This invention relates to a dust particle inspection apparatus and a device manufacturing method using the same. The present invention is suitably usable for inspection of the state of presence/absence a foreign substance such as a dust particle (presence/absence of or the size of such particle) or the height of such dust particle (magnitude in a direction of a normal to a surface where the particle is adhered to), upon a substrate such as a mask, for example, having a circuit pattern formed thereon, particularly in the field of the manufacture of devices such as semiconductor devices (e.g., IC or LSI), CCDS, liquid crystal panels or magnetic heads, for example. Thus, the present invention is suitably applicable to the manufacture of high-precision devices such as described above.
Particularly, the present invention provides specific advantageous effects when the same is applied to a semiconductor exposure method called a xe2x80x9cProximity X-ray Lithographyxe2x80x9d (hereinafter, xe2x80x9cPXLxe2x80x9d) wherein an X-ray beam of a wavelength 7-10 angstroms emitted from an electron accumulation ring (synchrotron radiation unit) is used as a light source and wherein a pattern of a mask is transferred to a wafer at a unit magnification while the mask and the wafer are dispose d opposed to each other with a gap of a few tens microns maintained therebetween.
Generally, in IC manufacturing processes, a circuit pattern formed on a substrate such as a mask is transferred to a wafer, being coated with a resist, by use of an exposure apparatus. If in this procedure there is a foreign substance such as a dust particle upon the surface of the substrate, such particle is also transferred in the transfer process to decrease the yield of the IC or LSI production.
In the conventional PXL procedure, a mask is disposed opposed to a wafer with a gap (clearance) of 10-30 microns kept therebetween, and the mask pattern is transferred to the wafer through Fresnel diffraction.
As regards exposure apparatuses of PXL type, an exposure apparatus having a largest exposure range of 52 mm square has currently been proposed. The largest exposure range of 52 mm square means that, even in the unit-magnification exposure, for a wafer of a size of 4 inches or larger, the whole surface of the wafer can not be exposed through a single exposure operation.
In the PXL exposure process, the whole surface of a wafer is exposed while the wafer is moved sequentially as in a repetition reduction exposure apparatus, called a xe2x80x9cstepperxe2x80x9d. Thus, in this respect, an exposure apparatus of PXL type may be called as a xe2x80x9cunit magnification X-ray stepperxe2x80x9d.
As regards the resolution, a result of 100 nm or less, or a result of 20 nm or less based on the alignment result, has been reported. Also, it is recognized that the PXL has a potential for an exposure process for devices of 1 gigabit or more. One special feature of the PXL is an X-ray mask.
Conventional X-ray mask manufacturing processes will now be explained, with reference to FIG. 5 and the like. Here, as regards the thicknesses of a mask, a wafer and a film thereon, for better understanding, they are illustrated in proportions different from practical proportions.
In the manufacture of a PXL mask, as shown in FIG. 5, a silicon (Si) wafer 30 is prepared as a substrate. Then, as shown in FIG. 6, a SiC film 31 of a thickness of 2-3 microns, called a membrane, is formed on the Si wafer 30.
When the SiC film 31 is produced on the Si wafer 30, practically the film is formed on the top and bottom faces of the substrate as well as the side face thereof. Since, however, the bottom face and the side face do not provide a function, in FIG. 6 and later, the films formed there are not illustrated.
Subsequently, as shown in FIG. 7, the surface of the SiC film 31 is flattened by polishing, whereby a SiC film 32 is provided. Then, as shown in FIG. 8, an ITO film or SiO2 film 33 is produced as an etching stopper and also for better affinity with an X-ray absorptive layer. Thereafter, as shown in FIG. 9, a material having a relatively high X-ray absorptivity such as W, Ta or Ta4B, for example, is applied with a thickness 0.3-0.5 micron, as an X-ray absorptive material 34. Then, as shown in FIG. 10, through various processes such as resist application, desired patterning with an electron beam patterning apparatus, development, etching, and resist separation, a pattern is defined by the X-ray absorbing material 34.
Subsequently, as shown in FIG. 11, a portion of the Si wafer 30 at a side thereof remote from the pattern is removed by back etching, such that X-rays can transmit through the Si wafer portion 35 corresponding to the exposure range. Finally, as shown in FIG. 12, the peripheral portion of the Si wafer 30 is mounted on a frame 36, by which an X-ray mask is accomplished.
It is well known that, in order to minimize the patterning error, an additional procedure may be performed after the step of FIG. 10, so that, as shown FIG. 13, the Si wafer 30 is mounted on a frame 36 and then, as shown in FIG. 14, the portion of the Si wafer 30 corresponding to the exposure range is back etched to enable transmission of X-rays therethrough. Thereafter, as shown in FIG. 15 and like FIG. 12, various processes such as resist application, desired patterning with an electron beam patterning apparatus, development, etching, and resist separation may be performed so that a pattern is defined by the X-ray absorbing material 34.
However, if the patterning process is performed after the frame 36 is mounted to the Si wafer 30, there may arise a problem that the Si wafer 30 and the frame 36 are detached from each other due to heat. In consideration of it, in many cases, the X-ray mask is produced by taking processes such as shown in FIGS. 5-12.
As regards the frame, it may be called as a xe2x80x9csupport ringxe2x80x9d. As for the material thereof, Pyrex or SiC is used. For mounting it to the membrane, an anodic bonding process or an adhesive agent is used.
A proposal has been made to use an integral type frame 37 (FIG. 15) wherein the frame 36 of FIG. 13 is made of the same material as the Si wafer 30, that is, to make the Si wafer substrate 30 and the frame 36 as a unit.
There is a problem peculiar to the PXL. That is, when a dust particle of a size larger than the exposure gap between a mask and a wafer is sandwiched between the mask (particularly, a SiC membrane) and the wafer, the SiC portion of the mask may be destroyed.
Seemingly, if the exposure gap is 10 microns, there is no possibility that a dust particle larger than 10 microns is present between a wafer and a mask. This is particularly so because a good yield rate is regarded absolutely important in the semiconductor manufacture. However, this is not correct. Particularly, if a dust particle is attached to a peripheral portion outside the effective area of a wafer or mask, a problem peculiar to the PXL arises.
As regards such dust particle adhered to the peripheral portion of a mask or wafer, it has not raised a critical problem since the current semiconductor manufacturing procedure uses, in most cases, an exposure apparatus called xe2x80x9coptical exposure apparatusxe2x80x9d wherein a pattern of a mask is projected and printed on a wafer through a projection optical system. In such optical exposure apparatus, there is a distance of 1 cm or more between the wafer and the projection optical system of the exposure apparatus. Further, the peripheral portion of a wafer is not used for the IC production. Therefore, as regards a dust particle at the wafer peripheral portion, no inspection process is currently performed.
However, according to the observation of the wafer peripheral portion made by the inventors of the subject application, in many cases there were large dust particles at the wafer peripheral portion. It has been found that, even in the semiconductor manufacture wherein a good yield rate is regarded absolutely important, in many cases there are large dust particles at the wafer peripheral portion.
As long as optical exposure apparatuses are used, no critical problem may arise from a dust particle at the wafer peripheral portion. However, the problem is just not yet known. To be exact, there is a possibility that such dust particle is displaced (separated) from the wafer peripheral portion, for some reason, to a wafer pattern portion (effective area) to cause a critical problem. A wafer inspection apparatus for inspecting any dust particle on a wafer having a pattern already formed thereon, may be used to perform the inspection, to prevent a decrease of the yield.
However, in the PXL procedure, the presence of a dust particle at the peripheral portion can cause a serious problem.
It is now assumed that, as shown in FIG. 16, a dust particle 13 is adhered to a peripheral portion of a wafer 2, and that a PXL exposure process is performed with a predetermined gap kept between the wafer and a mask 1. When the wafer 2 is thereafter moved such as shown in FIG. 17 for exposure of a region near the peripheral portion of the wafer 2, a force is applied to the dust particle 13 attached to the wafer 2.
At this time, since the dust particle 13 contacts to a portion of the Si material 30 of the mask 1, not having been back etched, this does not cause breakage of the mask 1.
However, if after the exposure the wafer 2 moves for exposure of another region thereof, since a force has been applied to the dust particle 13, the motion of the wafer 2 causes separation of the dust particle 13 from the wafer 2. Thus, the dust particle 13 also moves to another place.
As the dust particle 13 moves, it may be attached to a region corresponding to the SiC portion 32 of the pattern 35 of the mask, having been back etched, as shown in FIG. 18. In that occasion, when the exposure and wafer motion process is repeated, a force is applied again to the dust particle 13. Since the SiC portion 32 there has a thickness of 2-3 microns, the SiC portion may be destroyed.
Although what described above concerns an example wherein a dust particle is adhered to a peripheral portion of the wafer 2, the same applies to a case where a dust particle is attached to a peripheral portion of the mask 1.
FIG. 19 illustrates it. That is, a dust particle 13 is attached to a mask 1 at a first shot of wafer exposure. A force is applied to the dust particle 13 as the same is sandwiched between the mask and the wafer 2. The mask 1 is not broken thereby because of a similar reason as the case where a dust particle is attached to the peripheral portion of the wafer, as described hereinbefore. As the wafer 2 moves thereafter, the dust particle 13 may be moved and adhered to a region corresponding to the SiC portion 32 where the Si material portion of the mask 1 has been back etched. In that occasion, the mask 1 may be destroyed, similarly.
As described above, a dust particle at a peripheral portion of a mask or wafer, which does not cause serious inconveniences in the optical exposure method, particularly when it is larger than the exposure gap, may cause a critical problem of breakage of a mask.
Further, even if the mask is not broken, if a dust particle being moved displaces onto a wafer and it is not detected by inspection for some other reason, the semiconductor device may not function well. Thus, it may cause a decreased yield rate. This is similar to the problem in the optical exposure method.
The inventors of the subject application have made investigations about dust particle inspection to the whole surface of a wafer and an X-ray mask, including the peripheral portion thereof, by use of a wafer dust particle inspection apparatus for detecting a dust particle on a wafer having a pattern already formed thereon.
As regards the wafer dust particle inspection apparatuses for detecting a dust particle on a wafer having a pattern already formed thereon, there is a type which is based on such detection principle that a polarized light is obliquely projected on a wafer so that, by a circuit pattern, the light is reflected while keeping its polarization characteristic, whereas, by a dust particle, the light is reflected with a non-polarized state.
Inspection apparatuses using this detection principle have already been developed as product machines, and they practically assure a high throughput that the detection time of only 1 minute or shorter is necessary for an 8-inch wafer, as well as a high reliability. They have contributed to higher yield rates.
However, as described above, in optical exposure apparatuses, no concern has been put on the peripheral portion of a wafer.
According to the investigations made by the inventors of the subject application, it has been found that, by using such wafer dust particle inspection apparatus, for both a mask and a wafer, a dust particle can be detected while being distinguished from an etched portion of the Si material at the periphery. However, it has also been found that, in the vicinity of the periphery of the mask, there is a possibility that a signal larger than one for a usual dust particle may be produced to cause an erroneous detection as the presence of a large dust particle.
The cause of such erroneous detection will be such as follows. As regards a mask, currently, no specific design has been made to the structure of the peripheral portion. Therefore, in a CVD apparatus, a dust particle may be adhered to a peripheral portion of a wafer when the same is supported. Also, there may be non-uniformness of SiC film thickness or of absorptive material film thickness, due to influences applied from the peripheral portion of the mask. Further, there may be a peeled film portion produced as the film is scratched by tweezers or the like during the mask handling, for example, when the mask is mounted on a frame.
If these particles or surface irregularities are measured by using a current dust particle inspection apparatus for a wafer having a pattern formed thereon, it may be discriminated that there is a large dust particle, being larger than 10 microns.
In current wafer dust particle inspection apparatuses, the size of a dust particle is discriminated on the basis of a correlation table for a particle and a corresponding signal output detected beforehand. In this detection principle, the dust particle is taken as providing isotropic light scattering. On the other hand, from the peeled film portion or from the film thickness irregularities, because of its complicated structure, the light may be refracted and scattered and, thus, it may be detected. In that occasion, a large output signal as compared with a signal output of isotropic scattering light from a dust particle, is detected consequently. Thus, as regards the peeled film or the like, it may be detected as being a large dust particle, being larger than its real size, such as more than 10 microns.
Further, it has been found that, in the peripheral portion of a wafer, a dust particle may be crushed during the conveyance or as the wafer is mounted on a carrier, such that a dust particle of a large size, although it has no height, may be detected.
However, such peeled film, film non-uniformness, or a flat dust particle on the wafer is not a particle which has a height of 10 microns or more. Thus, it has no potential of causing breakage of the mask.
It is accordingly an object of the present invention to provide a dust particle inspection apparatus and a device manufacturing method using the same, by which a dust particle having a potential of causing breakage of a mask and a dust particle not having such potential can be discriminated and inspected, to thereby facilitate the manufacture of large integration devices.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.