Dark line CD and XY-CD improvement method of the variable shaped beam lithography in mask or wafer making

(1) Field of the Invention

This invention relates to forming images in a layer of resist material with a variable shaped electron beam and more specifically to methods of improving critical dimension and XY critical dimension control.

(2) Description of the Related Art

Electron beam exposure systems using an electron beam with a variable shape are often used in direct exposure of a layer of resist material formed on a substrate such as an integrated circuit wafer or a mask substrate. However critical dimension control can be inadequate for the cutting of the dark line around the circumference of the pattern. Methods providing improved control of dark line critical dimension and XY critical dimension are important.

U.S. Pat. No. 5,578,821 to Meisberger et al. describes an electron beam inspection system and method. In the inspection configuration there is a comparison system for comparing the pattern on a substrate with a second pattern.

U.S. Pat. No. 4,698,509 to Wells et al. describes a pattern generator for supplying beam deflection and blanking signals in an electron beam lithography system.

U.S. Pat. No. 5,808,892 to Tu describes an electron beam processing method for improving micron and submicron line quality and resolution.

U.S. Pat. No. 5,888,682 to Nakasuji describes methods and apparatus for accurately performing proximity-effect correction.

Electron beam exposure systems are often used in direct exposure of a layer of resist material formed on a substrate such as an integrated circuit wafer or a mask substrate. Advantages in processing efficiency can be gained by using an electron beam having a variable shape. The variable shaped electron beam, however, has less critical dimension control and beam size stability than an electron beam using a fixed beam shape. This loss of critical dimension control can be a problem at the outer circumference of a pattern element.

It is a principle objective of this invention to provide a method of using an electron beam exposure system having an electron beam with a fixed shape and a variable shape to form patterns in a layer of resist on an integrated circuit wafer while maintaining adequate critical dimension control and beam stability.

It is another principle objective of this invention to provide a method of using an electron beam exposure system having an electron beam with a fixed shape and a variable shape to form patterns in a layer of resist on a mask substrate while maintaining adequate critical dimension control and beam stability.

These objectives are accomplished by first setting the electron beam to a fixed beam size and shape. The fixed beam used has a square shape with a width set to provide optimum XY critical dimension control. A frame pattern is then formed around the outer perimeter of the original pattern, hereinafter referred to as the first pattern. The width of the frame pattern is a first distance, wherein the first distance is the width of the fixed beam which provides optimum XY critical dimension control. The fixed beam is then used to expose the frame pattern in a layer of resist. A remainder pattern which is the frame pattern and the first pattern subtracted from the full exposure field is then exposed in the layer of resist using an electron beam having a variable size and shape. This method maintains the advantages of the variable shape electron beam for most of the pattern exposure while maintaining critical dimension control at the critical dimension region of the first pattern.

FIG. 1 shows a top view of pattern elements of an original pattern.

FIG. 2 shows a cross section view of a substrate having a layer of resist formed thereon.

FIG. 3 shows a diagram of an electron beam exposure system.

FIG. 4 shows a cross section of the electron beam taken along line 4 — 4 in FIG. 3 .

FIG. 5 shows a top view of the original pattern converted to the original pattern surrounded by a frame pattern of this invention.

FIG. 6 shows a block diagram of the system used to form the digital design data for the frame pattern and the full exposure field except for the frame pattern and center pattern.

Refer now to FIGS. 1-6 for a description of the preferred embodiments of this invention. FIG. 1 shows a number of first pattern elements 12 each having an outer perimeter 11 . FIG. 2 shows a substrate 30 having a layer of opaque material 18 formed on the substrate 30 and a layer of resist material 10 formed on the layer of opaque material 18 . The pattern elements 12 are unexposed areas in the layer of resist 10 which has been exposed using an electron beam exposure system. The substrate 30 can be any suitable substrate but in this example the substrate is either an integrated circuit wafer or a mask substrate. In the case where the substrate 30 is a mask substrate the substrate 30 will be formed of a transparent material such as quartz and the opaque material 18 will be a material suitable for forming opaque mask elements, such as chrome. In the case where the substrate 30 is an integrated circuit wafer the wafer 30 will be a material such as silicon and the opaque material 18 will be a material suitable for forming conducting electrodes, such as aluminum, aluminum copper, polysilicon, or the like. The X and Y directions are indicated in FIG. 1 . The layer of resist 10 can be either a positive or negative resist.

FIG. 3 shows a diagram of an electron beam exposure system used to expose the layer of resist 10 which is formed on the substrate 30 . The substrate 30 is placed on a substrate holder 32 which is capable of moving the substrate 30 in X and Y directions. The X direction is indicated in FIG. 3 and the Y direction is perpendicular to the paper. The electron beam exposure system has a source of electrons 34 producing an electron beam 36 . The beam is shaped and positioned using a first aperture 38 , a second aperture 40 , and a shape deflector 42 . The shape deflector 42 uses either a magnetic field or an electric field to adjust the beam size and shape.

Conventionally the electron beam exposure system uses a variable beam size and shape, adjusting the beam in various portions of the pattern. A cross section of the electron beam is shown in FIG. 4 . As shown in FIG. 4 the beam has a rectangular shape, having a width 14 and a length 16 . In this example the beam width is between about 2.0 and 0.01 microns and the beam length is also between about 2.0 and 0.01 microns.

The variable beam size causes problems in critical dimension, CD, control, particularly at the outer perimeter 11 of the first pattern elements 12 . The variable beam size can change with time and different beam sizes have a linearity problem. In a test pattern of 121 test points formed in the same mask the X-CD had a range of 0.0364 microns with a 3-sigma of 0.0223 microns; the Y-CD had a range of 0.1036 microns with a 3-sigma of 0.0639 microns; and the XY-CD had a mean of −0.022 microns, a range of 0.101 microns, and a 3-sigma of 0.064 microns at after develop inspection.

In this invention the first step is to choose an optimum beam which is the beam size giving the best XY-CD is chosen and the beam is fixed at this size. In this example the beam size with the best XY-CD is a square beam having a width 14 and length 16 both equal to about 1.5 microns. With this fixed beam size the test pattern of 121 test points formed in the same mask the X-CD had a range of 0.0294 microns with a 3-sigma of 0.0191 microns; the Y-CD had a range of 0.0316 microns with a 3-sigma of 0.0191 microns; and the XY-CD had a mean of 0.005 microns, a range of 0.0364 microns, and a 3-sigma of 0.0213 microns at after develop inspection.

With this optimum beam of about 1.5 microns by 1.5 microns the beam size is very stable with time. For a clear line of 1.5 microns the test pattern of 121 test points formed in the same mask the X mean at 22 minutes was 1.5323 microns at 22 minutes, 1.5306 microns at 56 minutes, and 1.527 microns at 99 minutes; and the Y mean at 22 minutes was 1.5347 microns at 22 minutes, 1.5337 microns at 56 minutes, and 1.5348 microns at 99 minutes at after develop inspection. The optimum beam is a square beam, in this example having a width of between about 2.0 and 0.01 microns.

The next step of the invention after choosing the optimum beam size as a square beam having a width of between about 2.0 and 0.01 microns is to form a frame pattern 24 around the outer periphery 11 of the first pattern elements 12 as shown in FIG. 5 . FIG. 5 shows the first pattern elements 12 with a frame pattern 24 surrounding the outer perimeter 11 of each of the first pattern elements 12 . Each frame pattern 24 also has an outer perimeter 25 . There is a first distance 26 between each frame pattern 24 outer perimeter 25 and the outer perimeter 11 of the first pattern element 12 surrounded by that frame pattern 24 . The first distance 26 is chosen to be equal to the width of the optimum square beam, in this example between about 2.0 and 0.01 microns.

The frame patterns 24 are then exposed in the layer of resist, reference number 10 in FIG. 3, using the optimum fixed beam. As shown in FIG. 5, the remainder of the entire exposure field 21 is the entire exposure field except for the regions of the exposure field occupied by the first pattern elements 12 and the frame patterns 24 . After the exposure of the frame patterns 24 has been completed, the remainder of the entire exposure field 21 is then exposed in the layer of resist, reference number 10 in FIG. 3, using a variable beam size. The layer of resist is then developed which removes the exposed resist for a layer of positive resist or the unexposed resist for a layer of negative resist. The unexposed resist is that part of the resist within the frame patterns 24 , which is the region occupied by the first pattern elements 12 , see FIG. 5 . The exposed resist is that part of the resist occupied by the frame patterns 24 and the remainder of the exposure field 21 .

Alternatively the remainder of the entire exposure field 21 , which is the entire exposure field except for the regions of the exposure field occupied by the first pattern elements 12 and the frame patterns 24 , are first exposed in the layer of resist, reference number 10 in FIG. 3, using a variable beam size. After the exposure of the remainder of the entire exposure field 21 has been completed, the frame patterns 24 are then exposed in the layer of resist, reference number 10 in FIG. 3, using the optimum fixed beam. The layer of resist is then developed which removes the exposed resist for a layer of positive resist or the unexposed resist for a layer of negative resist. The unexposed resist is that part of the resist within the frame patterns 24 , which is the region occupied by the first pattern elements 12 , see FIG. 5 . The exposed resist is that part of the resist occupied by the frame patterns 24 and the remainder of the exposure field 21 .

In this invention the critical dimension control is maintained by exposing the critical regions of the frame patterns 24 with the square optimum beam, in this example having a width of between about 2.0 and 0.01 microns. The advantages of the variable beam size are retained for the less critical region of the remainder of the entire exposure field 21 , which is the region outside the frame patterns 24 and the first pattern elements 12 .

FIG. 6 shows a block diagram of the system used to form the design data for the pattern elements 24 and the remainder of the exposure field 21 from the design data for the first pattern elements 12 , which is provided. As shown in FIG. 6, the provided first element design data 60 are fed into a computer processor 62 . The computer processor 62 then determines the remainder of the exposure field digital design data 66 using the first pattern element digital data 60 by adding the first distance around the outer perimeters of each of the first pattern elements. The computer processor then forms the frame pattern digital design data 64 for the remainder of the exposure field by subtracting the first patterns and the frame patterns.

While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the invention.