Semiconductor Exposure Apparatus

1. Field of the Invention

This invention relates to a semiconductor exposure apparatus used in an integrated circuit manufacturing process and having chucks designed to suppress the magnification error for the pattern of the preceding step when the pattern of a mask is transferred onto a semiconductor wafer.

2. Description of the Prior Art

In integrated circuit manufacturing, the step of projecting and transferring a mask pattern onto a semiconductor wafer substrate is done plural times. At this time, it is necessary that the mask pattern of the next step be highly accurately aligned over the entire surface of the wafer relative to the mask pattern transferred onto the wafer in the preceding step.

Generally, a semiconductor wafer is adsorbed to a wafer chuck which is a support means for fixedly supporting the wafer and, once the wafer is held to the wafer chuck, the vacuum is not released until exposure is completed. Heretofore, the wafer chuck has been made of a metal, such as aluminum or stainless steel, which is very sensitive to the thermal expansion and contraction caused by fluctuation of the ambient temperatures as compared with the wafer. That is, it has been confirmed that when the wafer chuck creates a minute amount of expansion and contraction with the wafer held to the wafer chuck, the wafer also creates expansion and contraction exceeding the elongation percentage intrinsic to the wafer. For example, let it be assumed that the temperature around the chuck has risen by 0.2.degree. C. from after a silicon wafer has been adsorbed to a wafer chuck made of aluminum until exposure is started, and the temperatures of the wafer and wafer chuck have also risen correspondingly. Heretofore, in such a case, due to the coefficient of linear expansion (2.5.times.10.sup.-6 /.degree.C.) of the silicon wafer, the amount of expansion of a wafer of 5 inches has been only about 0.06 .mu.m and this has been considered to be a negligible amount. At this time, the wafer chuck (whose coefficient of linear expansion is 23.times.10.sup.-6 /.degree.C.) expands to about 0.6 .mu.m and the wafer is pulled by the friction force with the chuck and exhibits an amount of elongation approximate thereto. Accordingly, if exposure is effected in such condition, a magnification error occurs between the pattern of the preceding step and the pattern of the current step and highly accurate alignment of the wafer over the entire surface thereof becomes impossible. In recent years, various methods of highly accurately aligning a mask pattern and a wafer pattern by controlling the temperature of the wafer have been devised. However, if the temperature of the wafer chuck is changed after the wafer has been fixed to the wafer chuck, a great amount of elongation occurs in the wafer chuck which therefore, the wafer is pulled thereby, and has led to the disadvantage that an amount of expansion and contraction greater than necessary occurs.

For this reason, it has been very difficult to achieve highly accurate alignment of the wafer over the entire surface thereof in a case where use is made of a wafer chuck made of a metal such as stainless steel or aluminum.

A similar tendency seems to occur not only in the wafer and wafer chuck but also in the mask and mask chuck although there is difference in degree.

It is an object of the present invention to prevent, after a body such as a mask or a wafer has been vacuum-adsorbed or held to a chuck, the held body from creating an amount of expansion and contraction exceeding the amount of expansion and contraction by its own temperature change due to a temperature change of the chuck.

It is another object of the present invention to provide a semiconductor exposure apparatus in which the magnification error for the pattern of the preceding step is suppressed when a mask pattern is transferred onto a wafer.

The invention will become fully apparent from the following detailed description thereof taken in conjunction with the accompanying drawings.

FIG. 1 illustrates a projection type semiconductor exposure apparatus which is an embodiment of the present invention.

FIG. 2 shows an embodiment of the wafer chuck according to the present invention which has a two-piece structure.

FIG. 3 shows an example of the arrangement for limiting the temperature of the chuck.

Referring to FIG. 1, a photomask 1 and a wafer 2 are vacuum-adsorbed that is, held by a negative pressure to a mask chuck 3 and a wafer chuck 4, respectively, by a vacuum source 6, and the mask pattern of the mask chuck 3 to which a light beam 7 for exposure is applied is projected onto the wafer 2 by a projection optical system 5.

Now, embodiments of the chuck of the present invention will be shown below. Herein, only a case where the material of the semiconductor wafer is silicon is considered.

In a first embodiment, ceramics, silicon or Hastelloy A is employed as the material of the wafer chuck. The standard composition of Hastelloy A is Ni 55-59%, Mo 18-12%, C 0.04-0.15% and Fe 18-22%. As regards the coefficients of linear expansion of these materials, the coefficient of linear expansion of ceramics, particularly, silicon nitride (Si.sub.3 N.sub.4) is 3.0.times.10.sup.-6 /.degree.C. (Industrial Materials, 30.25 (1982)), that of silicon is 2.5.times.10.sup.-6 /.degree.C. (Physical Constant Table) and that of Hastelloy A is 2.7.times.10 .sup.-6 /.degree.C. (Mechanical Engineering Handbook, 6th Edition), and these are substantially coincident with the coefficient of linear expansion (2.5.times.10.sup.-6 /.degree.C.) of silicon wafer. The use of a wafer chuck formed of these materials results in obtainment of the following effect.

First, let it be assumed that the wafer is held to the wafer chuck, whereafter the temperature around the chuck is changed by some influence or other and the temperatures of the wafer chuck and the wafer are correspondingly changed. Heretofore, the wafer chuck has been made of aluminum or stainless steel and particularly in the case of aluminum, the wafer chuck has a coefficient of linear expansion about ten times as great as that of silicon and therefore, even if the temperature change is small, the amount of expansion and contraction of the chuck is significant and the wafer also is pulled by the expansion and contraction of the chuck, and such temperature change leads to a magnification error. Accordingly, by using as the material of the wafer chuck the aformentioned material having a coefficient of expansion approximate to the coefficient of expansion of the wafer, the amounts of expansion and contraction of the wafer chuck and the wafer are made smaller than the amount of expansion and contraction of the conventional chuck for some temperature change and are made innegligible and, since substantially the same amount of expansion and contraction occurs to the wafer and the chuck simultaneously, the stress by heat does not remain in the wafer due to the expansion and contraction thereof.

Further, in an apparatus wherein the temperature of the wafer or the wafer chuck is controlled to purposely achieve the expansion and contraction of the wafer and thereby correct the magnification error, even when a temperature change is imparted to the wafer after being fixed to the wafer chuck, if the wafer chuck is formed of said material, the expansion and contraction of the wafer is not hampered by the chuck and therefore, an accurate preestimate of the amount of expansion and contraction thereof becomes possible and highly accurate correction of the magnification error becomes possible.

However, where ceramics or silicon is used for the wafer chuck, it would be very difficult to form the entire chuck of such material because these materials are not easy to machine. However, the entire chuck need not always be formed of a single material, but only the portion thereof which holds the wafer may be formed of a required material and the other portion thereof may be formed of a conventionally used metal, for example, whereby a sufficient effect may be obtained. Of course, however, the portion of the wafer chuck which holds the wafer must be of a sufficient thickness not to transfer the deformation of the metal to the wafer. An example of the wafer chuck having such two-piece structure is shown in FIG. 2. In FIG. 2, reference character 4a designates the metal portion of the wafer chuck and reference character 4b denotes the ceramics portion of the wafer chuck.

Description will now be made of a second embodiment in which the material of the mask chuck is ceramics or invar. Soda-lime glass, LE-30 or quartz glass is generally used as the material of the photomask, but herein LE-30 and quartz glass will be considered. Among ceramics, the coefficient of linear expansion of silicon carbide (SiC) is 4.0.times.10 .sup.-6 /.degree.C. (which somewhat differs depending on the type of crystal and the sintering method) and this is substantially coincident with the coefficient of linear expansion 3.7.times.10 .sup.-6 /.degree.C. of LE-30, and the coefficient of linear expansion of invar is 0.9.times.10 .sup.-6 /.degree.C. (Mechanical Engineering Handbook, 6th Edition; invar has its coefficient of the components thereof) and this is approximate to the coefficient of linear expansion 0.5.times.10 .sup.-6 /.degree.C. of quartz glass. The standard composition of invar is C<0.20%, Mn 0.5%, Ni 36% and Fe remainder. Accordingly, by forming the mask chuck by the use of these materials, there is obtained an effect similar to that of the above-described first embodiment.

Description will now be made of a third embodiment in which super invar, which is a metal of low expansion, is used as the material of the mask chuck or the wafer chuck, the coefficient of linear expansion of this material is -0.01.times.10 .sup.-6 /.degree.C. (Mechanical Engineering Handbook, 6th Edition), which is a very samll value as compared with coefficient of linear expansion of silicon wafer or quartz glass mask. Super invar comprises invar with Co added thereto and its coefficient of linear expansion is lowest for Co 5%, Ni 31.5% and Fe 63.5%. Therefore, if a mask chuck or a wafer chuck formed of super invar, expansion and contraction will hardly occur in the two chucks even if the ambient temperature fluctuates after the mask or the wafer has once been fixed to the chuck and accordingly, the expansion and contraction of the photomask or the wafer substrate fixed thereto by vacuum adsorption will also be suppressed. Thus, the use of a mask chuck or a wafer chuck formed of super invar makes the expansion and contraction of the mask or the wafer very stable for over temperature and easily prevents occurrence of the magnification error resulting from temperature change.

As described above, by a material having an absolutely low coefficient of linear expansion equal to or smaller than that of the photomask or the wafer being used as the material of the mask chuck or the wafer chuck, the planar expansion and contraction of the mask and wafer resulting from temperature changes is reduced as compared with the conventional case where metal chucks are used, and the magnification error occurring when a mask pattern is projected and transferred onto the wafer becomes smaller and thus, more highly accurate alignment can be achieved over the entire surface of the wafer.

In the above-described embodiments, silicon single crystal has typically been used as the material of the semiconductor wafer substrate, but the same conception can also apply to other wafer substrates such as GaAs, SoS, quartz substrate, etc. and a sufficient effect can be expected by carrying out the present invention.

In the foregoing, description has been made of the embodiments in which the coefficient of linear expansion of the holding portion of the chuck is of a small absolute value substantially coincident with the coefficient of linear expansion of the mask or the wafer, but where the coefficient of linear expansion of the portion of the chuck is of a great absolute value substantially coincident with the coefficient of linear expansion of the mask or the wafer, such chuck can be used by controlling the temperature of the mask or the wafer.

Also, where the coefficient of linear expansion of the adsorbing portion of the chuck is smaller than the coefficient of linear expansion of the mask or the wafer and the absolute value to that coefficient of linear expansion is of a great absolute value, such chuck is usable by temperature control.

Controlling the temperture of the mask or the wafer is useful not only where the coefficient of linear expansion of the holding portion of the chuck is of a high absolute value, but also where it is of a low absolute value.

The above-described temperature control may be effected in a manner as shown, for example, in FIG. 3. FIG. 3 shows a semiconductor exposure apparatus in which only the wafer chuck is temperature-controlled. The wafer chuck 4 fixedly supports the wafer 2 by vacuum and, in the interior thereof, there are provided a heating resistor 9 which is a heating means for making the temperature of the wafer chuck higher by several degrees than the room temperature, a cooling air inlet 11 and a cooling air outlet 12 for circulating air of normal temperature as cooling air, and a platinum temperature measuring resistor 8 for detecting the temperature of the interior of the chuck.

The mask pattern and the wafer pattern are aligned and at the same time, the magnification errors of the two patterns are read, and the temperature width to be varied is calculated from the values of the magnification errors and the coefficient of linear expansion of the wafer, and then a temperature having this value added to the current temperature of the chuck is set by a temperature control device 13.

Thereafter, the temperature control device 13 is operated and the temperature of the wafer chuck is controlled so that the difference between the temperature detected by the platinum temperature measuring resistor 8 and the set temperature becomes zero. In FIG. 3, reference numeral 10 designates a radiator plate and reference numeral 14 denotes a power source.

In the above-described embodiment, air cooled by a cooling device, instead of the air of normal temperature, may be used as the cooling air. In such case, the cooling speed of the chuck is further increased and the temperature control below the room temperature becomes possible and therefore, efficient correction of the magnification error in a wider range is possible.

The cooling device referred to herein need not always have the temperature controlling function, but a sufficient effect may be provided even by a simple cooling device utilizing, for example, the adiabatic expansion of air.