
the absorber section 14 of the mask 6 are irradiated by X-rays 8 of 5 
mJ/cm.sup.2 which is represented as S in FIG. 6B. Next, the second 
irradiating step is conducted, to irradiate X-rays 8' of 30 mJ/cm.sup.2 
uniformly over the entire surface of the positive resist layer in the 
apparatus 200. Therefore, the total amount of X-rays (W' in FIG. 6A) in 
the portions of the positive resist layer under the transmitting section 
becomes 60 mJ/cm.sup.2 (30 mJ/cm.sup.2 +30 mJ/cm.sup.2) which is above the 
sensitivity value D.sub.0 (45 mJ/cm.sup.2). On the other hand, the total 
amount of X-rays (S' in FIG. 6B) in the portions of the positive resist 
layer under the absorber section becomes 35 mJ/cm.sup.2 (5 mJ/cm.sup.2 +30 
mJ/cm.sup.2) which is below the sensitivity value 45 mJ/cm.sup.2 
(D.sub.0). 
Fourth embodiment: 
The process in the same as the second embodiment sown in FIGS. 4A to 4C. 
However, a positive X-ray resist of the third embodiment is used. At first 
the uniform irradiation by X-rays 8' of 30 mJ/cm.sup.2 (Y in FIGS. 7A and 
7B) without a mask is performed in the apparatus 200. Next, in the 
conventional X-ray lithography apparatus 100, the second irradiation by 
X-rays 8 is conducted by using the mask 6. As shown in FIG. 7A, portions 
of the positive resist layer under the transmitting section of the mask 6 
is irradiated by X-rays of 30 mJ/cm.sup.2 (W'-Y) in the second step, and 
the total of X-rays W' irradiated on such portions becomes 60 mJ/cm.sup.2 
(30 mJ/cm.sup.2 +30 mJ/cm.sup.2), which is beyond the sensitivity value 
D.sub.0 (45 mJ/cm.sup.2). On the other hand, as shown in FIG. 7B, portions 
of the positive resist layer under the absorber section of the mask 6 are 
irradiated by X-rays of 5 mJ/cm.sup.2 (S'-Y) in the second step, and the 
total of X-rays (S') irradiated on such portions becomes 35 mJ/cm.sup.2 
(30 mJ/cm.sup.2 +5 mJ/cm.sup.2), which is below the sensitivity value 
D.sub.0.