Abstract:
A method and apparatus for annealing semiconductor substrates is disclosed. The apparatus has a pulsed energy source that directs pulsed energy toward a substrate. A homogenizer increases the spatial uniformity of the pulsed energy. A pulse shaping system shapes the temporal profile of the pulsed energy. A pulse circulator may be selected using a bypass system. The pulse circulator allows a pulse of energy to circulate around a path of reflectors, and a partial reflector allows a portion of the pulse to exit the pulse circulator with each cycle. The pulse circulator may have delaying elements and amplifying elements to tailor the pulses exiting from the circulator.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims benefit of U.S. Provisional Patent Application Ser. No. 61/495,872, filed Jun. 10, 2011, incorporated herein by reference. 
     
    
     FIELD 
       [0002]    Embodiments disclosed herein relate to methods and apparatus for manufacturing semiconductor devices. More specifically, apparatus and methods of annealing semiconductor substrates are disclosed. 
       BACKGROUND 
       [0003]    Thermal annealing is a commonly used technique in semiconductor manufacturing. A material process is generally performed on a substrate, introducing a material desirous of including in the substrate, and the substrate is subsequently annealed to improve the properties of the materially changed substrate. A typical thermal anneal process includes heating a portion of the substrate, or the entire substrate, to an anneal temperature for a period of time, and then cooling the material. In some cases, a portion of the material is melted and resolidified. 
         [0004]    Pulse laser annealing is an attractive method of annealing semiconductor substrates. Pulsed laser energy provides a degree of control over the annealing process not afforded by omnibus annealing processes such as RTP. Common methods of generating laser pulses do not offer full flexibility to design pulse energies, durations, and intensity profiles that may be needed for some processes. For generating very short pulses of laser energy, generating means are mostly limited to q-switches, prism compressors, gratings and the like that offer limited flexibility in designing energy pulses. 
         [0005]    Thus, there remains a need for new ways to generate and control pulsed energy for thermal processing. 
       SUMMARY 
       [0006]    A thermal processing apparatus is disclosed that has a pulsed energy source and a pulse circulator. The pulse circulator has at least a first and a second reflector, each of which may be a partial reflector. Each reflector has a reflective surface. The first reflector is positioned to receive energy reflected from the reflective surface of the second reflector at a reflective surface of the first reflector, and reflect the energy toward the second reflector. The second reflector transmits a portion of the energy incident on the reflective surface thereof. 
         [0007]    The pulse circulator may also have circuit mirrors to increase the optical path length of the pulse circulator. The circuit mirrors may be actuated to vary the optical path length of the pulse circulator. Delay optics and amplifiers may be included in the pulse circulator. 
         [0008]    The thermal processing apparatus may also include a homogenizer that increases spatial uniformity of an energy pulse, and a pulse shaping system for adjusting the temporal profile of a pulse. Multiple energy sources may be used to form a single pulse. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    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. 
           [0010]      FIG. 1A  is a plan view of a thermal processing apparatus according to one embodiment. 
           [0011]      FIG. 1B  is a schematic view of a pulse shaping system according to another embodiment. 
           [0012]      FIG. 1C , is a schematic view of a homogenizer according to another embodiment. 
           [0013]      FIG. 2  is a schematic view of a pulse circulator according to another embodiment. 
       
    
    
       [0014]    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. 
       DETAILED DESCRIPTION 
       [0015]      FIG. 1A  is a plan view of a thermal processing apparatus  100  according to one embodiment. An energy source  102 , which may be a laser source, forms an energy pulse  104 . The energy source  102  may be a single laser or a plurality of lasers with joining optics to produce a single beam or pulse from the plurality of lasers. The energy source  102  may produce electromagnetic energy having a wavelength between about 200 nm and about 2,000 nm, such as between about 500 nm and about 1,000 nm, for example about 532 nm or about 810 nm. In an embodiment featuring a plurality of lasers, each laser may have the same wavelength, or some or all of the lasers may have different wavelengths. In one embodiment, the output of four frequency-doubled Nd:YAG lasers is merged into a single laser beam for pulsed output. It should be noted that any or all of the lasers may be continuous wave, pulsed, q-switched, and the like. 
         [0016]    The energy pulse  104  is directed to an optional pulse shaping system  106 . The pulse shaping system  106  subjects the energy pulse  104  to transformations that change the temporal shape of the pulse, or the intensity of the pulse as a function of time. The pulse shaping system  106  may divide the energy pulse  104  into sub-pulses using splitters, direct the sub-pulses through different paths having different path lengths, and then recombine the sub-pulses using combiners. Such a pulse shaping system may be used to modify the native temporal pulse shape produced by the energy source  102 , if desired. 
         [0017]      FIG. 1B  schematically illustrates one embodiment of a pulse shaping system  106 . The pulse shaping system of  FIG. 1B  may comprise a plurality of mirrors  152  (e.g., 16 mirrors are shown) and a plurality of beam splitters (e.g., reference numerals  150 A- 150 E) that are used to delay portions of a laser energy pulse to provide a composite pulse that has a desirable pulse characteristics (e.g., pulse width and pulse profile). In one example, the laser energy pulse may be spatially coherent. A pulse of laser energy is split into two components, or sub-pulses  154 A,  154 B, after passing through the first beam splitter  150 A. Neglecting losses in the various optical components, depending on the transmission to reflection ratio in the first beam splitter  150 A, a percentage of the laser energy (i.e., X %) is transferred to the second beam splitter  150 B in the first sub-pulse  154 A, and a percentage of the energy (i.e., 1-X %) of the second sub-pulse  154 B follows a path A-E (i.e., segments A-E) as it is reflected by multiple mirrors  152  before it strikes the second beam splitter  150 B. 
         [0018]    In one example, the transmission to reflection ratio of the first beam splitter  150 A is selected so that 70% of the pulse&#39;s energy is reflected and 30% is transmitted through the beam splitter. In another example the transmission to reflection ratio of the first beam splitter  150 A is selected so that 50% of the pulse&#39;s energy is reflected and 50% is transmitted through the beam splitter. The length of the path A-E, or sum of the lengths of the segments A-E (i.e., total length=A+B+C+D+E as illustrated in  FIG. 1B ), will control the delay between sub-pulse  154 A and sub-pulse  154 B. In general by adjusting the difference in path length between the first sub-pulse  154 A and the second sub-pulse  154 B a delay of about 3.1 nanoseconds (ns) per meter can be realized. 
         [0019]    The energy delivered to the second beam splitter  150 B in the first sub-pulse  154 A is split into a second sub-pulse  156 A that is directly transmitted to the third beam splitter  150 C and a second sub-pulse  156 B that follows the path F-J before it strikes the third beam splitter  150 C. The energy delivered in the second sub-pulse  154 B is also split into a third sub-pulse  158 A that is directly transmitted to the third beam splitter  150 C and a third sub-pulse  158 B that follows the path F-J before it strikes the third beam splitter  150 C. This process of splitting and delaying each of the sub-pulses continues as each of the sub-pulses strikes subsequent beam splitters (i.e., reference numerals  150 D-E) and mirrors  152  until they are all recombined in the final beam splitter  150 E that is adapted to primarily deliver energy to the next component in the thermal processing apparatus  100 . The final beam splitter  150 E may be a polarizing beam splitter that adjusts the polarization of the energy in the sub-pulses received from the delaying regions or from the prior beam splitter so that it can be directed in a desired direction. 
         [0020]    In one embodiment, a waveplate  164  is positioned before a polarizing type of final beam splitter  150 E so that its polarization can be rotated for the sub-pulses following path  160 . Without the adjustment to the polarization, a portion of the energy will be reflected by the final beam splitter and not get recombined with the other branch. In one example, all energy in the pulse shaping system  106  is S-polarized, and thus the non-polarizing cube beam splitters divide incoming beams, but the final beam splitter, which is a polarizing cube, combines the energy that it receives. The energy in the sub-pulses following path  160  will have its polarization rotated to P, which passes straight through the polarizing beam splitter, while the other sub pulses following path  162  are S-polarized and thus are reflected to form a combined beam. 
         [0021]    In one embodiment, the final beam splitter  150 E comprises a non-polarizing beam splitter and a mirror that is positioned to combine the energy received from the delaying regions or from the prior beam splitter. In this case, the beam splitter will project part of the energy towards a desired point, transmit another part of the energy received towards the desired point, and the mirror will direct the remaining amount of energy transmitted through the beam splitter to the same desired point. One will note that the number of times the pulse is split and delayed may be varied by adding beam splitting type components and mirrors in the configuration as shown herein to achieve a desirable pulse duration and a desirable pulse profile. While  FIG. 1B  illustrates a pulse shaping system design that utilizes four beam delaying regions, which contain a beam splitter and mirrors, this configuration is not intended to be limiting as to the scope of the invention. 
         [0022]    Referring to  FIG. 1A , the thermal processing apparatus  100  also has an optional homogenizer  108  for increasing the spatial uniformity of the energy  104 . The homogenizer  108  employs elements that reduce or eliminate spatial coherency of the energy  104 , increase the number of spatial modes of the energy  104 , or spatially randomize the energy  104 . One or more refractive arrays, such as lens arrays, may be transmissively coupled with one or more focusing or defocusing elements, such as lenses, to increase the spatial uniformity of energy density of the energy  104  to about 10% or better, for example about 5% or better. 
         [0023]      FIG. 1C  is a schematic view of a homogenizer  108  according to one embodiment. The homogenizer of  FIG. 1C  receives an incident beam A 1  of spatially coherent electromagnetic energy and produces a uniform energy field at the image plane B 1 . A beam integrator assembly  178 , which contains a pair of micro-lens arrays  172  and  174  and lens  176 , homogenizes the energy passing through the beam integrator assembly  178 . It should be noted that the term micro-lens array, or fly&#39;s-eye lens, is generally meant to describe an integral lens array that contains multiple adjacent lenses. The beam integrator assembly  178  of  FIG. 1C  generally works best using an incoherent source or a broad partially coherent source whose spatial coherence length is much smaller than a single micro-lens array&#39;s dimensions. In short, the beam integrator assembly  178  homogenizes the beam by overlapping magnified images of the micro-lens arrays at a plane situated at the back focal plane of the lens  176 . The lens  176  should be well corrected so as minimize aberrations including field distortion. Also, the size of the image field is a magnified version of the shape of the apertures of the first micro-lens array  172 , where the magnification factor is given by F/f 1  where f 1  is the focal length of the micro-lenses in the first micro-lens array  172  and F is the focal length of lens  176 . 
         [0024]    In one example, a lens  176  that has a focal length of about 175 mm, and micro-lenses in the micro-lens array having a 4.75 mm focal length, are used to form an 11 mm square field image. One will note that many different combinations for these components can be used, but generally the most efficient homogenizers will have a first micro-lens array  172  and second micro-lens array  174  that are identical. The first and second micro-lens arrays  172  and  174  may be spaced a distance apart so that the energy density (Watts/mm 2 ) delivered to the first micro-lens array  172  is increased, or focused, on the second micro-lens array  174 . To avoid damaging the second micro-lens array  174  by focusing energy density of the second micro-lens array  174  exceeding the damage threshold of the any component of the second micro-lens array  174 , the second micro-lens array  174  is spaced a distance d 2  from the first micro-lens array  172  equal to the focal length of the lenslets in the first micro-lens array  172 . 
         [0025]    In one example, each of the first and second micro-lens arrays  172  and  174  contains 7,921 micro-lenses (i.e., an 89×89 array of lenslets) that are a square shape and that have an edge length of about 300 microns. The lens  176 , which may be a Fourier lens, is generally used to integrate the image received from the micro-lens arrays  172  and  174  and is spaced a distance d 3  from the second micro-lens array  174 . 
         [0026]    A random diffuser  170  may be placed within the homogenizer  108  so that the uniformity of energy A 5  leaving the homogenizer  108  is improved in relation to the incoming energy A 1 . In this configuration, the incoming energy A 1  is diffused by the placement of a random diffuser  170  prior to the energy A 2 , A 3  and A 4  being received and homogenized by the first micro-lens array  172 , second micro-lens array  174  and lens  176 , respectively. The random diffuser  170  will cause the pulse of incoming energy (A 1 ) to be distributed over a wider range of angles (α 1 ) to reduce the contrast of the projected beam and thus improve the spatial uniformity of the pulse. The random diffuser  170  generally causes the light passing through it to spread out so that the irradiance (W/cm 2 ) of energy A 3  received by the second micro-lens array  174  is less than without the diffuser. The random diffuser  170  is also used to randomize the phase of the beam striking each micro-lens array  172  and  174 . This additional random phase improves the spatial uniformity by spreading out the high intensity spots observed without the diffuser. 
         [0027]    In general, the random diffuser  170  is a narrow angle optical diffuser that is selected so that it will not diffuse the received energy in a pulse at an angle greater than the acceptance angle of the lens that it is placed before. In one example, the random diffuser  170  is selected so that the diffusion angle α 1  is less than the acceptance angle of the micro-lenses in the first micro-lens array  172  or the second micro-lens array  174 . In one embodiment, the random diffuser  170  comprises a single diffuser, such as a 0.5° to 5° diffuser that is placed prior to the first micro-lens array  172 . In another embodiment, the random diffuser  170  comprises two or more diffuser plates, such as 0.5° to 5° diffuser plates that are spaced a desired distance apart. In one embodiment, the random diffuser  170  may be spaced a distance d 1  away from the first micro-lens array  172  so that the first micro-lens array  172  can receive substantially all of the energy delivered in the incoming energy A 1 . 
         [0028]    Referring to  FIG. 1A , the thermal processing apparatus  100  further comprises a pulse circulator  116 . The pulse circulator  116  receives a pulse of energy and circulates the energy to generate a delay of all or part of the incoming pulse. The pulse circulator  116  employs elements that may include splitters, partial reflectors, total reflectors, adjustable reflectors, and the like, to circulate the energy pulse. 
         [0029]    In one aspect, the pulse circulator employs optical elements to circulate a pulse of electromagnetic energy. The pulse circulator may have a first reflector, for example a one-way mirror, that receives an incoming pulse, a second reflector, for example a partial mirror, that receives the pulse from the first reflector, and one or more circuit mirrors that direct energy reflected from the second reflector back to the first reflector. The second reflector transmits a certain percentage of the energy received from the first reflector each time the energy circulates, resulting in a portion of the original energy pulse being transmitted out of the pulse circulator  116  each time the energy travels around the pulse circulator  116  until the energy is effectively extinguished. Thus, in some embodiments, the pulse circulator  116  may be a pulse divider. 
         [0030]      FIG. 2  is a schematic view of a pulse circulator  200  usable in the thermal processing apparatus  100  according to one embodiment. The pulse circulator  200  has a first reflector  202  with a transmitting surface  202 A and a reflecting surface  202 B. The transmitting surface  202 A allows light incident on the transmitting surface  202 A to pass through the first reflector  202 , and the reflecting surface  202 B reflects light incident on the reflecting surface  202 B. 
         [0031]    The pulse circulator  200  also has a second reflector  204  that transmits a portion of radiation incident on the second reflector  204  and reflects a portion of the incident radiation. The first reflector  202  is positioned to receive radiation reflected from the second reflector  204  on the reflecting surface  202 B of the first reflector  202  and reflect the radiation back to the second reflector  204 . 
         [0032]    One or more circuit reflectors  206  may be included in the pulse circulator  200 . Two circuit reflectors  206  may be used to direct light reflected from the second reflector  204  to the reflective surface  202 B of the first reflector  202 . Light entering the pulse circulator  200  through the transmissive surface  202 A of the first reflector  202  cycles around the reflectors of the pulse circulator  200  following a circuit path  220 . Every time the energy cycles around the circuit path  220  to the second reflector  204 , a portion of the energy is released from the pulse circulator  200  in a sub-pulse  225 , leaving the remaining energy to cycle. The pulse circulator  200  thus converts a single pulse of incoming energy into a series of sub-pulses of declining intensity. Intensity of the sub-pulses declines geometrically according to the transmissivity of the second reflector  204 . 
         [0033]    Referring back to  FIG. 1A , the thermal processing apparatus  100  also includes a substrate support  120  for positioning a substrate to be subjected to the pulsed energy  104 . A bypass system  114  may be included to allow the pulse circulator  116  to be bypassed and the energy  104  sent directly to the substrate on the substrate support  120 . In this way, the thermal processing apparatus  100  may be used to direct a pulse of energy  104  to a substrate for thermal processing and to direct a series of sub-pulses of declining intensity to the substrate before or after the thermal processing. 
         [0034]    The bypass system  114  may be selected by a switchable reflector  110 , for example an LCD mirror or a microelectromechanical device, that may be switched from essentially full transmission to essentially full reflection by applying a voltage from a power source  112 . When the switchable reflector  110  is energized, the surface of the switchable reflector  110  facing the incoming energy becomes reflective, directing the incoming energy to the bypass system  114 . The bypass system  114  contains reflectors that direct the energy around the pulse circulator  116  to a second switchable reflector  118  that aligns the energy from the bypass system  114  toward the substrate support  120 . The switchable reflectors  110  and  118  are generally operated synchronously so that when the switchable reflector  110  is reflective, the switchable reflector  118  is also reflective, and when the switchable reflector  110  is transmissive, the switchable reflector  118  is also transmissive. 
         [0035]    In operation, the thermal processing apparatus  100  may be configured to direct pulses of processing radiation to the substrate support  120  to thermally treat a substrate positioned on the substrate support  120 . Following the thermal treatment, the thermal processing apparatus  100  may be configured to direct pulses of cool-down radiation to the substrate support  120  to cause a controlled cooling of the substrate following the thermal treatment. In one aspect, each cool-down pulse transfers energy to the substrate surface, increasing its temperature or slowing its rate of cooling in the area affected by the energy. 
         [0036]    The pulse circulator  116  of  FIG. 1A  or  FIG. 2  may be useful for thermal processing methods featuring controlled cooling of a substrate. In some such methods, cooling is controlled after heating to adjust the final properties of the substrate following the treatment. Using the pulse circulator  116  of  FIG. 1A  or the pulse circulator  200  of  FIG. 2 , energy may be added to the substrate at a controlled rate as the substrate cools to influence the rate of different morphology processes, and therefore influence the morphology of the final product. 
         [0037]    The pulse circulator  116  may be configured to produce a series of pulses spaced apart by a rest duration. The rest duration may be selected to allow the substrate temperature in the area affected by the cool-down pulses to decline by a specified amount. A cool-down pulse may then raise the temperature of the affected area by an amount less than the temperature decline during the immediately prior rest duration. The cool-down pulses generally have an intensity defined by the following relationship: 
         [0000]        I   n   =I   0 (1 −T ) n    
         [0000]    where I n  is the intensity of the n th  pulse, I 0  is the intensity of the incident pulse, and T is the transmissivity of the second reflector  204 . In one aspect, the path length of the pulse circulator  116  may be set such that the initial intensity I 0  of the pulse entering the pulse circulator  116  is substantially the same as pulses used in thermal processing of the substrate, and the rest duration between each pulse allows the thermal energy of the affected area of the substrate to decline by a desired amount between the cool-down pulses. 
         [0038]    In one embodiment, the thermal processing includes melting a portion of the substrate surface, and the subsequent cool-down pulses perform a controlled solidification or recrystallization of the substrate surface at a rate below the natural rate of solidification due to radiation and dissipation of surface energy of the substrate alone. Each pulse delivered during thermal processing may perform a controlled melting of a portion of the substrate surface, progressing a melt front through a depth of the surface. Then, a portion of the cool-down pulses may each perform a controlled remelt of a portion of the substrate surface, progressing a solidification front through the depth of the surface. In order to perform such a method, the switchable reflectors  110  and  118  are energized to bypass the pulse circulator  116  while the thermal processing pulses are delivered. Any number of thermal processing pulses may be delivered during the thermal processing operation. The switchable reflectors  118  may then be de-energized and a pulse of energy routed through the pulse circulator  116  to perform a controlled cooling of the processed surface. 
         [0039]    In one aspect, the circuit reflectors  206  of  FIG. 2  may be adjustable. The circuit reflectors  206  may be carried on a support  208  that is coupled to a track  210  by a linear actuator  212 . Limiters  214  may be provided to limit the range of motion of the actuator  212 , if desired. The configuration of  FIG. 2  allows adjustment of the path length of the pulse circulator  200  by moving the circuit reflectors  206  closer to or further from the first and second reflectors  202  and  204 . Adjusting the path length of the circulator affects the interval of time between pulses emerging from the second reflector  204 . 
         [0040]    Delay may also be introduced into the pulse circulator  200  by including an optical element with an elevated refractive index compared to the ambient medium of the pulse circulator  200 . Such optical elements include solids, liquids, and gases, and the degree of delay may be modulated by adjusting the thickness of the refractive medium through which the light passes. In one example, a delay optic  216  of varying thickness may be disposed along the optical path of the pulse circulator  200 . The thickness of the delay optic  216  is usually stepped, rather than angled, to maintain a perpendicular incidence of the light on the delay optic  216  to avoid redirection of the light by refraction. A 1 cm thick piece of glass (n≈1.5) disposed in a 1 m optical path will add about 0.5% to the interval between pulses emerging from the second reflector  204 . A 1 cm thick piece of transparent carbon (i.e. diamond, n≈2.4), will add about 1.4% to the interval in a 1 m circuit. The thickness of the material may be stepped, and the delay optic  216  actuated by a linear actuator  218  to position a selected step in the optical path to select a delay value. The delay optic  216  may be a single substance or a composite. In one aspect, the delay optic  216  may be a shaped vial of fluid having a desired refractive index. 
         [0041]    The intensity relationship between each cool-down pulse may be further influenced by adding optical elements to the pulse circulator  200 . In one aspect, an amplifier  222  may be added to the path of energy circulating in the pulse circulator  200 . The amplifier  222  is generally a medium susceptible to stimulated emission at wavelengths similar to, or equal to, the wavelength of the circulating energy. For example, if the circulating energy is produced by a Nd:YAG laser, the amplifier  222  may be an Nd:YAG crystal. The amplifier  222  may be pumped prior to circulating a pulse through the pulse circulator  200 , such that energy passing through the amplifier  222  causes the amplifier  222  to emit radiation substantially coherent with the incident energy. The exact decay profile of pulses emerging from the pulse circulator  200  may thus be adjusted by adding energy to each pulse at a controlled rate. 
         [0042]    In some aspects, the amplifier  222  may be operated as a pulse intensifier. For example, as the pulse circulates through the pulse circulator  200 , the amplifier may be recharged with each pass, adding more energy to the pulse with each pass such that each pulse exiting the pulse circulator  200  has greater intensity than the last. In one embodiment, a second pulse circulator may be integrated with the pulse circulator  200  to circulate a charging pulse in synchronization with the circulating pulse. Alternately, the amplifier  222  of the pulse circulator  200  may be pumped by a pulsed light source. 
         [0043]    In other embodiments the amplifier  222  may be charged at a frequency different from the oscillation frequency of the circuit such that a pulse circulates multiple times between charges applied to the amplifier. In such embodiments, the pulse circulator  200  produces pulses having a periodic intensity pattern, with the intensity of the pulses rising and falling according to the relationship between the circulation frequency and the charging frequency of the amplifier. 
         [0044]    The amplifier  222  may also have reflectors  224  to form an oscillator cavity in the amplifier  222  to allow for a broader range of amplification options. A first reflector  224 A will usually be a total reflector while a second reflector  224 B may be a partial reflector with fixed or variable transmissivity. The properties of the oscillator cavity may be varied, along with the optical path length of the pulse circulator  200 , to provide pulses having virtually any periodicity and intensity pattern. In one aspect, the pulse circulator  200  may be operated as a ring laser. 
         [0045]    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.