1. Field of the Invention
The present invention relates to a method of generating mask distortion data used in a lithography step in producing a semiconductor device, an exposure method using the same, and a method of producing a semiconductor device using the same.
2. Description of the Related Art
Masks having a common point are used in lithography using charged particles, such as an electron beam (EB) and an ion beam, under development as next generation lithography (NGL) which follows after present photolithography (wavelengths of 248 nm and 193 nm are dominant) used in mass production of a semiconductor device.
The common point is that, by deeply etching a substrate to be a mask from the back surface side to leave a thin film (membrane) having a thickness of 10 nm to 10 μm or so, mask blanks as a mask before being formed a pattern are formed and a pattern to be transferred are arranged to the obtained thin film.
Since such a mask includes a membrane region having low mechanical strength, not to mention a method of forming a pattern of high positional accuracy (IP: image placement), a technique of measuring distortion of a mask and correcting an IP error by using the information is important.
Proximity electron lithography (PEL) and electron projection lithography (EPL) are particularly dominant among NGL using an EB.
General explanations on an electronic optical system and mask of the two methods are described on the PEL in “Journal of Vacuum Science and Technology”, B17, p. 2897 (1999) by T. Utsumi and on the EPL in “Japanese Journal of Applied Physics”, 34, p. 6658 (1999) by H. C. Pfeiffer.
Among masks used in the above PEL and EPL, (1) those having a transfer pattern formed by an opening of a membrane are called stencil masks (for example, refer to “Japanese Journal of Applied Physics”, 34, p. 6658 (1999) by H. C. Pfeiffer) and (2) those having a transfer pattern formed by a scattering body, such as a metal thin film, are called a scattering membrane mask (for example, refer to “Journal of Vacuum Science and Technology”, B15, P. 2130 (1997) by L. R. Harriott).
The former stencil mask is used for both of the PEL and EPL, while the scattering membrane mask cannot be used for the PEL. It is because the PEL generally uses a low speed EB of several keV or less and the incident EB on the membrane is all absorbed.
Since these masks include a membrane region having low mechanical strength, a mask structure of not configuring the overall mask region with one membrane but configuring with a large number of small sectional membranes divided by a lattice shaped beam has been proposed. For example, a PEL mask is described in the Japanese Unexamined Patent Publication No. 2003-59819 and an EPL mask is described in the U.S. Pat. No. 5,523,580.
FIG. 1 is a schematic perspective view of a mask having a lattice shaped beam as above. The lattice shaped beam 100b composes a plurality of recessed portions 100a and, thereby, a thin film (membrane) 102 is divided to a large number of small sectional membranes. In the thin film 102, small regions sectionalized by the beam 100b respectively become pattern regions PA formed with a pattern.
As a method of producing a mask having the above beam structure, a method of using wet etching by an alkali solution of KOH, etc. and a method of using reactive ion etching are described in the U.S. Pat. No. 6,428,937.
FIG. 2A is a schematic sectional view of the above stencil mask.
It has a multilayer structure of forming on a silicon substrate 110 a buried oxide layer 111 as an interlayer having an etching stopper function and a silicon thin film (SOI: silicon on insulator) 112 to be a membrane. A part of the substrate 110 corresponding to the membrane is formed with recessed portions 110a sectionalized to be a lattice shape by being etched from the back surface to configure the lattice shaped beam 110b. 
Also, the SOI layer 112 is formed with through holes P along a mask pattern.
FIG. 2B is a schematic sectional view of the above scattering membrane mask.
A silicon nitride thin film 121 to be a membrane is formed on the silicon substrate 120, and a part of the substrate 120 corresponding to the membrane is formed with recessed portions 120a sectionalized to be a lattice shape by being etched from the back surface to compose the lattice shaped beam 120b. 
Also, a scattering body pattern 124, for example, made by a chrome film 122 having a film thickness of 10 nm and a tungsten film 123 having a film thickness of 50 nm is formed along the mask pattern.
As a method of transferring a pattern of a mask used in the PEL and EPL on a wafer at a high accuracy, a method of measuring distortion of the mask and correcting it when transferring can be considered.
This concept itself is not new and has been also used in photolithography, and IP accuracy of a produced photomask is measured routinely by a coordinate measuring device called “LMS IPRO” made by LEICA Corporate or “lightwave XY-6i” made by Nikon Corporation.
For example, when a magnification error of 3 ppm is measured on a mask pattern, the error can be corrected by finely adjusting an optical system of an exposure apparatus when exposing the photomask with a stepper or a scanner.
However, in lithography using charged particles, such as the PEL and EPL, since incident particles can be deflected at high accuracy and high speed by an electrostatic/magnetic field lens, it is considered that correction at a higher accuracy becomes possible.
In the case of the PEL, a main deflection lens and a sub deflection lens are combined to deflect the EB. Here, by scanning the EB by the main deflection lens on the mask region and changing an incident angle of the EB by the sub deflection lens in real time, distortion of the mask can be corrected (for example, refer to the U.S. Pat. No. 4,334,156).
On the other hand, in the case of the EPL, individual membrane sectionalized by the beam 100b as shown in FIG. 1 is defined as a subfield, and a method of transferring the subfield by EB irradiation at one time on a wafer and forming a device pattern by successively connecting the subfields on the wafer is applied. For example, a method of correcting distortion for each subfield is disclosed in the Japanese Unexamined Patent Publication No. 2000-124114.
In the case of the PEL, there is an advantage that, by giving information of distortion over the whole mask region as a map and compensating between measurement data points by a function of a higher order, distortion of a higher order can be corrected in addition to linear distortion of magnification, rotation and orthogonality.
On the other hand, in the case of the EPL, only linear distortion of each subfield can be corrected.
In either case of the above PEL and EPL, accurate measurement of mask distortion is important at first. However, a mark for a coordinate measuring device, such as the above LMS IPRO, generally cannot be arranged on a region arranged with a device pattern.
Accordingly, in the case of a photomask, a mark for coordinate measurement is arranged on a part corresponding to a so called scribe line, that is, a chip periphery region (a margin region when detaching a chip on a wafer by dicing).
FIG. 3 is an example of a layout of arranging the marks for coordinate measurement on the scribe line. Coordinate measurement marks MK are arranged on the scribe line SL for sectionalizing the chip region.
Reliability of distortion data becomes higher when increasing the number of coordinate measurement marks to increase measurement points and, furthermore, uniformly distributing them allover the mask region, however, due to the limit that the coordinate measurement marks can be arranged only on the scribe line, only limited number of coordinate measurement marks could be used as shown in FIG. 3.
In the EPL, as shown in FIG. 1, by utilizing the fact that the mask is originally divided to small sectionalized subfields (typically, about 1 mm square), it has been proposed to arrange coordinate measurement marks on a beam between the subfields (refer to the U.S. Pat. No. 6,040,095).
As explained above, since only linear distortion of subfields is corrected in the EPL, it is sufficient to measure marked coordinates at four corners of each subfield. The marks on the beam are not transferred to the wafer, so that it is possible to increase measurement points and improve accuracy of distortion measurement. The U.S. Pat. No. 6,040,095 discloses a formation method and arrangement method of the marks on the beam.
In the method described in the above U.S. Pat. No. 6,040,095, however, accuracy of measuring distortion is limited, so it is difficult to use the method in device production using the PEL and EPL.
Namely, the method of the U.S. Pat. No. 6,040,095 stands on an assumption that beam marks and an actual device pattern in a membrane displace by the same distortion function, so that disposition of the latter can be corrected based on measurement data of the former. However, this assumption is physically not self-evident, moreover, it became clear that it stood only approximately in our actual measurement.
Accordingly, more accurate measurement of distortion of the membrane is not possible only by measuring the beam marks. Particularly, in the case of the PEL, since a pattern on the mask is transferred on the wafer without reduction, an effect of a distortion data error is supposed to be larger than that in the case of the EPL. Thus, it is difficult to apply the method described in the U.S. Pat. No. 6,040,095 as it is.