Patent Grant
8692151
1. Field of the Invention
Embodiments described herein relate to thermal processing of semiconductor substrates. More specifically, embodiments described herein relate to methods and apparatus for laser thermal annealing of semiconductor substrates.
2. Description of the Related Art
Laser thermal processing is a commonly used technique in the semiconductor industry. Semiconductor substrates are subjected to laser thermal treatment for various recrystallization, activation, and annealing processes related to integrated circuits, large area panels such as flat-panel displays and solar panels, photonic media, and magnetic media. Laser processing is frequently selected for large heating rates achievable with laser means, and the rapid throughput available.
In most instances, a substrate to be processed is positioned on a support in a laser apparatus, and the laser is focused to a spot on the substrate. The substrate is then moved to position the laser spot at successive locations on the substrate until all desired locations on the substrate are processed. Positioning is normally accomplished using a precision x-y stage for supporting the substrate. The substrate may also be moved in a z-direction to maintain focus of the laser spot on the substrate.
As the size of devices on semiconductor substrates declines with Moore's Law, the need for accurate control of positioning and focus increases. Imprecision in positioning the laser may result in devices desirous of thermal processing being missed, while other locations not desirous of thermal processing are irradiated. In addition, uniform treatment becomes increasingly difficult as imprecision in focus of the radiation becomes larger relative to device size. These trends rapidly increase the difficulty of accurately positioning the stage in the x, y, and z-directions. Thus, there is a continuing need for thermal processing apparatus and methods that yield improved precision of location and focus.
Embodiments disclosed herein provide an apparatus for thermally treating a semiconductor substrate, the apparatus having a process chamber, a rotatable substrate support disposed in the process chamber, a light source capable of producing a beam of light having substantially uniform intensity coupled to the process chamber at a beam entry point, a constant optical path length beam location assembly disposed in the chamber in optical communication with the beam entry point, the constant optical path length beam location assembly having a plurality of movable optical components, and a controller coupled to the substrate support and the constant optical path length beam location assembly, the controller configured to position the optical components of the beam location assembly and a selected portion of a substrate on the substrate support such that the beam illuminates the selected portion and the optical path length for the beam is substantially the same for all portions of the substrate.
Other embodiments provide a beam location apparatus for a laser annealing chamber, the beam location apparatus having a beam targeting optical assembly, a beam targeting detector, and a controller configured to sequentially target a beam of laser radiation to a plurality of locations with a substantially constant optical path length.
Other embodiments provide a method of targeting a beam of laser radiation at a location on a surface, including staging the target location by rotating the surface to an accessible orientation, locating a beam capture mirror over the target location, determining a reflection point of a beam steering mirror such that the optical path length of the beam reflecting from the beam steering mirror and the beam capture mirror to the surface is substantially equal to a target optical path length, moving the beam steering mirror to the reflection point, rotating the beam steering mirror to direct the beam to the beam capture mirror, and rotating the beam capture mirror to direct the beam to the target location.
So that the manner in which the above-recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.
To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. It is contemplated that elements disclosed in one embodiment may be beneficially utilized on other embodiments without specific recitation.
Embodiments described herein generally provide an apparatus for thermally treating a semiconductor substrate.
The beam 114 is generally shaped by the radiation source 110 to have an intensity profile that is uniform across a cross-section of the beam 114. In one embodiment, the beam 114 has an intensity profile with uniformity of about 2% or less. An example of a radiation source which may be used to generate a radiation beam useable in the embodiments disclosed herein is described in United States Patent Publication No. 2009/0032511, published Feb. 5, 2009, and incorporated herein by reference. The beam 114 may also be shaped into an image that has a certain outline when projected on a surface. In one embodiment, the beam 114 may have a circular cross-sectional shape. In other embodiments, the beam 114 may have a cross-sectional shape that is square, rectangular, or oval. The cross-sectional shape of the beam 114 may be generated by including an aperture having the desired shape in the final optical element 112 of the radiation source 110.
The apparatus 100 further comprises a beam location assembly 134 disposed in the chamber in optical communication with the beam entry point 132. The beam location assembly 134 targets the radiation beam 114 to a target location on a substrate disposed on the rotatable substrate support 104. The beam location assembly 134, which may be a beam targeting optical assembly, comprises a plurality of movable optical components. In the embodiment of
The beam steering mirror 122 is movable along a first y-guide 120 in the y-direction by a linear actuator (not visible in the view of
The beam location assembly 134 is generally configured as a constant path length beam location assembly. As such, the mirrors 108 and 122 of the beam location assembly 134 are controlled by a controller 106 coupled to the linear actuators of the mirrors 108 and 122. The controller 106 moves the mirrors 108 and 122, and rotates the substrate support 104, to position the beam of radiation 114 at a target location while maintaining a constant optical path length for the radiation beam 114. The constant path length is useful for maintaining cross-sectional area and intensity profile of the radiation beam 114. Variation of the path length from exposure to exposure may result in non-uniform treatment from die to die or at different locations of a single die.
The radiation source 110 may be a continuous wave or pulsed laser or a plurality of continuous wave or pulsed lasers. The radiation source 110 may blend, shape, or combine output of multiple lasers in any convenient way, for example by generating pulses from multiple lasers with desired frequency and phase difference, and blending the pulses using pulse-stretching optics to produce energy pulses of arbitrary shape and temporal energy profile. The radiation source 110 may additionally include polarizing optics. In operation in a laser thermal annealing apparatus, the radiation beam 114 may be scanned across the surface of a substrate by continuous motion of the mirrors 108/122, or the mirrors may be stationary while the beam 114 irradiates the substrate, and moved when the beam 114 is shut off in a stepping process, or any combination thereof.
The rotational actuators 124 and 130 may rotate the mirrors 122 and 108 about one, two, or three axes depending on the degrees of freedom required by the particular embodiment. For example, in an embodiment such as the embodiment of
Such an embodiment may be useful when the substrate has dies that are not oriented along radii of the substrate. For example, a silicon wafer may have rectangular dies oriented in a rectilinear grid across the surface of the wafer. If the rectangular dies are to be processed using a rectangular laser spot, it is generally desired that the orientation of the laser spot align with the orientation of the dies. Rotating the substrate support 104 any angular distance that is not a multiple of 180 degrees changes the orientation of the dies with respect to the orientation of the laser spot. Thus, an embodiment of a beam location assembly that enables processing of all dies on one side of the diameter D2 may be useful for such processes.
In an embodiment wherein the laser spot has no particular orientation, such as a laser spot having a circular cross-section, the degrees of freedom of the mirrors 122 and 108 may be further constrained, for example by constraining the beam capture mirror 108 to move with the beam steering mirror 122 in the y-direction. Constant path length of the beam 114 may be maintained in such an embodiment by rotating the substrate support 104 by a determined amount for each target location to be processed. In such an embodiment, portions of a substrate not desirous of thermal processing may be exposed to annealing radiation from the laser spot due to mismatch between the profile of the laser spot and the shape of the target location. Masking the substrate, for example with a high emissivity coating, may protect such locations on the substrate.
Alternately, the radiation source may include a rotatable aperture, for example included in the final optical element 112 of the radiation source 110, to yield an oriented laser spot, for example a rectangular laser spot, that is rotatable to follow the rotation in orientation of the target location. The controller 106 may communicate with the rotatable aperture through a rotational actuator. An aperture that shapes the beam 114 emerging from the radiation source 110 generally comprises a thermally stable or refractory material, such as ceramic, to withstand prolonged and/or repeated irradiation. The surface of the aperture facing the interior of the radiation source 110 will generally be subjected to prolonged and/or repeated irradiation by the laser or plurality of lasers housed in the radiation source. Thus, either the interior surface of the aperture, or the entire aperture, will comprise a thermally resistant or refractory material. In some embodiments, thermal effects of the radiation incident on the interior surface of the aperture may be mitigated by scattering the radiation from a roughened surface or by coating the surface with a reflective or high-emissivity material. In other embodiments, the rotatable aperture may be cooled by forming channels for a cooling fluid through portions of the aperture.
The distance from the beam steering mirror 122 to the beam capture mirror 108, projected onto the x-z plane in the view of
In an embodiment in which the mirrors 108/122 move together in the y-direction, the beam capture mirror 108 moves in the x-direction to compensate the distance “a” from the beam entry point to the beam steering mirror 122. In this embodiment, rotating the mirrors is not required, and the target location moves along a radius of the substrate support at a 45° angle to the x-axis of the apparatus. To reach all possible target locations of a substrate on the substrate support, the support is rotated to bring target locations along the beam access line. In such cases, rotating the substrate support may rotate orientation of target locations, so a non-oriented beam is used, or a rotatable aperture is used to adjust beam orientation.
Precision may be enhanced in some embodiments by providing incremental positioning adjustment, rotational adjustment, and focus adjustment for the radiation source.
A variable focus lens useful for most embodiments will be fast to adjust, and will generally be thermally resistant. In one embodiment, a liquid cell variable focus lens having conduits for flowing the liquid through the lens may be used. In another embodiment, the variable focus lens may incorporate channels or conduits for flowing cooling fluid through the lens outside the liquid cell. In other embodiments, a liquid crystal lens may be used.
The actuator 212 may be controlled by a controller such as the controller 106 of
The detector 210 may be a photon detector of any convenient type, including a camera. Types of photon detectors that may be used include a CCD matrix and a photodiode array. The detector 210 may additionally be a light intensity uniformity detector in some embodiments.
In alternate embodiments, the substrate support may comprise a precision rotator mounted on a precision x-y stage to enable x-y and rotational motion of a substrate disposed on the support. Adding the x-y positioning may improve processing throughput in some embodiments by enabling gross positioning by the x-y stage and fine positioning by the precision optics described herein. In another alternate embodiment, beam positioning and focus may be enhanced using an imaging device to view the optical path of the radiation beam through the aperture or final optical element of the radiation source 110. A CCD matrix may be incorporated with the radiation source 110, for example, to collect radiation reflected from the substrate along the optical pathway. The controller 106 may use data from the CCD matrix to generate control signals for any of the actuated devices described herein to improve positioning and focus of the beam.
In other embodiments, a plurality of beam capture mirrors and beam steering mirrors may be used with a single substrate support to address target locations at all points on a substrate without rotating the substrate. In one exemplary embodiment, a first beam steering mirror may be used with a first beam capture mirror to cover target locations over one-half of the substrate, moving as described above to maintain a first constant path length. The first beam steering mirror may then be parked in a position to steer the beam to a second beam steering mirror that moves with a second beam capture mirror to cover the remaining target locations at a second constant path length different from the first constant path length.
Embodiments disclosed herein also provide a method of targeting a beam of laser radiation at a location on a surface. The method comprises staging the target location by rotating the surface to an orientation in which the target location is accessible to a beam location optical assembly and locating a beam capture mirror over the target location. The beam capture mirror will reflect a beam of laser radiation from a source to the target location.
In one embodiment, the optical path length of the radiation beam is kept substantially constant for all target locations on the surface. The source location for the beam impinging on the beam capture mirror is identified by determining a reflection point of a beam steering mirror such that the optical path length of the beam reflecting from the beam steering mirror and the beam capture mirror to the target location is substantially equal to a target optical path length. The beam is directed toward the beam steering mirror from a fixed point, so the optical path length depends only on the position of the beam steering mirror, the beam capture mirror, and the location of the target. The beam steering mirror is moved to the identified reflection point, and the two mirrors rotated as needed to align in optical communication. A controller may be used to synchronize movement of the various elements concurrently to improve targeting speed.
To improve precision in targeting, various venire adjustments may be made. For example, the fixed point source of the beam may be adjusted after detecting the accuracy of the initial positioning. If maintaining a particular focal length is desired, the length of the optical path may be detected using any convenient detector, such as any of the methods described herein, and the focus may be adjusted using a variable focus lens in communication with a controller. Finally, if the beam has a desired rotational orientation, that orientation may be adjusted precisely by providing a rotatable aperture under the control of a controller.
In other embodiments, a lens may be used as the final adjustment of the beam before it reaches the target location. The lens may be a position adjustment lens only, such as a flat prism inclined at an angle to the direction of propagation of the beam, or the lens may be an optically active lens, such as a projection lens. The lens or prism may be moved over a target location, receive a beam of radiation from a source such as any of the mirrors described herein, and direct the beam precisely to the target location. The lens or prism may be positioned as described herein, and may be rotated to achieve a needed orientation.
In the embodiment of
Subject to the configuration described above, the rotation position of the substrate support 410 is determined as follows. With the beam 402 travelling parallel to the substrate surface from the beam entry point 404 to the second mirror 414, define a Cartesian coordinate plane 418 parallel to the substrate surface through the beam center. Calculations are simplified if the coordinate plane 418 is defined such that the x and y axes are parallel to the sides of the die 408 in the reference position 408r. Let (xs,ys) be the coordinate position of the center of the beam at the beam entry point 404. Let (x1,y1) be the target position of the center of the first mirror 406. Let (xc,yc) be the center 412 of rotation of the substrate support 410. Let (x2,y2) be the position of the center of the die 408 at the target position 408t. Let (x0,y0) be the position of the center of the die 408 at the reference position 408r. The substrate 416 may be oriented to a reference position in which the die 408 is in the reference position 408r by rotation prior to processing. Alternately, an offset may be detected between the orientation of the substrate and the reference orientation.
The position (x2,y2) is calculated such that the sum of the distances from (xs,ys) to (x1,y1) and (x1,y1) to (x2,y2) is a constant and such that the sides of the die 408 at the target position 408t are aligned with the rectangular cross-sectional shape of the laser beam 402. Let the constant beam length be denoted BL. A rotation angle θ for the substrate 416 to move the die 408 from the reference position 408r to the target position 408t is calculated as follows:
The position (x2,y2) may be calculated from the position (x0,y0) by simple transformation from polar to Cartesian coordinates. If the substrate was not physically oriented at the reference orientation, the detected offset angle may be added or subtracted, as appropriate, to the rotation angle θ prior to determining the position (x2,y2).
The second mirror 414 is moved to the position (x2,y2) by an x-y positioner (as in
y1=y2+(x2−xs)tan θ.
The first mirror 406 is moved to position (x1,y1) by a linear positioner.
The second mirror 414 is held at a constant 45° angle with respect to the x-y plane, and is rotated about an axis perpendicular to the x-y plane by a rotational actuator to engage the beam reflected from the first mirror 406. The rotation angle α of the second mirror 414 with respect to the x-axis is the same as the angle θ. The first mirror 406 is held at a constant perpendicular orientation with respect to the x-y plane and rotated about an axis perpendicular to the x-y plane by a rotational actuator to reflect the beam 402 toward the second mirror 414. The rotation angle γ of the first mirror 406 with respect to the x-axis is given by
Positioning the substrate and the mirrors according to the method described above ensures the optical path length of the beam remains constant for all processing locations on a substrate. A controller 420 may be coupled to the mirrors 406 and 414, and to the substrate support 410 to accomplish the motions determined by the equations above. The controller may be configured with software designed to perform the above calculations and send control signals to actuators that move the mirrors 406 and 414 and the substrate support 410 based on the calculated positions.
The substrate support 410 is generally rotated by a precision rotational actuator with a certain defined precision. The imprecision in rotational position imparted by the actuator may be translated into a positioning error of up to about 40 μm in a 300 mm wafer embodiment. Such errors may be corrected, in one embodiment, by finely adjusting the position of the beam shaping aperture or beam cutter described above. If the aperture is located at the beam entry point 404, the aperture may be rotated or moved laterally in two directions to correct for fine positioning errors. Piezoactuators may be used for such fine adjustment. Errors in positioning the substrate may be detected and measured using photonic detectors such as cameras, CCD matrices, or photodiode arrays.
While the foregoing is directed to embodiments of the invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof.
This application claims benefit of U.S. Provisional Patent Application Ser. No. 61/319,777 filed Mar. 31, 2010, and incorporated herein by reference in its entirety.
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