Statistical Illuminator

Abstract

The technology disclosed relates to an illumination source including numerous laser diodes. In particular, it relates to extending the duty cycle and/or reducing the frequency of component replacement by detecting failure of one or more individual laser diodes and compensating for the failure, without replacing the laser diodes. The technology disclosed can be used in cases of catastrophic laser diode failure by changing the power of remaining laser diodes to restore illumination to the coherence function similar to the pre-failure illumination field. Particular aspects of the technology disclosed are described in the claims, specification and drawings.

Description

RELATED APPLICATION

[0001] This application claims the benefit of U.S. Provisional Application No. 61/158,310, filed 6 Mar. 2009, which is hereby incorporated by reference.

[0002] This application is related to US patent application entitled "Rotor Imaging System and Method with Variable-Rate Pixel Clock"; and US patent application entitled "Variable Overlap Method and Device for Stitching Together Lithographic Stripes"; and US patent application entitled "Lithographic Printing System with Placement Corrections", all filed contemporaneously. The related applications are incorporated by reference.

BACKGROUND OF THE INVENTION

[0003] The technology disclosed relates to an illumination source including numerous laser diodes. In particular, it relates to extending the duty cycle and/or reducing the frequency of component replacement by detecting failure of one or more individual laser diodes and compensating for the failure, without replacing the laser diodes.

[0004] The Micronic Laser development team has pioneered a variety of platforms for microlithographic printing. An established platform for the Sigma machine is depicted in FIG. 2. A rotor printing platform is described in recently filed patent applications. A drum printing platform is described in other patent applications.

[0005] One printing mechanism designed for these platforms uses swept beams that are modulated as they traverse the surface of the workpiece, applying energy as a paintbrush applies color. Another printing mechanism design freezes the motion of the workpiece with the flash and stamps two dimensional patterns on the workpiece, exposing a radiation sensitive layer in a manner similar to block printing a pattern. Printing with stamps is an intricate process that typically overlaps multiple writing passes.

[0006] Illuminators are a major part of the operating cost of many microlithographic printing systems. Accordingly, the opportunity is ever present to develop new illuminators. New illuminator designs may deliver increased power, extended lives, failure tolerance and decreased maintenance.

SUMMARY OF THE INVENTION

[0007] The technology disclosed relates to an illumination source including numerous laser diodes. In particular, it relates to extending the duty cycle and/or reducing the frequency of component replacement by detecting failure of one or more individual laser diodes and compensating for the failure, without replacing the laser diodes.

[0008] The technology disclosed can be used in cases of catastrophic laser diode failure by changing the power of remaining laser diodes to restore illumination to the coherence function similar to the pre-failure illumination field. Particular aspects of the technology disclosed are described in the claims, specification and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] FIG. 1: Partially coherent projection system using an SLM and relays in the illuminator and projection paths.

[0010] FIG. 2: Generic writer or printer using a one-dimensional SLM as disclosed.

[0011] FIG. 3: An example illuminator using an array of light sources S1-S8.

[0012] FIG. 4a: Gaussian distribution of light from the plane of the sources.

[0013] FIG. 4b: Depicting the coherence function with the failed laser diode.

[0014] FIG. 4c: Depicting the solution for a failed laser diode.

[0015] FIG. 5: A flow chart depicting the iteration process for maintaining a constant value for illumination field.

DETAILED DESCRIPTION

[0016] The following detailed description is made with reference to the figures. Preferred embodiments are described to illustrate the technology disclosed, 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.

[0017] The technology disclosed uses a one or two dimensional array of laser diodes with individually controlled power feeds and radiation outputs as an illumination source. Based on our analysis, we believe that 15 laser diodes is a good minimum number to permit the system to continue operation after catastrophic failure of one or more laser diodes, while continuing to satisfy a selected coherence function and afford a printing fidelity. Our analysis demonstrates that seven or eight laser diodes is too few to permit reduction of output from one of the remaining good laser diodes, to reestablish symmetry. By catastrophic failure, we mean that one or more laser diodes suffers a reduction in output to less than or equal to 20% of its initial output. By continuing to operate, we mean that the array of laser diodes can be used without replacing the failed laser diode. Coupled to an array of 15 or more laser diodes, we describe a detection and recovery method device to avoid the inconvenience of interrupted production and increase the time between replacement of illumination elements or illumination sources.

[0018] FIG. 3 depicts an example illuminator using an array of light sources S1-S8. The number of sources can vary from case to case, e.g. depending on the number of sources necessary to reach the desired total optical power. The sources may be laser diodes, e.g. with wavelength approximately 405, 375, or 360 nm. The number of sources needs to be larger or much larger than shown, at least 15 laser diodes ranging upward to 30, 60, 120, or 200 sources. The sources may be discrete, mounted in mechanic modules or be part of laser bars. The sources are incoherent to each other, e.g. by being selected to have a slightly different wavelength. The light from each source is collimated to fall on the SLM from one direction. The sources together create a partially coherent illumination on the SLM, which is beneficial for resolution and image contrast in an image subsequently formed of the illuminated SLM. The partial coherence may be described by a coherence function, well known to the skilled optical engineer and described in Born & Wolf: "Principles of optics" and other textbooks. The coherence factor can be calculated by means of the Zernike-van Cittert formula found in the textbooks, relating the coherence function to the angular spread of the light impinging on it.

[0019] The coherence function may have an approximately Gaussian or sin(x)/x shape. The exact shape is a compromise between desired imaging properties and technical limitation of the illuminator. An approximately Gaussian shape assumes an approximate Gaussian distribution of light from the plane of the sources. An example is shown in FIG. 4a. The filled dots show the actual power from each laser diode and the open dots the maximum allowed power from the same laser diodes. The laser diodes are shown with an even spatial distribution, but it is possible to use varying distances, thereby creating the desired power distribution with few laser diodes, and/or most laser diodes operating closer to their maximum power. The graph to the right in FIG. 4a shows the resulting coherence function.

[0020] The laser diodes have a limited life and the cost of the laser diodes is a large part of the cost of ownership of the disclosed laser writer. Typically the laser diodes fade slowly during life, but catastrophic failure also happens. Such a failure is shown in the example in FIG. 4b. Having a laser diode fail can cause a number of problems.

[0021] First, if the deterioration of the coherence function is large enough to affect image properties, the writing system may need to be taken down for repair immediately, upsetting production planning. If the repair cannot be affected immediately, the system may be down for hours or longer until a skilled service person with the proper spare parts arrives.

[0022] Second, if the light sources, e.g. laser diodes, are mounted in modules or form part of the same array component, one failed source means changing a whole module or array, incurring higher replacement costs.

[0023] In addition, having a tunable fault-tolerant scheme may allow laser diodes or laser diode arrays with less tight specifications to be used. To be able to run the system with laser diode arrays with some laser diodes performing out of spec may save cost and in some cases even make it possible to use lasers which cannot be reproducibly produced, e.g. at shorter wavelengths.

[0024] FIG. 4b shows the coherence function with the failed laser diode in the left figure. The figure shows the actual coherence function ("Act") and the intended one ("Ref") and the difference magnified ten times ("10*Diff"). The horizontal scale may, in some embodiments, be equal to the number of pixels in the SLM. The difference between intended and actual coherence function translates to errors in the image, e.g. in the balance between the size of small and large features. The figure also shows the phase angle of the complex coherence function in milliradians. A tilted phase angle from mirror to mirror is the same as an apparent tilt in the illumination and will give problems with the landing angle of the light in the image, i.e. the image gets a displacement sideways when focus is changed.

[0025] The problem of failing laser diodes or laser diodes out of specification may be solved as shown in FIG. 4c. The power of some of the other laser diodes has been changed to restore the coherence function to be more similar to the intended one. To avoid problems with the landing angle the distribution of power is made symmetrical by the lowering of the power of the laser diode symmetrical to the failed one on the other side of the optical axis. Doing this improves landing angle, but amplifies other image errors, like the large-small balance. Laser diodes close to the laser diodes with low power are adjusted to a higher power. In a general procedure each may have a different limit and the distribution may be more complicated than shown in the figure.

[0026] The adjustment of the power to the laser diodes may be done automatically by calculation of the coherence function or even the properties of the image and finding, e.g. by iteration, laser diode currents that minimize the resulting errors.

[0027] Another possibility is to specify momenta of different orders for the light intensity and bringing the momenta within bounds by modifying the drive currents to the laser diodes. In some cases it may not be possible to recreate the desired momenta, coherence functions, or image properties at the same total power. In those cases, a lower power may be set and the writing speed of the laser writer reduced, in order to keep it running until a repair can be done. Likewise it may be possible to run some laser diodes beyond their safe power levels in order to keep the system running until a repair can take place, thereby eating into the lifetime of the laser diodes slightly, but avoiding unscheduled downtime.

[0028] The light source may be measured constantly or at short regular intervals using an array of detectors or a camera. The image may be brought to the camera by means of a beam sampling mirror or grating always present in the system.

[0029] The tuning of the light source currents may be automated in the background by the following procedure. FIG. 5 depicts an iteration process used to calculate a coherence function and solve for a new possible distribution of laser diode power. First, measure the light distribution 513 in the source plane. Second, calculate a quality function 515, which may be first, second, etc. or momenta of the light distribution; or the coherence function from Zernike-van Cittert, or the image of one or more features in the image. Next, solve the derived coherence function to determine a new possible distribution of laser diode power 517 using the quality index. If the quality index is within allowed bounds 519 then check to see if it is close to the boundary limit 521. If yes, then alert the operator 519 to schedule the repair. However, if the quality index is not within the allowed bounds then alert the operator 539 that the system is out of bounds.

[0030] In the following paragraphs, we describe systems that use 2D and 1D SLMs that require illumination services.

[0031] A generic projection system is illustrated by FIG. 1. It has an object 1, which can be a mask or one or several SLMs, and a workpiece 2, e.g. a mask blank, a wafer or a display workpiece 2 device. Between them is a projection system 3 that propels 4 onto the image 5. The object is illuminated by an illuminator 6. The projection system consists of one or several lenses (shown) or curved mirrors. The NA of the projection system is determined by the size of the pupil 8. The illuminator 6 includes a light source 17 illuminating the illumination aperture 19. Field lenses 10 and 11 are shown but the presence of field lenses is not essential for the function. The imaging properties are determined by the size and intensity variation inside the illuminator aperture 9 in relation to the size of the pupil 8.

[0032] The basic projection system in 1a can be realized in many equivalent forms, e.g. with a reflecting object as shown in FIG. 1. The imaging power of the optical system can be refractive, diffractive or residing in curved mirrors. The reflected image can be illuminated through a beam splitter 12 or at an off-axis angle. The wavelength can be ultraviolet or extending into the soft x-ray (EUV) range. The light source can be continuous or pulsed: visible, a discharge lamp, one or several laser sources or a plasma source. The object can be a mask in transmission or reflection or an SLM. The SLM can be binary or analog; for example micromechanical, using LCD modulators, or using olectrooptical, magnetooptical, electroabsorbtive, electrowetting, acoustooptic, photoplastic or other physical effects to modulate the beam.

[0033] The Sigma7300 mask writer made by Micronic Laser Systems AB further includes as an Excimer laser 17, a homogenizer 18, and relay lenses 13 forming an intermediate image 14 between the SLM and the final lens. The pupil of the final lens is normally located inside the enclosure of the final lens and difficult to access, but in FIG. 1 there is an equivalent location 15 in the relay. The smallest of the relay and lens pupils will act as the system stop. There is also a relay in the illuminator providing multiple equivalent planes for insertion of stops and baffles. In some implemantations, the Sigma7300 has a catadioptric lens with a central obscuration of approximately 16% of the open radius in the projection pupil.

[0034] FIG. 2 is a rendering of the Sigma system, using a two-dimensional SLM as disclosed. A light source 205 (arc lamp, gas discharge, laser, array of lasers, laser plasma, LED, array of LEDs etc.) illuminates a one-dimensional SLM 204. The reflected (or transmitted in the general case) radiation is projected as a line segment 203 on a workpiece 201. The data driving the SLM changes as the workpiece is scanned 207 to build up an exposed image. A strongly anamorphic optical system 206 concentrates energy from multiple mirrors in a column (or row) to point in the image and the entire two-dimensional illuminated array forms a narrow line segment 203 that is swept across the workpiece. In one dimension, the anamorphic optics demagnify the illuminated area, for instance, by 2.times. to 5.times., so the a 60 millimeter wide SLM would image onto a line segment 30 to 12 mm long. Along the short dimension, the anamorphic optics strongly demagnify the column of mirrors to focus onto a narrow area such as 3 microns wide, i.e. essentially a single resolved line. Alternatively, the area could be 1 or 5 microns wide or less than 10 microns wide. Focus onto a 3 micron wide area could involve an 80.times. demagnification, from approximately 240 microns to 3 microns. The anamorphic optical path demagnifies the row of mirrors to an extent that individual mirrors are combined and not resolved at the image plane.

Some Particular Embodiments

[0035] The technology disclosed may be practiced as a method or device adapted to practice the method. The technology disclosed may be an article of manufacture such as media impressed with logic to carry out computer-assisted method or program instructions that can be combined with hardware to produce a computer-assisted device.

[0036] One embodiment is a method of extending the life of an illumination source upon catastrophic failure of one or more illumination elements among 15 or more elements. The method includes operating an illuminator that combines radiation output from 15 or more illumination elements. The illuminator distributes initial power to the elements that produces initial radiation output levels from the elements. The illuminator also combines the initial radiation output levels to produce an overall illumination field from the illuminator that satisfies a quality function. Next, there is detection of failure of a first illumination element that reduces output from the first element to less than 20 percent of its initial output level. The power distribution to and output from one or more non-failing illumination elements is reduced to restore symmetry in the overall illumination field. The power distribution to and output from at least some of the illumination elements is increased to restore quality of the overall illumination field, as measured by the quality function.

[0037] In alternate embodiments, the illuminator combines radiation from 15 up to 200 illumination elements. The illumination elements can also have varying spatial distribution.

[0038] One aspect of the technology disclosed, applicable to any of the embodiments above, is expressing said quality function as an approximately Gaussian distribution. Alternately, the quality function can also be expressed as an approximately sin(x)/x distribution.

[0039] Another aspect of the technology disclosed is automatically detecting, reading and increasing power distribution.

[0040] In another embodiment, the illuminator operates with the 15 or more illumination elements after the first illumination element fails, without replacing the failed first illumination element.

[0041] In yet another embodiment, failure of a second illumination element is detected, applying the reducing and increasing steps to compensate for the failure of the second illumination element, and continuing to operate the illuminator with the 15 or more illumination elements after the first and second illumination elements have failed, without replacing the first or second illumination elements.

[0042] Any of the methods described above or aspects of the methods may be embodied in a self correcting illuminator system. The system includes an illuminator that includes 15 or more illumination elements and optics that combine radiation output from the illumination elements, a power supply coupled to the illumination elements that distributes power to the illumination elements, sensors optically coupled to the radiation output, a controller coupled to the sensors and controlling the power supply, the controller including program instructions that set an initial power level for the illumination elements, wherein initial output levels from the illumination elements produce an overall illumination field from the illuminator that satisfies a quality function. The controller also detects failure of a first illumination element that reduces output from the first element to less than 20 percent of its initial output level. The controller is further responsive to the detected failure, reduce power distribution to and output from one or more non-failing illumination elements to restore symmetry in the overall illumination field and also responsive to the detected failure, increase power distribution to and output from at least some of the illumination elements to restore quality of the overall illumination field, as measured by the quality function.

[0043] One aspect of the technology disclosed is illumination elements having even spatial distribution. Alternately, the illumination elements can also have varying spatial distribution.

[0044] Another aspect of the technology disclosed is expressing said quality function as an approximately Gaussian distribution. Alternately, the quality function can also be expressed as an approximately sin(x)/x distribution.

[0045] While the technology is disclosed by reference to the preferred embodiments and examples detailed above, it is understood that these examples are intended in an illustrative rather than in a limiting sense. Computer-assisted processing is implicated in the described embodiments, implementations and features. Accordingly, the disclosed technology may be embodied in methods for reading or writing a workpiece using at least one optical arm that sweeps an arc over the workpiece, systems including logic and resources to carry out reading or writing a workpiece using at least one optical arm that sweeps an arc over the workpiece, systems that take advantage of computer-assisted control for reading or writing a workpiece using at least one optical arm that sweeps an arc over the workpiece, media impressed with logic to carry out, data streams impressed with logic to carry out reading or writing a workpiece using at least one optical arm that sweeps an arc over the workpiece, or computer-accessible services that carry out computer-assisted reading or writing a workpiece using at least one optical arm that sweeps an arc over the workpiece. It is contemplated that modifications and combinations will readily occur to those skilled in the art, which modifications and combinations will be within the spirit of the disclosed technology and the scope of the following claims.

Claims

1. A method of extending the life of an illumination source upon catastrophic failure of one or more illumination elements among 15 or more elements, the method including: operating an illuminator that combines radiation output from 15 or more illumination elements, including distributing initial power to the elements that produces initial radiation output levels from the elements; and combining the initial radiation output levels to produce an overall illumination field from the illuminator that satisfies a quality function; detecting failure of a first illumination element that reduces output from the first element to less than 20 percent of its initial output level; reducing power distribution to and output from one or more non-failing illumination elements to restore symmetry in the overall illumination field; increasing power distribution to and output from at least some of the illumination elements to restore quality of the overall illumination field, as measured by the quality function.

2. The method of claim 1, further including the illuminator combining radiation from up to 200 illumination elements.

3. The method of claim 1, wherein the illumination elements have even spatial distribution.

4. The method of claim 1, wherein the illumination elements having varying spatial distribution.

5. The method of claim 1 further including expressing said quality function as an approximately Gaussian distribution.

6. The method of claim 1, further including expressing said quality function as an approximately sin(x)/x distribution.

7. The method of claim 1, wherein said detecting, reading and increasing power distribution can be done automatically.

8. Method of claim 1, further including operating the illuminator with the 15 or more illumination elements after the first illumination element has failed without replacing the first illumination element.

9. The method of claim 1, further including detecting failure of a second illumination element, applying the reducing an increasing steps to compensate for the failure of the second illumination element, and continuing to operate the illuminator with the 15 or more illumination elements after the first and second illumination elements have failed, without replacing the first or second illumination elements.

10. A self-correcting illuminator system including: an illuminator that includes 15 or more illumination elements and optics that combine radiation output from the illumination elements; a power supply coupled to the illumination elements that distributes power to the illumination elements; sensors optically coupled to the radiation output; a controller coupled to the sensors and controlling the power supply, the controller including program instructions that set an initial power level for the illumination elements, wherein initial output levels from the illumination elements produce an overall illumination field from the illuminator that satisfies a quality function; detect failure of a first illumination element that reduces output from the first element to less than 20 percent of its initial output level; responsive to the detected failure, reduce power distribution to and output from one or more non-failing illumination elements to restore symmetry in the overall illumination field; and further responsive to the detected failure, increase power distribution to and output from at least some of the illumination elements to restore quality of the overall illumination field, as measured by the quality function.

11. The system of claim 10, wherein the quality function is an approximately Gaussian distribution.

12. The system of claim 10, wherein the quality function is an approximately sin(x)/x distribution.

13. The system of claim 10 wherein the illumination elements have even spatial distribution.

14. The system of claim 10 wherein the illumination elements have varying spatial distribution.