WO2007031105A1

WO2007031105A1 - Alignment method with compensation of non linear errors

Info

Publication number: WO2007031105A1
Application number: PCT/EP2005/009975
Authority: WO
Inventors: Tomas ÖSTRÖM
Original assignee: Micronic Laser Systems Ab
Priority date: 2005-09-16
Filing date: 2005-09-16
Publication date: 2007-03-22

Abstract

The present invention relates to alignment of a writing system and a workpiece (11). In particular, it relates to alignment to write a second layer pattern on a workpiece (11) that has a first layer pattern, using an SLM (21). It extends to producing a mask or reticle, and to producing a layer of a device using the mask or reticle. Particular aspects of the present invention are described in the claims, specification and drawings.

Description

NON LINEAR PSM ALIGNMENT METHOD

FIELD OF THE INVENTION

[0001] The present invention relates to alignment of a writing system and a workpiece. In particular, it relates to alignment to write a second layer pattern on a workpiece that has a first layer pattern. It extends to producing a mask or reticle, and to producing a layer of a device using the mask or reticle.

BACKGROUND OF THE INVENTION

[0002] The development of semiconductor lithography has been exponential since the early 60ies and the produced features are getting smaller every second or third year, at the same time as the circuits get faster and more complex. The errors in lithography can broadly be classified as placement and size errors, or "registration" and "critical dimension", ("CD") in the jargon of the trade. There is a more or less fixed relation between the errors that can be allowed in the pattern and the size of the smallest features in the pattern. A rule of thumb is that on the mask the placement of figures has to be within 5% of the design rule and the size of the features should be within 2.5% of the design rule. These are surprisingly small numbers, but have been justified by both theory and experiments.

[0003] A variety of lithographic techniques have been developed that use a mask or reticle that has been patterned twice. The second patterning may produce a phase shift structure. The word "registration" is used in the mask industry meaning misregistration from a reference grid, normally an ideal mathematical grid. In the past, registration of the finished product to an ideal mathematical grid has not been necessary. If all layers (approximately 25 in a semiconductor chip and 6 in a TFT) are printed using the same type of exposure station, systematic and equal behavior of the exposure station will cancel, since every layer is distorted in the same way. However, when resolution is pushed in order to achieve circuit speed and packaging density, the cost of lithography is rising rapidly, both because of higher tool cost and because of more expensive masks. To make production more economical the display, mask and chip manufacturer are trying to use more sophisticated technology than needed for each layer, so called mix-and-match. Different layers may be printed using different types of exposure tools with different error characteristics. [0004] It is increasingly important that the second patterning of the mask or reticle be accurately aligned to the first. This is particularly important with alternating phase shift structures, which may be produced in more than two patterning steps and with different exposure tools having different characteristics as described above. One problem with the prior art technique of aligning and patterning said second layer on top of said first layer is that only linear errors may be compensated for. This is performed by aligning the workpiece in the exposure tool by means of measuring alignment marks provided beside a die area. However, as the masks tends to be larger and the dimensions smaller it may become more and more difficult to achieve the needed registration between different layers.

[0005] There is a need in the art for compensation of non linear errors in the first layer when patterning said second layer in order to improve a matching of structures between said two layers.

SUMMARY OF THE INVENTION [0006] The present invention relates to alignment of a writing system and a workpiece. In particular, it relates to alignment to write a second layer pattern on a workpiece that has a first layer pattern. It extends to producing a mask or reticle, and to producing a layer of a device using the mask or reticle. Particular aspects of the present invention are described in the claims, specification and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] FIG. 1 depicts a sample layout of alignment marks around a die on a mask.

[0008] FIG. 2 depicts a pattern generating apparatus according to prior art.

DETAILED DESCRIPTION

[0009] The following detailed description is made with reference to the figures. Preferred embodiments are described to illustrate the present invention, not to limit its scope, which is defined by the claims. Those of ordinary skill in the art will recognize a variety of equivalent variations on the description that follows.

[0010] The term "exposure equipment" as here employed should be broadly interpreted as referring to any type of radiation exposure source such as electromagnetic radiation, including ultraviolet (UV) radiation (e.g., having a wavelength of 365, 248, 193, 157 or 126 nm) and extreme ultraviolet (EUV) radiation (e.g., having a wavelength in the range of 5-20 nm), as well as particle beams, such as ion beams or electron beams. [0011] The exposure equipment may also be of a type wherein the substrate is immersed in a liquid having a relatively high refractive index, e.g., water, so as to fill a space between a final element of a proj ection system and a substrate.

[0012] One of the sub-functions in the Micronic Sigma 7300 mask writer is the second layer alignment system for writing of phase shift masks. The strategy chosen for performing PSM alignment is to use the DUV writing laser together with the spatial light modulator (SLM) to create a light stamp image, which is reflected on the first layer alignment marks. The reflected image is captured and measured with a DUV-sensitive

CCD camera. Using the writing laser avoids position offsets coming from misalignment of multiple laser sources.

[0013] Attenuated and alternating phase shift mask (PSM) have become widely used resolution enhancement techniques to overcome shrinking kl factors. Even more aggressive PSM techniques are being developed. A common denominator for the techniques is that the PSM is manufactured in at least two writing steps, where the mask is developed, etched and re-coated with new resist between two exposures. To expose resist twice, the mask writer must be able to align the second level exposure, so that the pattern matches the first level. The first level exposure is often made with an electron beam mask writer. The second level exposure may be made with another type of exposure tool, for instance a laser beam mask writer.

[0014] For attenuated PSM, the required layer-to-layer alignment accuracy is typically several hundreds of nm. The required pattern accuracy (resolution, CD accuracy, placement accuracy) is also very loose for the second layer and it can therefore be printed e.g. on lower-end i-line laser pattern generators (PGs). The required layer-to-layer alignment accuracy for alternating PSM is much tighter. The requirements on pattern accuracy are also tighter to avoid phase errors on the PSM. For the initial alternating PSM applications, i-line laser PGs are still commonly used. However, as the feature sizes shrink for the 90-nm, 65-nm and lower technology nodes, the pattern accuracy from i-line laser PGs will not match the requirements on the second layer of the alternating PSM or other more aggressive PSM techniques.

[0015] One alternative to the i-line laser PG for the second layer is to use the same electron beam mask writer as in the first layer, an approach that has been demonstrated by R. Plontke, L. Bettin, D. Beyer, J. Butschke, M. Lmscher, C. Koepernik, B. Leibold, A. Vix, P. Voehringer, in "Avoidance/Reduction of Charging Effects in Case of Partially Insufficient Substrate Conductivity when using ESPACER 300Z", 20th European Mask Conference on Mask Technology for Integrated Circuits and Micro-Components, GMM- FB 43, pp 233-240, 2004. If the mask writer can align the second layer correctly, this should assure a good match between the two layers patterns. The disadvantage of using an electron beam system for the second layer is the creation of charges in the relief of the first layer pattern, charges that reduce the accuracy of the electron beam. One suggested method to avoid the charging is to apply a conducting film under or on top of the second layer resist, an extra process step that could lead to increased number of defects and reduced production yield.

[0016] Another attractive alternative to the i-line laser PG is a laser PG that uses a

DUV wavelength. The short wavelength together with pattern fidelity enhancement techniques give pattern accuracy that very well matches the electron beam printed first layer, as described by H. Martinsson, J. Hellgren, N. Eriksson, M. Bjuggren, T. Sandstrom, in "Transparent corner enhancement scheme for a DUV pattern generator", Photomask Japan 2003, Proceedings SPIE vol. 5130, pp 297-308, 2003. As optical exposure is used for the second layer, charging effects don't occur and there is no need to apply any conducting film. The second layer must still be aligned to the first layer and the DUV PG must have a very accurate alignment system. The Sigma7300 DUV PG from Micronic Laser Systems uses 248 nm wavelength and an imaging technique similar to a 248 nm lithography scanner. It provides the resolution and pattern accuracy needed to avoid phase defects between the first and second layer and it has a very accurate second layer alignment system. [0017] The Sigma 7300 second layer alignment system uses the pattern generating spatial light modulator (SLM), [see WO 99/45439 which is incorporated herein by reference,] illuminated with the writing 248 nm DUV Laser and CCD camera looking at the reflected image from the substrate containing the first layer alignment pattern. See T. Sandstrom, P. Askebjer, J Sellander, R. Zerne, A. Karawajczyk, 'Tattern Generation with SLM Imaging", 21st Annual BACUS Symposium on Photomask Technology, Proceedings SPIE vol. 4562, pp 38-44, 2001. Using the same laser when measuring, as during writing, avoids optical alignment differences between measuring and writing. Only small, dedicated areas on the mask are exposed and there is reduced risk of affecting the real patterns of the PSM. Since the image grabbing is fast, even with a lot of robustness increasing redundancy, the yield of the system is very high. [0018] Figure 2 illustrates an example embodiment according to a prior art pattern generating apparatus 20, which also could be used as a measuring apparatus, including means to write a pattern 21, e.g., an SLM, rotating polygon plus a Acousto optical modulator (AOM) as used in the Alta machines or an Acousto optical modulator (AOM) and an Acousto optical deflector (AOD) as used in the Omega machines, directing a laser beam from a laser, and means 22 to measure the height Hz between the apparatus 20 and a glass plate 11 with the surface 13 on which the pattern is to be written is placed upwards on a support 23, a so called stage. The pattern writing means 21 may be translated over the entire surface of the stage, which movement may be implemented in a number of ways. Figure 2 illustrates one way where the stage is provided with means to move it in relation to the pattern writing means 21 in the x direction, and where the pattern writing means 21 is attached to a sliding support 24 arranged on a beam 25 to move the pattern writing means in the Y direction. Other possible ways to implement the translation of the pattern writing means is to provide the means to move the stage in both x and y direction with a non-moving pattern writing means, or the pattern writing means could be provided with means to move in both x and y direction with a non moving stage. [0019] The apparatus 20 may also be provided with an angled foot plate 26 arranged at a constant distance above the surface 13 of the glass plate 11 by means of an air cushion 27. The foot plate 26 and the pattern writing means 21 may be attached to the sliding support 24 via a flexible attachment 28, to allow the distance between the sliding support 24 and the pattern writing means/foot plate to vary dependent on the roughness of the surface 13 of the glass plate 11. The varying distance in the z direction, i.e., the height hz, may be measured to calculate the roughness of the surface 13 in the z direction. The size of the foot plate that is parallel to the surface 13 of the glass plate 11 has an opening for a laser beam from the pattern writing means 21 and may be relatively large, e.g., 5 mm on each side, since the purpose of the measurement is to detect deviations in height over a relatively large distance. The air cushion beneath the foot plate will act as an auto focus device for the pattern generating apparatus due to the constant distance between the foot plate and the glass plate. [0020] The invention should however not be limited to this kind of pattern generating apparatus using air cushion as an auto focus device, but other types of systems that will provide focus for the system could be used.

[0021] In one embodiment a reference surface is determined against which the difference in height hz is calculated. The reference surface may have any desired shape as long as the shape of the reference surface is maintained unchanged. In one example embodiment said shape of the reference surface is a flat plane.

[0022] In another embodiment according to the present invention said apparatus further having the capability of measuring the characteristics of the pattern in the die of the first written layer. Only aligning the workpiece, which already has at least one layer printed thereon, by using said alignment marks, presumes that the errors outside the die are equal to the errors inside the die. i.e., that the errors at the edges of the workpiece, where the alignment marks are provided, are linear and equal to the errors inside the same die. However, most likely there are non linear errors in the pattern of the at least first layer which have to be corrected for in order to meet the requirements of registrations when a second layer is to be printed on top of said at least first layer. Said non linear errors may come from an uneven thickness of the resist layer, clamping errors, dose variations of the exposure tool, vibrations, non ideal surface (roughness) of the workpiece, lens errors, and systematic errors from the machine writing the at least first layer etc. [0023] A surface map may be computed by measuring a number of points of said first layer of pattern. The more measuring points used in the computation the better an approximation of the measured surface will be to the real situation. However, more measuring points will result in longer measurement times. The distance between adjacent measurement points should not exceed a predetermined distance, which is dependent on the required accuracy for the measurement to get a reasonable good result from the measurement. An example of maximum distance between adjacent measurement points maybe 100 mm.

[0024] Said surface map may be computed by first collecting information about a plurality of measurement points. [0025] In a first embodiment said surface map is determined by introducing a measurement plate having an appropriate measurement pattern provided on its surface. The measurement may be performed by relaying a laser beam, for instance the laser beam from the exposure laser source in the machine which is to be written said second layer on top of said first layer. A surface map may be stored for a plurality of different types of exposure equipments which have provided said at least first layer on said workpiece. Such exposure equipments may be different kind of electron beam(s) exposure equipment, laser beam(s) exposure equipment or any type of exposure equipment. Different brands may produce different surface maps, different types from the same manufacturer may also provide different surface maps, even different machines of the same brand and type may produce different surface maps. These surface maps may be stored and used in the correction when writing said second layer on top of said first layer. Said second layer may be printed with a different kind of exposure equipment than said at least first layer. Said second layer may be written by using information about the systematic linear and/or non linear behavior of said at least first layer of the pattern, which is most likely written by using a first type of exposure equipment, and information about linear and/or non linear behavior of the second type of exposure equipment which is going to be used for printing said second layer on top of said at least first layer of the pattern. [0026] In another embodiment according to the present invention the measurements may be performed by relaying a laser beam from said first layer onto a detector while the workpiece is aligned on the stage and ready to be provided with said second layer exposure. By collecting information about the position of a number of features in said first layer pattern and compare said measurement result with the pattern data used to print said first layer pattern, one can make up a map of how the first layer pattern is distorted, in essentially the same way as used when making up the surface map using said measurement plate. The features to be detected in said first layer pattern may be "special" features surrounded by different types of features. The advantage with detecting said special features is that thy may quickly be detected and not mixed up with adjacent similar features. If measuring on a "common" feature surrounded by common features you may run into a situation where you measure a first "common" feature displaced to be in a position of a second "common2 feature". Such a measurement will result in a wrong surface map and will thereby result in a wrong correction of the second layer printed on top of said first layer. [0027] The relayed signal and detector used for detecting said signal from the first layer pattern within the die may be the same tools used for analyzing the alignment marks outside said die. The measurement may be performed before applying the resist to be exposed with said second layer pattern or after applying said resist for said second layer. In the latter case, i.e., when measuring on the first layer pattern coated with the resist to be patterned with said second layer, a wavelength to be used may or may not expose said resist. A wavelength not exposing said resist may come from a separate laser source or the same laser source as used for exposing the resist. In the case of using the same laser source as used for exposing the resist, said laser source may be manipulated in some way, one type of manipulation may be the frequency doubling technique. [0028] In another embodiment according to the present invention, alignment marks are not only provided outside said die but also on so called "free positions" inside said die. hi such way, one may use the alignment marks within the die for computing said surface map instead of measure on real features in said first layer pattern. In an alternative embodiment according to the present invention, said surface map is computed by using information from both the factual position of features as well as the factual positions of alignment marks inside said die.

[0029] The surface map between measurement points are approximated by using well known techniques, such as linear approximation, second grade equation approximation etc.

[0030] An overview of second layer alignment provides an introduction to figures that are described below, hi one embodiment, figure 1, the first layer alignment marks that are used contain 2 coarse align marks 410, 16 fine align marks 430 and 2 reserved areas 420. Different numbers of coarse alignment marks, fine alignment marks and reserved areas can readily be used. The reserved areas are used for reference background normalization and CCD alignment. The coarse align strategy is improves overall performance and efficiency. The fine align marks are simple cross patterns with 2 μm line width. A pattern position in the Sigma 7300 is defined by the stage position and the pattern position on the SLM. To achieve an absolute position of the reflected first layer pattern CCD image, the coordinate system must be transferred from the SLM to the CCD. This is achieved by the CCD alignment displaying a pattern on the SLM₅ which is reflected on a reserved chrome area on the substrate back to the CCD. This technique makes the system practically offset free. Iterating the coarse align measurements and the fine align cross pattern measurements creates a temporary surface map transformation containing translations, scales, orthogonality and rotation which is used during the second layer printing.

[0031] One of ordinary skill will understand that areas reserved or allowed for alignment marks may be adjusted, translated, bent or changed in dimensions. These allowed areas can accommodate rough align marks, fine align marks and blank marks. For instance, two rough align marks can be placed anywhere within allowed areas, preferably on opposite sides. The rough align marks may be 1.5 mm square, with a reserved border of 0.5 mm on each side, hi addition, two blank marks, 1.5 mm square or another size, can be placed anywhere within the allowed areas. Sixteen fine align marks can be accommodated, preferably four per allowed area, with a minimum separation between marks of 1 cm and a minimum separation between first and last marks in an area of 10 cm. These marks may be 1 mm square.

[0032] One alignment sequence begins with aligning the SLM and the CCD camera. With a workpiece in place, a rough alignment is conducted using the rough alignment marks. The pattern of these marks reduces the searching required to locate the fine alignment marks positioned elsewhere in the allowed areas. The fine alignment marks are measured next, to provide data to calculate the origin, scale, ortho and rogation of the mask. [0033] With some or all of these statistics calculated, the measurement can be verified and a second mask layer written.

[0034] After docking in the pattern 11 flanks are measured. These flanks include the two bars of two consecutive V-marks. Since the two bars have a different width they can easily be identified. After this identification the pitch between the V-marks as well as the pitch of the V-marks themselves are calculated. For the further calculations the coordinates of the present position (xθ, yθ) are stored.

[0035] Since these measurements are affected by a possible rotation the rotation of the pattern (j) has to be measured. This rotation can be estimated by making a second measurement on another x-coordinate. In order to make a safe movement it must first be decided in which direction to move not to end up outside the pattern. By analyzing the measured pitch of the V-marks it can be concluded if the head is docked above or below the center of the pattern. Knowing this it is safe to move in the direction towards central regions of the pattern.

[0036] If the rotation of the plate is large there is a risk of loosing track of the pattern when moving in x. To avoid this, a small initial movement (50 microns) gives a rough estimation of the rotation. With this estimation it is now safe to move 2 mm, giving a higher angular resolution.

[0037] With the rotation known the measured pitches are adjusted to a few trigonometric relations. The pitch of the two V-mark give the x-offset to the "origin" of the pattern (Dx). The pitch between the marks identifies their numbers in the array (index). This index in turn gives the y-offset to the pattern "origin" (Dy).

[0038] The calculated offsets are only valid in the coordinate system of the pattern.

By including the pattern rotation the pattern origin can be transformed into the coordinate system of the stage (x_prigin, y_origin). [0039] A position accuracy of 10 —20 microns was achieved with rotations up to at least five degrees. The goal pattern was in these tests placed roughly 3 cm from the roughalign pattern

[0040] One aspect of the present invention is that the same illumination source may be used for alignment and for printing of patterns. For alignment, the source may be projected onto the workpiece using the SLM or, alternatively, using an additional mirror. When the SLM is used for projecting illumination during alignment, the amount of illumination can be controlled by appropriate orientation of the mayor's, as has been described in previous applications assigned to Micronic Laser. [0041] One embodiment of present invention is a method for writing a second layer of a pattern aligned to a first layer of the pattern on a workpiece using a spatial light modulator (SLM). This method includes loading the workpiece which includes the first layer of pattern, at least one pre-alignment mark, and at least one alignment mark, onto a stage in the pattern generator. The method further includes aligning the SLM with a CCD camera, detecting at least one pre-alignment mark using the CCD camera, detecting at least one alignment mark using the CCD camera and writing a second layer of pattern on the workpiece using information about the detected at least one alignment mark. [0042] In the method described, a variety of marks may be used. One suitable pre- alignment mark includes horizontal and vertical gridlines, where the horizontal and vertical gridlines have an increasing pitch or decreasing spatial frequency. The change in pitch or frequency may be linear, exponential or logarithmic.

[0043] Yet another embodiment is method of producing a phase shifted mask or reticle, according to the any of the prior embodiments and further including exposing a layer of radiation sensitive material on the workpiece after a first patterning of the workpiece. This method also includes patterning phase shifting structures on the workpiece using the exposed layer. Again, any of the aspects of embodiments mentioned above also can be applied to or combined with this embodiment. [0044] Of course, the present invention can alternatively be practiced as a device including logic and resources adapted to carry out the methods described above. The device may be a pattern generator, a module installed the pattern generator, or an external device in communication with the pattern generator. Or, it may be practiced is an article of manufacture impressed with machine-readable code adapted to carry out the methods described above.

Claims

1. A method of writing at least a second layer of a pattern over a first layer of the pattern on a workpiece, wherein the workpiece includes at least one alignment mark, the method including: providing information about the at least one alignment mark; providing information about a surface map of the first layer of the pattern inside a die of said workpiece, said first layer of the pattern is exposed by a first layer exposure equipment; aligning said first layer of the pattern in a second layer exposure equipment by using the information about the at least one alignment mark; and compensating for at least non-linear errors in said first layer of the pattern by using the information about the surface map of the first layer of the pattern inside the die of said workpiece when writing the at least a second layer of the pattern.

2. The method according to claim 1, wherein said information about said surface map is provided from a separate substrate.

3. The method according to claim 2, wherein information about different surface maps from different types of exposure equipments are pre-stored and ready to be used to compensate for at least non-linear errors in said first layer of the pattern when writing the at least a second layer of the pattern.

4. The method according to claim 1, wherein said at least one alignment mark is provided outside said die.

5. The method according to claim 1 , wherein at least one alignment mark is provided inside said die.

6. The method according to claim 1, wherein the surface map is measured before applying a resist layer for writing said second layer.

7. The method according to claim 1, wherein the surface map is measured after applying a resist layer for writing said second layer.

8. The method according to claim 1 or 6, wherein said information of said surface map is provided by detecting at least one reflected beam of electromagnetic radiation from said workpiece.

9. The method according to claim 7, wherein said information of said surface map is provided by detecting at least one reflected beam of electromagnetic radiation from said workpiece, where said resist is essentially unaffected by the wavelength of said beam of electromagnetic radiation.

10. The method according to claim 1 , wherein said compensation for linear errors includes calculating at least translation and rotation parameters of the alignment.

11. The method according to claim 1 , wherein said at least one alignment mark is provided on the workpiece when applying said first layer of the pattern.

12. The method according to claim 1, wherein the final product of said first and second layer of the pattern is a phase shift mask.

13. The method according to claim 1, wherein the electromagnetic radiation is detected by a CCD camera.

14. The method according to claim 1, further including: capturing an image of said die area of the workpiece by a CCD camera