KR100452732B1 - Assist features for use in lithographic projection - Google Patents

Abstract

In mask patterns for devices such as DRAMs that include a nearly regular array of isolated features, auxiliary features are placed to make the array more symmetrical. If the isolated feature is not at every point in the regular unit cell but at the maximum point, the auxiliary feature may be placed at a point in the unit cell that is not occupied by the isolated feature. Isolated features may represent contact holes.

Description

Assist FEATURES FOR USE IN LITHOGRAPHIC PROJECTION}

The present invention

An illumination system for supplying a projection beam of radiation;

A first object table provided with a first object holder for holding a mask;

A second object table provided with a second object holder for holding a substrate; And

Using a lithographic projection apparatus comprising a projection system for drawing the irradiated portion of the mask onto a target area of the substrate,

Providing a mask with a pattern to the first object table;

Providing a substrate having a radiation sensing layer to the second object table; And

A mask having an assist feature is used in a conventional device manufacturing method comprising drawing the irradiated portion of the mask to a target area of the substrate.

For simplicity of explanation, the projection system will hereinafter be referred to as the "lens". However, the term should be broadly interpreted as encompassing various types of projection systems, including refractive optics, reflective optics, catadioptric systems, and charged particle optics, for example. The illumination system may also include elements that operate according to any of the principles of directing, shaping or controlling the projection beam of radiation, and in the following description these elements will be referred to collectively or individually as "lenses". . In addition, the first and second object tables may be referred to as "mask tables" and "substrate tables", respectively.

As used herein, "radiation" and "beam" are terms used to encompass all forms of electromagnetic radiation and include ultraviolet (eg, having a wavelength of 365 nm, 248 nm, 193 nm, 157 nm or 126 nm). , And extreme ultraviolet (EUV). In principle, these terms also include X-rays, electrons and ions. The present invention is also described herein using a coordinate system in the orthogonal X, Y and Z directions, wherein the rotation about an axis parallel to the I direction is denoted by R i . In addition, unless otherwise required in the context, the term "vertical" (Z) as used herein refers to the direction normal to the substrate or mask surface or parallel to the optical axis of the optical system, rather than to any particular orientation of the device. It is. Similarly, the term "horizontal" means a direction parallel to the substrate or mask surface or perpendicular to the optical axis, ie normal to the "vertical" direction.

Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In this case, the mask (reticle) may contain a circuit pattern corresponding to each layer of the integrated circuit, which pattern may be a target region or an exposure region (silicon wafer) on a substrate (silicon wafer) coated with a radiation sensing material (resist) layer. In general, a single wafer is formed with a whole network consisting of several adjacent target areas, and these target areas are continuously irradiated one at a time through a reticle. In one type of lithographic projection apparatus, each target region is irradiated by exposing the entire mask pattern onto the target region at once. Such an apparatus is commonly referred to as a wafer stepper. An alternative apparatus, commonly referred to as a step-and-scan apparatus, progressively scans the mask pattern under a projection beam in a predetermined reference direction ("scanning" direction), while the same as the scanning direction. Each target area is irradiated by synchronously scanning the substrate in the opposite or opposite direction. Since the projection system generally has a magnification factor M (generally <1), the speed V at which the substrate table is scanned is a factor M times that at which the mask table is scanned. More detailed information relating to the lithographic apparatus described herein can be found in International patent application WO 97/33205.

Generally, a lithographic apparatus has one mask table and one substrate table. However, devices with at least two independently movable substrate tables have also been developed. For reference, multi-stage devices are disclosed, for example, in International Patent Applications WO 98/28665 and WO 98/40791. The basic operating principle of this multi-stage device is that while the first substrate table is in an exposure position below the projection system for the exposure of the first substrate on the table, the second substrate table is moved to the loading position and exposed. The substrate may be taken out, the new substrate may be picked up, an initial measurement of the new substrate may be performed, and immediately after the exposure of the first substrate is completed, the new substrate may be transferred to an exposure position under the projection system. Repeat the cycle. In this way, the throughput of the device can be substantially increased, and the cost of maintaining the device is improved. This principle applies equally when only one table is used which is moved between the exposure position and the loading position.

It is known to provide a so-called "assist feature" in the mask to enhance the image projected onto the resist and eventually improve the developed device. The auxiliary feature is not intended to appear in the developed pattern in the resist but rather is provided to the mask in order to take advantage of the diffraction effect so that the developed image is closer to the desired circuit pattern. Generally speaking, an auxiliary feature means a "sub-resolution" that is at least one dimension smaller than the smallest feature of the mask to be actually decomposed on the wafer. Auxiliary features may have dimensions defined as the smallest width of a feature or the minimum distance between features in a mask and sometimes a fraction of a "critical dimension" which is the resolution limit of the lithographic projection apparatus in which the mask will be used. However, since the mask pattern is generally projected at a magnification of less than 1, such as 1/4 or 1/5, the auxiliary feature on the mask may have a larger physical dimension than the minimum feature on the wafer. Two kinds of auxiliary features are known. Scattering bars are lines with sub-resolution widths placed on one or both sides of an isolated conductor to mimic the proximity effect that occurs in areas with dense patterns. A serif is an additional area of various shapes placed at the corners of the conductor line and at the corners of the end or square features to make the ends or corners of the line more angled or rounded as desired. A secondary feature called "is considered to be in the form of a serif). Further information regarding the use of scattering bars and serifs can be found, for example, in US 5,242,770 and US 5,707,765, which are also incorporated herein by reference.

Contact holes or vias in integrated circuits are particularly problematic in drawing. Because contact holes often must be formed through a large number of, or relatively thick, process layers already formed on the wafer, they must be patterned into a relatively thick photoresist layer and increase the depth of focus in the aerial image of the mask pattern.

It is an object of the present invention to provide an improved mask that allows better drawing of dense features regularly or irregularly spaced, such as contact holes, as well as a method of making such a mask and a device using said improved mask. It is to provide a method for producing.

1 shows a lithographic projection apparatus in which an embodiment of the invention may be used,

2 is a view showing a part of a mask pattern for printing contact holes in the manufacture of a dynamic random access memory (DRAM);

3 is a view showing a part of a mask pattern for printing a contact hole in a DRAM according to the first embodiment of the present invention;

4 is a graph of partial intensity of an aerial image generated by the mask pattern of FIGS. 2 and 3;

5 shows a portion of an alternative mask pattern for printing contact holes;

6 is a view showing a part of a mask pattern according to a second embodiment of the present invention;

FIG. 7 is a graph of partial intensity of an aerial image generated by the mask pattern of FIGS. 5 and 6;

8 shows a portion of a mask pattern for printing rectangular features;

9 illustrates a portion of a mask pattern for printing rectangular features, according to a third embodiment of the present invention;

10 is a contour plot of the intensity of a portion of an aerial image generated by the mask pattern of FIGS. 8 and 9;

11 is a view showing a portion of a mask pattern according to the fourth embodiment of the present invention.

According to the invention, in the manufacture of a device, a mask for a lithographic projection apparatus comprising a plurality of isolated areas representing a feature to be printed and contrasting with the background, the isolated areas are generally arranged in an array. ,

A mask for a lithographic projection apparatus is provided, further comprising a plurality of auxiliary features that are smaller than the isolated area and arranged to make the array more symmetrical.

Auxiliary features are ideally placed to reduce asymmetry in the aerial image by reducing asymmetry in the array and to reduce all asymmetrical aberrations due to odd aberrations such as 3-wave and coma. In an array that already has some degree of symmetry, the secondary feature may add additional symmetry or make the unit (raw) cell more symmetrical in itself. In this way, the present invention can improve the imaging symmetry which is not highly dependent on the exposure apparatus and the illumination or projection settings. In many cases, an isolated feature may appear to occupy a regular array of grid points (although the isolated feature may deviate slightly from the ideal grid point), but at least one point of the unit (or primitive) cell is empty. have. In this case, the auxiliary feature is placed at at least some of the empty grid points.

For example, an array of features in a regular mask by itself, having one or more rotational symmetry or reflective symmetry, is necessarily limited and therefore lacks translational symmetry. In other words, the unit cell at the edge of the array has fewer neighboring unit cells and thus is not exactly equivalent to the middle unit cells. Thus, auxiliary features in accordance with the present invention may be placed around the outside of the array to form pseudo-neighbors for the outermost features within the array. Of course, secondary features are placed inside the array so that composite auxiliary features at different locations, for example, make the array more rotationally symmetrical or reflective symmetrically, and the environment of the outermost feature is more likely to be visible to the interior feature. It can be used as an array in which auxiliary features are placed outside the array to make them similar.

The auxiliary features must be at least one dimension small enough so that they are detectable in the aerial image and partially expose the light (energy sensitive) resist, so that they do not appear in the developed pattern of the resist. Thus, the auxiliary feature is generally less than the line width of the mask pattern and the resolution limit of the lithographic apparatus in which the mask will be used.

The isolated region may represent, for example, a contact hole to be formed in the DRAM array. Isolated regions and auxiliary features may be transparent regions on a relatively opaque background or vice versa. In reflective masks, isolated areas and auxiliary features will have different reflectances from the background. In a phase shift mask, isolated regions and auxiliary features may introduce different phase shifts and / or different attenuations than the background. The secondary feature need not have the same "tone" as the isolated region.

According to yet another aspect of the present invention, a method of fabricating a mask for a lithographic projection apparatus, the method comprising fabricating a device, the method comprising forming a plurality of isolated regions representing features to be printed and contrasting with the background; In the mask manufacturing method in which the divided areas are generally arranged in an array,

And forming a plurality of auxiliary features that are smaller than the isolated region and are arranged to make the array more symmetrical.

The position, shape, and size of the secondary features are calculated for the aberrations at the wavefront to be generated by the pattern without the secondary features, and then the positions for the secondary features that reduce the expected aberrations, especially three- and one-wave (cosmetic) aberrations. And so on.

According to another form of the invention,

An illumination system for supplying a projection beam of radiation;

A first object table provided with a first object holder for holding a mask;

A second object table provided with a second object holder for holding a substrate; And

A method of manufacturing a device using a lithographic projection apparatus comprising a projection system for drawing an irradiated portion of a mask on a target area of a substrate,

In manufacturing a device, a patterned mask is provided to said first object table, said mask comprising a plurality of isolated areas representing features to be printed and contrasting with the background, said isolated areas being generally arranged in an array. step;

Providing a substrate having a radiation sensing layer to the second object table; And

Drawing the irradiated portion of the mask on the target region of the substrate,

The mask is provided with a plurality of auxiliary features that are smaller than the isolated area and are positioned to make the array more symmetrical.

In the manufacturing process using the lithographic projection apparatus according to the present invention, the pattern of the mask is drawn on a substrate on which a layer of energy sensing material (resist) is applied even at a minimum. Prior to this imaging step, the substrate may go through various processes such as priming, resist application and soft bake. After exposure, there is another process such as post-exposure bake (PEB), development, hard bake and measurement / inspection of imaged features. This series of procedures is used, for example, as the basis for patterning each layer of the IC device. This patterned layer is then subjected to several processes to process each layer, such as etching, ion implantation (doping), metallization, oxidation, chemical-mechanical polishing, and the like. If several layers are required, the whole process or its modified process will have to be repeated for each new layer. As a result, there will be an array of devices (dies) on the substrate (wafer). These devices are separated from each other by a technique such as dicing or sawing, and each of these devices can be mounted to a carrier and connected to a pin or the like. Additional information on such a process can be obtained, for example, from "Microchip Fabrication: A Practical Guide to Semiconductor Processing (3rd edition, Peter van Zant, McGraw Hill Publishers, 1997, ISBN 0-07-067250-4)." Can be.

Although reference is made herein to specific applications of the device according to the invention in the manufacture of integrated circuits, it will be apparent that such devices have many other possible applications. For example, the apparatus may be used in the manufacture of integrated optical systems, induction and detection patterns for magnetic region memories, liquid crystal display panels, thin film magnetic heads, and the like. As those skilled in the art relate to these alternative applications, terms such as "reticle", "wafer" or "die" as used herein are more general, such as "mask", "substrate" and "target area", and the like. It should be considered that the term is being replaced.

While the disclosure herein concentrates on lithographic apparatus and methods using masks used to pattern a beam of radiation entering a projection system, the invention described herein employs general "patterning means" for patterning the beam of radiation. It can be seen to extend to lithographic apparatus and methods. The term " patterning means " is broadly interpreted as meaning a means that can be used to impart a patterned cross section corresponding to a pattern to be formed in a target area of a substrate to an incident radiation beam, and as used herein, " It is also used in the term "light valve". In general, the pattern will correspond to a specific functional layer in the device to be formed in the target area, such as an integrated circuit or other device. Examples of such patterning means include the following in addition to the mask on the mask table.

Programmable mirror array. An example of such a device is a matrix-addressable surface with a viscoelastic control layer and a reflective surface. The basic principle of such a device is that incident light is reflected as diffracted light in (eg) addressed areas of the reflecting surface, while incident light is reflected as non-diffracted light in unaddressed areas. Using an appropriate filter, the undiffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind. In this way, the beam is patterned according to the addressing pattern of the matrix-addressable surface. The required matrix addressing can be performed using any suitable electronic means. More information about such mirror arrays can be obtained, for example, from US Pat. Nos. 5,296,891 and 5,523,193, which are incorporated herein by reference.

Programmable LCD Array. An example of such a structure is disclosed in US Pat. No. 5,229,872, which is incorporated herein by reference.

Such general patterning means in the context of the present specification and claims should be construed to imply an alternative meaning for the mask.

Hereinafter, the present invention will be described with reference to exemplary embodiments and the accompanying drawings.

Lithographic projection apparatus

1 schematically depicts a lithographic projection apparatus that may be used with the present invention. The device,

A radiation system Ex, IL for supplying a projection beam PB of radiation (for example UV or EUV radiation). In special cases the radiation system also comprises a radiation source LA;

A first object table (mask table) MT having a mask holder for holding a mask MA (e.g., a reticle) and connected to first positioning means for accurately positioning the mask with respect to the item PL. );

A second object table (substrate) having a substrate holder for holding the substrate W (for example, a resist coated silicon wafer) and connected to second positioning means for accurately positioning the substrate with respect to the item PL; Table) (WT); And

Projection system (" lens ") PL (e.g., refractive system, kata) for drawing the irradiated portion of the mask MA to the target area C (consisting of one or more dies) of the substrate W; Dioptric system or reflective optical system).

As shown, the device is of a transmissive type (eg with a transmissive mask). In general, however, it may also be reflective (e.g. with a reflective mask). Alternatively, the apparatus may employ other kinds of patterning means, such as the above mentioned programmable mirror array or LCD.

The radiation source LA (eg, lamp, laser or discharge plasma chamber) produces a projection beam of radiation. The beam enters the lighting system (illuminator) IL directly or after conditioning means, for example beam expander Ex, into the lighting system. The illuminator IL comprises adjusting means AM for setting the outer and / or inner radial magnitude (commonly referred to as sigma-outer and sigma-inner, respectively) of the beam intensity distribution. It also typically includes a variety of other devices such as integrators (IN) and capacitors (CO). In this way, the beam PB incident on the mask MA has a uniformity and intensity distribution in its cross section.

1, the radiation source LA is placed in the housing of the lithographic projector (e.g. sometimes as if the radiation source LA is a mercury lamp), but it is far from the lithographic projection apparatus and the radiation it produces The beam may be brought into the device (eg by means of a suitable directional mirror). In the case where the radiation source LA is an excimer laser, it is more likely to be the latter. The present invention and claims encompass both of these cases.

Subsequently, the beam PB passes through the mask MA, which is fixed on the mask table MT. The beam PB passing through the mask MA passes through the lens PL to focus the beam PB on the target area C of the substrate W. By means of the second positioning means (and interferometer measuring means IF), the substrate table WT can be accurately moved to position different target areas C in the path of the beam PB, for example. Similarly, the first positioning means is capable of accurately positioning the mask MA with respect to the path of the beam PB, for example, after mechanically withdrawing the mask MA from the mask library or during scanning. Can be used. In general, the movement of the objective tables MT, WT will be done by a long stroke module (coarse positioning) and a short stroke module (fine positioning), although not clearly shown in FIG. However, in the case of a wafer stepper (as opposed to a step-and-scan apparatus), the mask table MT may only be connected or fixed to a short stroke actuator.

The apparatus described above can be used in two different modes:

In the step mode, the mask table MT is basically kept stationary, and the entire mask image is projected to the target area C at once (ie in a single "flash"). Subsequently, the substrate table WT is shifted in the x and / or y directions so that different target regions C may be irradiated by the beam PB.

In scan mode, the same scenario applies, except that the predetermined target area C is not exposed in a single "flash". Instead, the mask table MT is movable in a predetermined direction (so-called " scanning direction &quot;, for example, y direction) at a velocity of v so that the projection beam PB is to scan all parts of the mask image and at the same time The substrate table WT simultaneously moves in the same direction or the opposite direction at a speed V = Mv , where M is the magnification of the lens PL (usually M = 1/4 or M = 1/5). In this way, a relatively large target area C can be exposed without degrading the resolution.

First embodiment

2 shows a portion of a conventional 6% attenuated phase shift mask pattern that can be used in the lithographic projection apparatus described above to form contact holes (vias) in a dynamic random access memory (DRAM) device. The mask pattern includes an array of transparent regions 110 on a substantially opaque background, and exposure radiation passes through the transparent regions 110 to expose a resist in a region where a contact hole is to be formed. It can be seen that the transparent area is arranged in such a manner that the group consisting of three (111, 112, 113) is regularly repeated. The contact holes are a square of 0.2 mu m and the spacing between the contacts is 0.2 mu m. A reference area 120 of 0.4 × 1.6 μm ² is also shown.

The inventors have found that these contact holes are not drawn accurately and are distorted and out of their normal position.

In accordance with the present invention, auxiliary features are added to the mask pattern such that the array of features is closer to symmetry. The proper location and dimensions of the secondary feature are determined experimentally by taking into account the Zernike coefficients representing the wavefront aberrations in the pattern and the image to be generated from the required secondary feature.

Wavefront aberrations can be expressed as a series of their angles,

Where r and θ are the radial and angular coordinates respectively (r is normalized), and m is an index indicating the contribution of the m th aberration. R and R 'are functions of r.

The aberration may be expressed in terms of Zernik development.

Where Z in each term is the Zernik coefficient and f in each term is the corresponding Zernik polynomial. The function f takes the form of the product of the polynomial of r and the sine or cosine of mθ. For example, the cosmetic aberration (m = 1) can be expressed as Zernik series of Z7, Z8, Z14, Z15, Z23, Z24, Z34, Z35, etc. F ₇ (r, θ)] is as follows.

Zernik development for low order aberrations is summarized in Table 1 below.

In particular, the inventors have found that substantial improvement can be achieved by arranging the auxiliary features to handle odd aberrations (odd m), in particular Z10 (3-wave) coefficients. Improvements can also be made by arranging the secondary features to reduce coma (one wave) aberration in the aerial image. The location, shape, and size of the auxiliary features can be determined using existing computer computing techniques, a program known as the technique, which is known as "Solid C", a software package distributed and distributed by Sigma-C GmbH, Germany. Simulate and model Other suitable software packages such as "Prolith" are known and may be used alternatively.

In the DRAM example, six repeating transparent regions (see FIG. 3) are made as two pairs of triplets (A, B, C and D, E, F) so that each element of the triplet is square (in this embodiment). Improvements are made by considering one of the four corners of the unit cell. According to the present invention, the auxiliary features 151 and 152 are placed in the fourth corner and constitute a transparent square of a size small enough not to be printed in the developed pattern. The square, for example, each side may be 0.12㎛. It is important to note that the secondary feature is visible in the aerial image and partially exposes the resist, but is washed away in the development of the resist.

The effect of the present invention can be seen in FIG. 4, which is a graph of the intensity in the aerial image generated by the mask pattern of FIGS. 2 and 3 corresponding to line 130. In FIG. 4, the solid line represents the intensity of the aerial image generated by the mask pattern (FIG. 3) according to the present invention, and the dotted line represents the intensity of the aerial image generated by the conventional mask pattern (FIG. 2). The asymmetry of the print can be represented by the distances L and R shown in FIG. 4, which are selected between 0.25 in any unit of the graph of FIG. 4 and measured at the resist threshold, the distance between peaks that will form a contact hole. Indicates. It can be seen from the present invention that the distance L is reduced so that the asymmetry represented by the distance difference L-R is also reduced.

Second embodiment

5 illustrates an alternative configuration of the contact hole 110. In this configuration, the pair of contact holes 211 and 212 are repeated to form a honeycomb structure. The contact hole is a square of 0.2 mu m and the spacing between adjacent contacts is 0.2 mu m. Also ^shown is a reference area 220 of 0.4 × 0.9 μm ² . According to the present invention, by adding an additional auxiliary feature 251 to the center of each honeycomb cell, the symmetry of the mask pattern is improved as shown in FIG. The auxiliary feature 251 may be, for example, a square having one side of 0.12 μm.

Along the line 230, an improvement provided as a second embodiment of the present invention can be seen in FIG. 7, which is a graph of the intensity of the aerial image generated by the mask pattern of FIGS. 5 and 6. As in FIG. 4, the solid line indicates the intensity of the image generated by the mask pattern (FIG. 6) according to the present invention, and the dotted line is by the conventional pattern (FIG. 5). It can be clearly seen that the aerial image generated by the pattern according to the invention is less asymmetric.

Third embodiment

Figure 8 shows a conventional "brick wall" pattern consisting of rectangles arranged in a staggered array. The rectangles are 0.2 × 0.5 μm ² , 0.2 μm apart from adjacent rectangles. The reference area 320 is 0.6 × 0.6 μm ² . As shown in FIG. 9, in accordance with the present invention, an auxiliary feature 350 is disposed between the rectangles 310. The auxiliary feature may likewise have, for example, 0.12 μm on one side.

10 is a contour plot at 0.25 (arbitrary unit) intensity threshold of the aerial image generated by unit cell 320. The fine dotted line represents the image generated by the present invention (FIG. 9), while the dashed-dotted line represents the image generated by the conventional mask pattern (FIG. 8). Advantages The dashed line represents the ideal aerial image of the mask pattern without triwave aberration. It can be clearly seen that the image provided by the present invention is closer to the ideal.

Fourth embodiment

11 shows a mask pattern according to a fourth embodiment of the present invention. This array consists of a regular rectangular array of features such as, for example, contact holes 410. The array itself is extremely regular and has a high degree of internal symmetry. However, comparing the features in cell 420 with the features in cell 421 shows that there are differences. Features 410 in cell 420 have neighbors on all sides, while features 410 in cell 421 have no neighbors outward of them. Thus, in accordance with the present invention, auxiliary features 450 are provided out of the array of features 410 to provide pseudo-neighbors to features at the edges of the regular array. Thus, cell 421 becomes more similar to cell 420. Accordingly, the translational symmetry of the array is such that the secondary feature is more similar to the array as seen from the print feature 410 in the middle of the array as viewed from the print feature 410 at the edge of the array. Improve.

The size of the printing feature 410 and the auxiliary feature 450 may be similar to that of the first to third embodiments and may be modified to suit the particular use of the invention required. If the printing features forming the array are not square, the secondary feature may also be non-square, but it must be small enough not to be printed. Auxiliary features around the outside of the array can be used in conjunction with auxiliary features within the array itself. Furthermore, if an image of a given auxiliary feature is affected by a feature farther than its nearest neighbor, then an array of additional auxiliary features may be provided as needed. In general, it is more desirable to provide auxiliary features around all sides of the array of printing features, but it is not necessary and / or practical to provide auxiliary features around the entire array if there is another feature near the array. In FIG. 11 the secondary feature appears to be spaced apart from the array at the same distance as the array pitch, but this distance may change to alter the effect of the secondary feature as intended.

While specific implementations of the invention have been described above, it will be apparent that the invention may be practiced otherwise than as described. The present invention is not limited to the above contents. It should be clear that the present invention can be applied to any mask pattern with (asymmetric) arrays or groups of features. The invention is also applicable to cases where the array contains only a portion of the mask pattern, for example in the manufacture of system-on-chip devices combining memory and logic or processors on one device.

According to the present invention, there is provided an improved mask for better drawing of regular or irregularly spaced and dense features such as contact holes, as well as methods of making such masks and manufacturing devices using the improved masks. A method is provided.

Claims (32)

In manufacturing a device, a mask for a lithographic projection apparatus comprising a plurality of isolated areas representing a feature to be printed and contrasting with the background, said isolated areas generally arranged in an array, And a plurality of auxiliary features smaller than said isolated area and arranged to make said array more symmetrical. The method of claim 1, And the isolated area is arranged in groups forming a unit cell, and wherein the auxiliary feature is arranged to make the unit cell more symmetrical. The method of claim 2, The isolated region is arranged at or near several points of at least one regular unit cell, and the auxiliary feature is disposed at a point of the regular unit cell that is not occupied by the isolated region. Mask for the device. The method of claim 3, Wherein said isolated region is disposed at three corner portions of a rectangular unit cell, and said auxiliary feature is disposed at a fourth corner portion. The method according to any one of claims 1 to 4, And the auxiliary feature is arranged to reduce the effect of at least one odd aberration at the wavefront generated by the mask pattern when illuminated by exposure radiation in the lithographic projection apparatus. The method according to any one of claims 1 to 4, The auxiliary feature is disposed along at least a portion of the edge of the array such that the surroundings of the features at or near the edge of the array are more similar to the surroundings of the features in the interior of the array. Mask for the projection device. The method according to any one of claims 1 to 4, The auxiliary feature is arranged to reduce the effect of three-wave and / or cosmetic (one-wave) aberrations at the wavefront generated by the mask pattern when illuminated by exposure radiation in the lithographic projection apparatus. Mask for lithographic projection apparatus. The method according to any one of claims 1 to 4, And the auxiliary feature has the same contrast to the background of the mask as the isolated area. The method according to any one of claims 1 to 4, And said isolated feature is more transparent than said background to exposure radiation of said lithographic projection apparatus. The method according to any one of claims 1 to 4, And the isolated feature is more reflective than the background to the exposure radiation of the lithographic projection apparatus. The method according to any one of claims 1 to 4, And wherein said isolated feature imparts a different phase shift than said background. The method according to any one of claims 1 to 4, And the auxiliary feature is smaller than the line width of the mask. The method of claim 12, And the auxiliary feature is smaller than a resolution limit of the lithographic projection apparatus. A method of making a mask for a lithographic projection apparatus, comprising: forming a plurality of isolated areas representing features to be printed and contrasting with the background in manufacturing the device, wherein the isolated areas are generally arranged in an array. In the mask manufacturing method, Forming a plurality of auxiliary features that are smaller than the isolated region and are arranged to make the array more symmetrical. The method of claim 14, Determining wavefront aberrations in the aerial image to be generated in the lithographic projection apparatus by means of the pattern of isolated regions; And Determining the location, shape, and size of the plurality of secondary features to reduce aberrations in the aerial image. The method of claim 15, Wherein the location of the plurality of auxiliary features is determined to reduce three-wave and / or one-wave (cometic) aberrations. An illumination system for supplying a projection beam of radiation; A first object table provided with a first object holder for fixing a mask; A second object table provided with a second object holder for fixing the substrate; And A method of manufacturing a device using a lithographic projection apparatus comprising a projection system for drawing an irradiated portion of a mask on a target area of a substrate, In manufacturing a device, a patterned mask is provided to said first object table, said mask comprising a plurality of isolated areas representing features to be printed and contrasting with the background, said isolated areas being generally arranged in an array. step; Providing a substrate having a radiation sensing layer to the second object table; And Drawing the irradiated portion of the mask on a target area of the substrate, And the mask is provided with a plurality of auxiliary features that are smaller than the isolated area and arranged to make the array more symmetrical. The method of claim 17, Said device comprises a memory array, in particular a dynamic random access memory array. A device manufactured according to the method of claim 17 or 18. The method of claim 17 or 18, The auxiliary feature having a maximum dimension less than 50% of the wavelength of the projection beam, preferably having a maximum dimension in the range of 30-40% of the wavelength of the projection beam. The method of claim 1, And wherein said auxiliary feature imparts a different phase shift than said background. The method of claim 1, And said isolated feature imparts a different attenuation than said background. The method of claim 1, And the auxiliary feature imparts different attenuation than the background. The method of claim 1, One or both of said auxiliary feature and said isolated area impart a different phase shift, attenuation, or tone than said background. Means recorded on a recording medium for instructing a computer to generate files corresponding to a mask used in a lithographic drawing process, the computer program for controlling the computer including the recording medium readable by the computer To Generation of the files, Forming a plurality of isolated areas representing a feature to be printed and contrasting with the background in manufacturing the device, the isolated areas are generally arranged in an array; And And forming a plurality of auxiliary features that are smaller than the isolated region and are arranged to make the array more symmetrical. The method of claim 25, The generation of the files, Determining wavefront aberrations in the aerial image to be produced in the lithographic projection apparatus by means of the pattern of isolated regions; And And determining the position, shape, and size of the plurality of secondary features to reduce aberrations in the aerial image. The method of claim 26, Wherein said location of said plurality of auxiliary features is determined to reduce one or both of three-wave and cosmetic aberrations. The method of claim 25, And wherein said isolated feature imparts a different phase shift than said background. The method of claim 25, And the auxiliary feature imparts a different phase shift than the background. The method of claim 25, And wherein said isolated feature imparts different attenuation than said background. The method of claim 25, And the auxiliary feature imparts different attenuation than the background. The method of claim 25, One or both of the auxiliary feature and the isolated area impart a different phase shift, attenuation or tone from the background.