CN113811981A - Method and optical system for processing semiconductor material - Google Patents

Method and optical system for processing semiconductor material Download PDF

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CN113811981A
CN113811981A CN202080034669.6A CN202080034669A CN113811981A CN 113811981 A CN113811981 A CN 113811981A CN 202080034669 A CN202080034669 A CN 202080034669A CN 113811981 A CN113811981 A CN 113811981A
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laser
pulse
laser pulse
semiconductor material
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S·格伯特
H·卡勒特
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Inneva Ltd
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
    • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
    • H01L21/02104—Forming layers
    • H01L21/02365—Forming inorganic semiconducting materials on a substrate
    • H01L21/02656—Special treatments
    • H01L21/02664—Aftertreatments
    • H01L21/02667—Crystallisation or recrystallisation of non-monocrystalline semiconductor materials, e.g. regrowth
    • H01L21/02675—Crystallisation or recrystallisation of non-monocrystalline semiconductor materials, e.g. regrowth using laser beams
    • H01L21/02678—Beam shaping, e.g. using a mask
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K26/00—Working by laser beam, e.g. welding, cutting or boring
    • B23K26/02—Positioning or observing the workpiece, e.g. with respect to the point of impact; Aligning, aiming or focusing the laser beam
    • B23K26/06—Shaping the laser beam, e.g. by masks or multi-focusing
    • B23K26/062—Shaping the laser beam, e.g. by masks or multi-focusing by direct control of the laser beam
    • B23K26/0622—Shaping the laser beam, e.g. by masks or multi-focusing by direct control of the laser beam by shaping pulses
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K26/00—Working by laser beam, e.g. welding, cutting or boring
    • B23K26/02—Positioning or observing the workpiece, e.g. with respect to the point of impact; Aligning, aiming or focusing the laser beam
    • B23K26/06—Shaping the laser beam, e.g. by masks or multi-focusing
    • B23K26/064—Shaping the laser beam, e.g. by masks or multi-focusing by means of optical elements, e.g. lenses, mirrors or prisms
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
    • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
    • H01L21/02104—Forming layers
    • H01L21/02365—Forming inorganic semiconducting materials on a substrate
    • H01L21/02367—Substrates
    • H01L21/0237—Materials
    • H01L21/02422—Non-crystalline insulating materials, e.g. glass, polymers
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
    • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
    • H01L21/02104—Forming layers
    • H01L21/02365—Forming inorganic semiconducting materials on a substrate
    • H01L21/02518—Deposited layers
    • H01L21/02521—Materials
    • H01L21/02524—Group 14 semiconducting materials
    • H01L21/02532—Silicon, silicon germanium, germanium
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
    • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
    • H01L21/02104—Forming layers
    • H01L21/02365—Forming inorganic semiconducting materials on a substrate
    • H01L21/02656—Special treatments
    • H01L21/02664—Aftertreatments
    • H01L21/02667—Crystallisation or recrystallisation of non-monocrystalline semiconductor materials, e.g. regrowth
    • H01L21/02691—Scanning of a beam
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
    • H01L21/67—Apparatus specially adapted for handling semiconductor or electric solid state devices during manufacture or treatment thereof; Apparatus specially adapted for handling wafers during manufacture or treatment of semiconductor or electric solid state devices or components ; Apparatus not specifically provided for elsewhere
    • H01L21/67005—Apparatus not specifically provided for elsewhere
    • H01L21/67011—Apparatus for manufacture or treatment
    • H01L21/67098—Apparatus for thermal treatment
    • H01L21/67115—Apparatus for thermal treatment mainly by radiation

Abstract

A method and an optical system for processing a layer of semiconductor material, in particular for generating a crystalline semiconductor layer, are disclosed. The method comprises the following steps: -providing a first laser beam (74) with first laser pulses (76) and a second laser beam (84) with second laser pulses (86), -shaping the first laser pulses (76) and the second laser pulses (86) by means of a beam shaping device into line-shaped laser pulses with a short axis and a long axis, -imaging the shaped line-shaped laser pulses on the semiconductor material layer by means of an imaging device into illumination lines with a short axis and a long axis, wherein the method further comprises the steps of: -setting the polarization direction of the first laser pulse (76) in the direction of the short axis of the illumination line (36), -setting the polarization direction of the second laser pulse (86) in the direction of the long axis of the illumination line, and-delaying the second laser pulse (86) in time with respect to the first laser pulse (76) by a predetermined time interval Δ t, the predetermined time interval Δ t being selected such that the illumination line imaged on the layer of semiconductor material has a combined temporal intensity profile (96) of the pulse shapes of the first maximum (M1) and the second maximum (M2).

Description

Method and optical system for processing semiconductor material
Technical Field
The present disclosure relates to a method for processing a semiconductor material, in particular for producing a crystalline semiconductor layer, and to an optical system for processing a semiconductor material, in particular for producing a crystalline semiconductor layer.
Background
For crystallization of a Thin Film layer used for manufacturing a Thin Film Transistor (TFT), for example, a laser is used. Silicon (Si), more precisely amorphous silicon (a-Si), is particularly relevant as the semiconductor to be processed. The thickness of the semiconductor layer is, for example, 50nm, and the semiconductor layer is usually located on a substrate, such as a glass substrate, or on another carrier.
The layer is irradiated with light from a laser, for example a pulsed solid-state laser. Here, light with a wavelength of, for example, 343nm is shaped into an illumination line and imaged on an imaging plane of the semiconductor material. The illumination line has a short (narrow) axis and a uniformly long beam axis. The minor or narrow axis has a gaussian or flat intensity profile.
The irradiation line is moved in the direction of the short axis over the semiconductor layer at a feed speed of typically about 5 to 50 mm/s. The power density of the beam (in the case of a continuous wave laser) or the pulse energy density (in the case of a pulsed laser) is set (einstellen) such that it is partially melted in the case of, for example, amorphous silicon and the melted silicon is subsequently solidified in a polycrystalline structure on the glass substrate starting from unmelted solid silicon. The time during which melting and solidification occurs is typically in the range of 10 to 100ns, and subsequent cooling of the film to room temperature typically requires hundreds of mus.
Uniform illumination intensity, i.e. uniformity of the spatial intensity distribution integrated along the short and/or long axis, is particularly important when irradiating (Bestrahlung) an amorphous silicon layer and converting it into a polycrystalline silicon layer. The more homogeneous or uniform the intensity distribution of the irradiation light, the more homogeneous or uniform the crystal structure of the thin film layer (e.g., the grain size of the polycrystalline layer) and the better the electrical characteristics of the final product, e.g., a thin film transistor, composed of the thin film layer. Due to the high mobility of electrons and positively charged holes, a uniform crystal structure may lead to, for example, a high conductivity.
Non-uniformities can occur particularly along the long beam axis and the short beam axis perpendicular to the long beam axis as the illumination line moves in the short axis direction over the semiconductor layer. This inhomogeneity is referred to as "Mura". The so-called "scanning Mura" originates from inhomogeneities along the beam axis and occurs in stripe-like inhomogeneities extending in the scanning direction or in the feed direction. Perpendicular to this is the so-called "shot (Mura") due to pulse-to-pulse intensity and energy density fluctuations.
In order to keep the "shot Mura" as small as possible, fluctuations in the energy density and intensity variations over time from laser pulse to laser pulse are to be kept as small as possible, for example by using lasers with very good pulse stability and by superimposing the laser beams of a plurality of laser sources. To keep the "scan Mura" as small as possible, the intensity of the illumination rays along the long axis should be made as uniform as possible. Further, it is known to reduce "scanning Mura" by reciprocally moving the irradiation line in the long axis direction by 1mm to 2mm to "smear (versemieren)" possible unevenness along the long axis during scanning by means of a mirror oscillating at a frequency of 10Hz to 200Hz around a rotation axis.
The so-called "nitrogen (sticksttoff) Mura" is also known, which derives from the fact that: on the exposed surface of the radiation substrate
Figure BDA0003344993190000021
I.e. the surface of the material layer to be processed, e.g. the semiconductor layer, the surface is flushed with nitrogen in order to reduce the oxygen concentration there to a value between 10ppm and 20ppm and thereby prevent oxidation of the material, e.g. silicon. For this purpose, a nitrogen blanket flow is passed directly over the material layer to be exposed (belichenden). In this case, inhomogeneities in the laminar flow can lead to inhomogeneities in the crystal structure, so-called "nitrogen Mura".
In order to produce a regular polycrystalline grain structure during the crystallization process, it is known to use surface interference effects which cause a modulated intensity distribution during exposure and, by repeated exposure during feeding, to strengthen the grain structure about the size of the wavelength of light. This effect is referred to as "Laser Induced Periodic Pattern Structure (LIPPS)". Thus giving a particle structure of about 0.3 μm to 0.4 μm at a wavelength of 343nm, for example. For linearly polarized light, the modulated intensity distribution is formed only in the direction of polarization, i.e. in the direction of the electric field vector. Experimental studies have shown that a regular structure along the long beam axis is formed when the light is polarized in the long axis, and accordingly this effect can be observed in the feed direction when the light is polarized in the feed direction.
Disclosure of Invention
The present disclosure proposes an improved method for processing semiconductor materials, in particular for producing a homogeneously crystallized semiconductor layer. The invention also proposes an improved apparatus for processing semiconductor material, in particular for producing a homogeneously crystallized semiconductor layer. The semiconductor layer crystallized homogeneously is in particular a semiconductor layer having a homogeneous grain size.
The present disclosure proposes a method having the features of claim 1 and an optical system having the features of claim 11.
A method for processing a layer of semiconductor material, in particular for producing a crystalline semiconductor layer, is disclosed, having the following steps:
providing a first laser beam with first laser pulses and a second laser beam with second laser pulses,
shaping the first laser pulse and the second laser pulse into a line-shaped laser pulse having a short axis and a long axis by means of a beam shaping device,
imaging the thus shaped linear laser pulses on the layer of semiconductor material as an illumination line having a short axis and a long axis by means of an imaging device,
-setting the polarization direction of the first laser pulse in the direction of the short axis of the illumination line,
-setting the polarization direction of the second laser pulse in the direction of the long axis of the illumination line, and
the second laser pulse is delayed in time with respect to the first laser pulse by a predetermined time interval Δ t, which is selected such that the irradiated line imaged on the layer of semiconductor material has a pulse-shaped combined time-intensity profile with a first maximum and a second maximum.
The order in which the method steps are presented herein above does not reflect the chronological order in which the steps are performed. Generally, the order in which the method steps are presented above does not reflect the temporal order in which the steps are performed. Usually, for example, setting the polarization direction in the beam path takes place where the single beam, i.e. the first and second beam laser, is split. The individual beams need to be shaped, homogenized and superimposed after the provision of the first and second laser beams. The step of time delaying may be performed before or after the polarization alignment of the first and second laser beams. In principle, the chronological order of the steps given can also be in other forms.
The semiconductor material to be processed may be, for example, a thin layer of amorphous silicon with a thickness of approximately 50nm coated on a carrier, for example a glass substrate.
The first and second laser beams are provided by at least one laser, as will also be described in detail below. The laser may be, for example, a UV solid-state laser emitting light with a wavelength of 343 nm. The first and second laser pulses have a full width at half maximum time (FWHM) in a range of 15ns to 20 ns. The first and second laser light pulses are generally linearly polarized here.
In a further method step, the polarization of the first and second pulses is set in a specific predefined direction, for example by means of a polarization device. The first laser pulse is thus linearly polarized in the direction of the minor axis of the illumination line and the second laser pulse is linearly polarized in the direction of the major axis of the illumination line. The pulses emitted by solid state lasers are typically linearly polarized. If the transmitted pulses are linearly polarized, then the polarization directions of the transmitted first and second pulses are rotated to predetermined, defined directions, respectively, in accordance with the methods of the present disclosure. This can be achieved by means of a polarizing means, such as a λ/2 plate, which has a corresponding orientation with respect to the light beam or pulse impinging on the λ/2 plate. In particular the polarization direction of the first pulse is rotated into the short axis direction, as will be described further below. Thus, the polarization of the first pulse is aligned in the short axis direction, i.e. the polarization is aligned almost only in the short axis direction, so that the component of e.g. linearly polarized light in the direction perpendicular to the short axis is only 1% (polarization ratio 100: 1), or e.g. the polarization ratio 95: 5. the polarization direction of the second pulse is rotated into the long axis direction perpendicular to the short axis direction, as will also be described below. The polarization of the second pulse is thus aligned in the long axis direction, i.e. the polarization is aligned almost only in the long axis direction, e.g. the component of the linearly polarized light in the direction perpendicular to the long axis is only 1% (polarization ratio 100: 1), or e.g. the polarization ratio 95: 5.
furthermore, the second pulse is delayed in time with respect to the first pulse by a predetermined time interval Δ t. The time delay is typically 5ns to 20 ns. The time interval Δ t is generally selected such that, in the imaging of the radiation onto the semiconductor material layer, which will be described further below, the superposition of the two pulses over time gives a single pulse. The temporal superposition results in a combined temporal intensity variation having a first maximum and a second maximum.
In a further method step, the first and second pulses are shaped into a laser line, i.e. a line-shaped laser pulse, by means of a beam shaping device. The beam shaping means may constitute anamorphic optics. The beam shaping device may for example have a lens array homogenizer which is based on the principle of splitting one or more incident laser beams into four sub-beams and subsequently spatially superposing them. The laser line has a minor axis and a major axis.
The laser line thus formed is imaged by means of an imaging device as an illumination line on an imaging plane of the semiconductor material. The illumination line likewise has a short axis and a long axis, the direction of which predetermines the direction of the polarization of the first and second pulses. The direction of the short and long axes of the illumination line generally coincides with the direction of the short and long axes of the line-shaped laser pulse. The length of the illumination line, i.e. the geometric extension in the direction of the long axis, is typically between 100mm and 1000mm, for example 100mm,250mm,750mm or 1000 mm. The illumination line can also be longer if the beam shaping device and/or the imaging device are configured accordingly. The width of the illumination line, i.e. the geometric extension in the direction of the short axis, is given as the half-value width (FWHM) in the case of a gaussian distribution and is typically between 20 μm and 200 μm. The width is measured at the intensity 90% position (90% full width) in the case of a flat distribution, typically between 20 μm and 200 μm.
In a further method step, the irradiation beam can be moved relative to the semiconductor material layer in the feed direction. Thus, since the feed direction corresponds to the short axis direction, the first pulse is linearly polarized in the feed direction. By scanning the semiconductor material layer with the radiation, the entire semiconductor material layer or at least a larger area of the semiconductor material layer can be exposed and thus processed. The carrier with the semiconductor material can be placed, for example, on a table that is moved in the feed direction and can thus be moved relative to the irradiation lines. The feed rate is usually 5mm/s to 50 mm/s.
According to a further aspect, the relative intensities of the first laser pulse and the second laser pulse are selected such that the ratio of the first maximum to the second maximum in the combined temporal intensity profile is in the range from 0.8 to 1.4, in particular in the range from 0.9 to 1.2, in particular 1.0. Since the combined time intensity profile is obtained by superimposing the temporal intensity variations of the first and second pulses, the ratio of the first maximum value to the second maximum value in the combined time intensity profile can be set by the intensities of the first and second pulses, also taking into account the time interval Δ t. It has been found that in combination with the defined polarization directions of the first and second pulses, as described above, a very uniform grain structure can be obtained in the formed (poly) crystalline semiconductor layer for a given area of the ratio of the first maximum to the second maximum.
The combined time intensity profile of the illumination lines may have a time half-value width between 40ns and 50ns, which is associated with the first maximum of the combined time intensity profile. The crystallization process is influenced by tens of ns by the relatively long pulse duration and promotes the formation of a uniform grain structure.
According to a further aspect, a first laser and a second laser are provided, which are arranged for emitting a first laser beam and a second laser beam, respectively, and the second laser pulse is delayed from the first laser pulse emission by a time interval Δ t under control. The delay may be achieved, for example, by an electronic delay of the trigger signal of the second laser relative to the trigger signal of the first laser.
Alternatively, a first laser is provided, which is designed to provide a laser beam having pulses and to divide the laser beam of the first laser into a first laser beam component and a second laser beam component, wherein the first laser beam component forms a first laser beam having first laser pulses and the second laser beam component forms a second laser beam having second laser pulses. In this alternative, therefore, a laser is provided which operates in pulsed mode and divides the laser beam emitted by it into a first laser beam and a second laser beam by means of a beam splitter.
In this alternative, the time delay of the second pulse relative to the first pulse may be achieved by phase shifting the first pulse relative to the second pulse by causing an optical path length of the second laser beam from the beam splitting point to the imaging plane of the semiconductor material to be greater than an optical path length of the first laser beam from the beam splitting point to the imaging plane of the semiconductor material.
In principle, it is of course also possible in a variant to use two lasers, the time delay of the second pulse relative to the first pulse being provided by a longer optical path of the second pulse.
According to another aspect, the first laser pulse may be one of a plurality of first laser pulses of the first laser beam and the second laser pulse may be one of a plurality of second laser pulses of the second laser beam, and wherein each of the plurality of laser pulses of the pulsed second laser beam is delayed in time by a predetermined time interval Δ t relative to another of the plurality of laser pulses of the pulsed first laser beam. The laser or lasers are thus operated in pulses and emit a plurality of laser pulses at a specific pulse repetition rate of, for example, 10 kHz. The laser pulses of the second laser beam are delayed in time with respect to the first laser pulses in such a way that the first laser pulses and the second laser pulses are always superimposed on a radiation beam in the form of pulses, having a first and a second maximum, which is imaged on the semiconductor material. Or in other words the semiconductor material is exposed to the radiation in pulses at the pulse repetition rate of the laser.
The feed rate, the pulse repetition rate of the first laser beam and the second laser beam, and the geometric half-value width of the irradiation beam in the direction of the short axis are selected such that the position of the semiconductor material is exposed to the irradiation beam a plurality of times. Or in other words, the semiconductor material is moved slowly relative to the irradiation line such that the geometric half-value width of the irradiation line in the direction of the short axis is large to correspond to the pulse repetition rate of the pulsed light beam or a multiple of the pulse repetition rate after a period of time, whereby the path of movement of the semiconductor material is small, so that previously exposed locations are exposed again or a plurality of times.
According to a further aspect of the method of the present disclosure, a third laser beam having third laser pulses and a fourth laser beam having fourth laser pulses are provided, the first, second, third and fourth laser pulses are shaped into line-shaped laser pulses having a short axis and a long axis by means of a beam shaping device, and the line-shaped laser pulses thus shaped are imaged as illumination lines on an imaging plane of the semiconductor material layer by means of an imaging device. Furthermore, the polarization direction of the third laser pulse is set in the direction of the short axis of the illumination line, the polarization direction of the fourth laser pulse is set in the direction of the long axis of the illumination line, and the fourth laser pulse is time-delayed relative to the third laser pulse by a predetermined time interval Δ t, which is selected such that the illumination line imaged on the semiconductor material layer has a pulse-shaped combined temporal intensity profile having a first maximum value and a second maximum value.
Thus, according to this aspect of the method, the four pulsed light beams are uniformly superimposed and imaged, wherein two of the four pulsed light beams are respectively made to have pulses linearly polarized in the short axis direction of the irradiation line and synchronized in time, and the other two of the four pulsed light beams are respectively made to have pulses linearly polarized in the long axis direction of the irradiation line and delayed in time with respect to the pulses of the first two pulsed light beams.
According to another aspect of the present disclosure, an optical system for processing a layer of semiconductor material, in particular for producing a crystalline semiconductor layer, is proposed, having:
a beam shaping device arranged for shaping the first laser pulses of the first laser beam and the second laser pulses of the second laser beam into line-shaped laser pulses having a short axis and a long axis,
an imaging device arranged for imaging the thus shaped line-shaped laser pulses on the layer of semiconductor material as an illumination line having a short axis and a long axis,
-polarizing means arranged for aligning the polarization direction of the first laser pulse in the direction of the minor axis of the illumination line and the polarization direction of the second laser pulse in the direction of the major axis of the illumination line, and
a delay device, which is provided for delaying the second laser pulse by a predetermined time interval Δ t relative to the first laser pulse, the predetermined time interval Δ t being selected such that the irradiated line imaged on the semiconductor material layer has a pulse-shaped combined temporal intensity profile having a first maximum and a second maximum.
The polarization means are therefore designed and arranged such that the first laser pulse comprises almost exclusively the polarization component in the direction of the short axis of the radiation line (polarization ratio of, for example, 1: 100) and the second laser pulse comprises almost exclusively the polarization component in the direction of the long axis of the radiation line (polarization ratio of, for example, 1: 100). The polarization means may comprise a first polarization means for the first laser beam with the first laser pulses and a second polarization means for the second laser beam with the second laser pulses. In this way, the first polarizing means is arranged in particular in the beam path of the first laser beam and the second polarizing means is arranged in particular in the beam path of the second laser beam.
The beam shaping means may constitute anamorphic optics. The beam shaping device may for example have a lens array homogenizer which is based on the principle of splitting one or more incident laser beams into four sub-beams and subsequently spatially superposing them. The laser line has a minor axis and a major axis.
The laser line thus formed is imaged by means of an imaging device as an illumination line on an imaging plane of the semiconductor material. The illumination line likewise has a short axis and a long axis, the direction of which predetermines the direction of the polarization of the first and second pulses. The direction of the short and long axes of the illumination line generally coincides with the direction of the short and long axes of the line-shaped laser pulse.
The polarizing means of the optical system may in particular comprise a first λ/2 plate which is arranged in the beam path of the first laser beam, in particular arranged before the beam shaping means, and is oriented with respect to the first laser pulses incident on the λ/2 plate such that the first laser pulses are linearly polarized in the direction of the short axis after passing through the λ/2 plate; and a second lambda/2 plate arranged in the beam path of the second laser beam, in particular arranged before the beam shaping device, and oriented with respect to the second laser pulses incident on the lambda/2 plate such that the second laser pulses are linearly polarized in the direction of the long axis after passing through the lambda/2 plate.
The first λ/2 plate is oriented such that it rotates the polarization direction of the first laser beam with the linear polarization of the first laser pulses into the short axis direction. The second lambda/2 plate is oriented in such a way that it rotates the polarization direction of the second laser beam with the second laser pulses into the long axis direction.
According to one variant, the delay device has a delay circuit for setting the trigger signal of the second laser for emitting the second laser beam with the second laser pulses at a time interval Δ t relative to the trigger signal of the first laser, wherein the first laser is set for emitting the first laser beam with the first laser pulses. The second trigger signal may be electronically delayed relative to the first trigger signal.
According to one variant, the delay device has a beam bypass, so that the optical path length of the second laser beam up to the imaging plane of the layer of semiconductor material is greater than the optical path length of the first laser beam up to the imaging plane of the layer of semiconductor material. According to this variant, the time delay is caused by the channel differentiation (gangunterschad) by the second laser pulse having a longer optical path length up to the superposition than the first pulse.
The first and second laser beams of the optical system may be provided by a first and second laser source, or may alternatively be provided by one laser source, the laser beam emitted by which is then divided into a first laser beam and a second laser beam by means of a beam splitter.
According to a further aspect, the beam shaping device may be arranged for shaping the first laser pulses of the first laser beam, the second laser pulses of the second laser beam, the third laser pulses of the third laser beam, and the fourth laser pulses of the fourth laser beam into line-shaped laser pulses having a short axis and a long axis; the imaging device may be arranged for imaging the thus shaped linear laser pulses on the layer of semiconductor material as illumination lines having a short axis and a long axis; the polarization means may be arranged to linearly polarize the third laser pulse in the direction of the minor axis of the illumination line and to linearly polarize the fourth laser pulse in the direction of the major axis of the illumination line; and the delay means may be provided for delaying the fourth laser pulse relative to the third laser pulse by a predetermined time interval Δ t, which is selected such that the irradiated line imaged on the semiconductor material layer has a pulse-shaped combined temporal intensity profile having a first maximum value and a second maximum value.
According to this aspect, the optical system comprises a four-beam arrangement in which four laser beams are uniformly superimposed and imaged as an illumination beam on the semiconductor material layer, wherein the polarizing means is arranged and disposed for causing two of the four laser beams to have pulses respectively linearly polarized in the short axis direction of the illumination beam and synchronized in time, and causing the other two of the four pulse beams to have pulses respectively linearly polarized in the long axis direction of the illumination beam and delayed in time with respect to the pulses of the first two pulse beams.
The disclosure also includes an apparatus for processing a layer of semiconductor material, in particular for producing a crystalline semiconductor layer, comprising an optical system according to the above aspect, wherein the apparatus is provided for moving the layer of semiconductor material relative to the irradiation line in a feed direction, wherein the feed direction corresponds to the direction of the minor axis of the irradiation line. The semiconductor material layer can be moved relative to the radiation by means of a feed device, such as a table, which is moved in the feed direction, on which the carrier with the semiconductor material layer is placed, so that a larger area up to the entire semiconductor layer is exposed to the radiation and is thus processed. The feed direction corresponds to the short axis direction, so that the polarization of the first pulse and/or the third pulse is oriented in the feed direction.
Drawings
The invention is explained in detail below with the aid of the figures. In the drawings, there is shown in the drawings,
fig. 1 shows a schematic representation of a layer of semiconductor material, which, for processing, is exposed with an irradiation line moving in a feed direction with respect to the layer of semiconductor material;
fig. 2a to 2c show the linear geometry of the imaged illumination line;
FIGS. 3a and 3b show schematic representations of the optical system of an apparatus for processing semiconductor layers, by means of which system an irradiation beam can be formed and imaged on a semiconductor material;
FIG. 4 shows a schematic diagram of an embodiment of an optical system in which a first laser beam and a second laser beam are provided by a beam splitter of the laser beams;
FIG. 5 shows a schematic diagram of an embodiment in which four laser beams are provided by four laser sources, wherein the pulses of two laser beams are emitted time-delayed from the pulses of the other two laser beams, respectively;
FIG. 6 shows a schematic diagram of an embodiment in which two laser beams are provided by two laser sources, wherein the pulses of one laser beam have a time delay relative to the pulses of the other laser beam;
fig. 7 schematically shows the intensity distribution of a combination of illumination lines over time, which is given by a uniform superposition of the individual pulses;
FIG. 8a shows a scanning electron microscope image of a crystalline silicon layer generated according to the disclosed method, an
Fig. 8b shows a scanning electron microscope image of a crystalline silicon layer processed according to the comparative method.
Detailed Description
In fig. 1 is schematically shown how a semiconductor material is irradiated with a laser beam to generate a uniformly crystallized layer according to the disclosed method. A layer 12 of semiconductor material to be processed is applied on a carrier 10, for example a glass substrate. In the example shown here, the semiconductor material to be processed is amorphous silicon. The thickness of the layer of semiconductor material 12 is typically about 50 nm.
A line-shaped laser beam 14 is imaged onto the semiconductor material and moved relative to the semiconductor material in the feed direction X such that the laser line 14 scans over and irradiates at least a partial area of the layer of semiconductor material 12. In the example shown here, the carrier 10 is moved in space with the semiconductor material layer 12 and thus relative to the stationary laser beam 14. The laser line 14 may be moved relative to the layer of semiconductor material 12 such that the entire layer of semiconductor material 12 is irradiated by the laser line 14. Generally, the laser line 14 can be moved relative to the layer of semiconductor material 12 such that a particular area is irradiated multiple times by the laser line 14. Typical feed rates are in the range of 5mm/s to 50 mm/s.
In the exemplary embodiment shown here, the propagation direction of the laser beam 14 is perpendicular to the surface of the semiconductor material layer 12, i.e., the laser beam 14 here strikes the surface of the semiconductor material layer 12 perpendicularly at an angle of incidence of 0 °.
Fig. 2a to 2c show the linear geometry of the laser beam 14. The intensity associated with a particular direction is shown in fig. 2a to 2c, respectively.
Fig. 2a shows the laser line intensity in the direction of the long axis, i.e. the intensity distribution 16 integrated along the short axis (along the x-axis), where the intensity distribution 16 thus integrated is shown along the long axis (along the y-axis). Conventionally, in the figures, the minor axis runs parallel to the x-axis and the major axis runs parallel to the y-axis. As can be seen in this figure, the distribution 16 is approximately rectangular, i.e. ideally uniformly formed along the major axis. The length of the illumination line in the y-direction may typically be between 100mm and 1000mm, for example 100mm,250mm,750mm or 1000mm, or more than 1000 mm.
The laser line intensity in the short axis direction, i.e. the intensity distribution 18, 20 integrated along the long axis (i.e. along the y-axis) is shown in fig. 2b and 2c, respectively, where such integrated intensity distribution is shown along the short axis (i.e. along the x-axis). The intensity in fig. 2b has a gaussian curve 18. Alternatively, the intensity may also be a flat curve 20 ("flat top"), i.e. a variation approximating a rectangle, as shown in fig. 2 c.
Typical intensity widths in the x-direction are between 20 μm and 200 μm. Here, the Width of the gaussian curve 18 in fig. 2b is given as the Full Width at Half Maximum (FWHM), which for the flat curve 20 in fig. 2c is the Width of the curve at 90% intensity equivalent to the Maximum intensity (FW 90%: Full Width at 90%).
If the irradiation line 14 is directed over the semiconductor material layer (e.g., a-Si)12 to be processed, the semiconductor material layer 12 is melted and solidified into a crystallized layer having improved electrical characteristics in a short time.
Fig. 3a and 3b schematically show an optical system 30 of an apparatus for processing semiconductor layers, by means of which system the irradiation lines 14 as described in connection with fig. 1 and 2 can be formed and imaged on a semiconductor material.
The optical system 30 has a beam shaping device 32, which will be described in detail below, the beam shaping device 32 being arranged to shape the laser beam such that a beam profile of the laser beam has a major axis and a minor axis; and an imaging device 34 arranged after the beam shaping device 32 in the beam path of the laser beam, the imaging device 34 being arranged for imaging the thus shaped laser beam as an irradiation line 36. In the example shown here, four laser beams impinge beam shaping device 32, namely first laser beam 38, second laser beam 40, third laser beam 42, and fourth laser beam 44. It is also possible according to the present disclosure for two laser beams to impinge on the beam shaping means 32, as will be described below by way of example. The number of laser beams is in principle not limited to 4 or 2 here, but any other number of laser beams is also possible and is included within the scope of the present disclosure.
In the example shown here, the laser radiation is laser radiation of a wavelength 343nm emitted by a plurality of UV solid-state lasers. In principle, however, other laser sources, in particular other solid-state laser sources, for example solid-state lasers emitting in the green spectral range, can also be used.
As already shown in fig. 1 and 2, the minor axis is parallel to the x-axis and the major axis is parallel to the y-axis, as also shown in fig. 3a and 3 b. The optical axis of the optical system extends parallel to the z-axis.
Fig. 3a shows the imaging characteristics of the optical system 30 in the y-direction, i.e. along the long axis of the shaped laser beam and the irradiation line, and fig. 3b shows the imaging characteristics of the optical system 30 in the x-direction, i.e. along the short axis of the shaped laser beam and the irradiation line.
The beam shaping means 32 of the optical system 30 in fig. 3a and 3b has anamorphic homogenizing optics (anamorphic homogenizing optics) 46, which homogenizes the intensity of the incident laser beam in the direction of the y-axis. The anamorphic homogenizing optics 46 comprises, for example, two cylindrical lens arrays arranged parallel to each other. These cylinder lens arrays divide the incident beam into individual sub-beams and superimpose them over the entire plane, thereby maximizing the homogenization of the laser radiation. In the case of multiple incident laser beams, each laser beam is divided into separate sub-beams and subjected to a uniform superposition. Such a homogeneous optical device is described in detail, for example, in the prior art DE 4220705 a1, DE 3829728 a1 or DE 10225674 a1 cited in the present application.
The beam shaping device 32 of the optical system 30 also has a condenser cylindrical lens 48 which is arranged in the beam path downstream of the anamorphic homogenizing optics 46, the condenser cylindrical lens 48 being provided to deflect the laser beam divided and homogenized by means of the anamorphic homogenizing optics 46 telecentrically onto the illumination line 36 and there superimpose it about the long axis, i.e. in the y direction. The combination of anamorphic homogenizing optics 46 and condenser cylindrical lens 48 therefore causes the incident laser radiation to be imaged uniformly as illumination line 36 on the imaging plane.
An imaging device 34 is arranged in the beam path after the condenser cylindrical lens 48, the imaging device 34 being provided for focusing the laser beam on the irradiation line 36 with respect to the short axis, i.e. in the x direction. Or in other words, the imaging device 34 images the laser beam as the irradiation line 36, where only the short axis of the beam profile is homogenized, and the homogenized long axis of the beam profile is not homogenized. The imaging device 34 may be, for example, a focusing cylindrical lens optic.
The combination of anamorphic homogenizing optics 46 and condensing cylindrical lens 48 may be an anamorphic optic or a portion of such an optic. They can in particular be part of an anamorphic optical device, as is depicted in fig. 4 to 6 of the document DE 102012007601 a1 cited in the present application with respect to the anamorphic optical device 42.
The beam shaping means 32 may therefore also comprise one or more of the following optical elements:
a first collimating cylindrical lens, designated with reference numeral 54 in DE 102012007601 a1, for collimating the laser beam emitted about the x-axis,
a second collimating cylindrical lens, designated by reference numeral 56 in DE 102012007601A 1, for collimating the laser beam emitted about the y-axis,
a cylindrical lens arranged in the beam path after the first collimating cylindrical lens, which is designated with reference numeral 58 in DE 102012007601 a1, for focusing the laser beam about the x-axis onto an intermediate image designated with reference numeral 60 in DE 102012007601 a1,
an intermediate collimating cylindrical lens arranged in the beam path after the first collimating cylindrical lens 54, which is designated with reference numeral 58' in DE 102012007601 a1, for collimating the laser beam of the first intermediate image, and/or
A further cylindrical lens, which is designated by reference numeral 62 in DE 102012007601 a1, arranged in the beam path after the first intermediate image, in particular after the intermediate collimating cylindrical lens, serves to focus the laser beam about the x-axis onto the second intermediate image, which is designated by reference numeral 64 in DE 102012007601 a 1.
The above-described anamorphic homogenizing optic 46 may be, or include, for example, the assembly 68 shown in fig. 4 to 6 of DE 102012007601 a 1.
The above-described condensing cylindrical lens 48 may be, for example, the condensing cylindrical lens 74 shown in fig. 4 to 6 of DE 102012007601 a1, or include such an assembly.
Finally, the imaging device 34 described above may be, for example, the assembly 66 shown in fig. 4 to 6 of DE 102012007601 a1, or comprise such an assembly.
Furthermore, the optical system may also have a polarizing device 50 for each laser beam 38, 40, 42, 44 incident on the anamorphic optics. The polarizing means are here optics for setting the polarizing means 50, for example a λ/2 plate in the beam path of each incident laser beam 38, 40, 42, 44. The optics 50 are disposed in the beam path before the anamorphic or anamorphic homogenizing optics 46. Each incident laser beam passes through optics 50 such that laser beams 38, 40, 42, 44 passing through optics 50 are linearly polarized in a defined direction. Specifically, the laser beam emitted by the laser is already linearly polarized, as in the example of a UV solid-state laser shown here, for example, and the orientation of the polarization is rotated in a defined direction by means of the optics 50. Here, the optical device 50, for example a λ/2 plate, is oriented with respect to the polarization direction of the incident linearly polarized light such that two of the four laser beams are linearly polarized in the long axis direction after passing through the optical device 50 and the remaining two of the four laser beams are linearly polarized in the short axis direction after passing through the optical device 50. Here, the feed direction corresponds to the direction of the short axis, so that the remaining two of the four laser beams are linearly polarized in the feed direction. More specifically, according to the present disclosure, optics 50 (e.g., λ/2 plates) disposed in the beam paths of first and second laser beams 38, 40 are oriented to polarize first and second laser beams 38, 40, respectively, in the short-axis direction, i.e., the x-direction, in the feed direction, and optics 50 (e.g., λ/2 plates) disposed in the beam paths of third and fourth laser beams 42, 44, respectively, are oriented to polarize third and fourth laser beams 42, 44, respectively, in the long-axis direction, i.e., the y-direction. In the example shown here, the laser is also operated in pulses, so that each pulse of the respective laser beam has the above-mentioned polarization direction of the respective laser beam.
The four laser beams 38, 40, 42, 44 may be laser radiation emitted by four laser sources, i.e. each laser beam corresponds to a separate laser source, respectively.
Alternatively, the laser beams 38, 40, 42, 44 can also be divided into a first sub-beam and a second sub-beam by beam splitting the laser beam emitted by one laser source by means of a beam splitter. The beam splitter may be configured such that the first sub-beam produced by the division is a transmitted beam and the second sub-beam is a transmitted beam, for example, each about 50%. For this purpose, polarizing optics, such as so-called thin-film polarizers, can be used, for example. A thin film polarizer is an optical substrate with a special coating that allows light with p-polarization (the plane of oscillation of the electric vector is parallel to the incident light beam and the perpendicular on the substrate surface) to pass through while light with s-polarization (the plane of oscillation of the electric vector is perpendicular to the incident light beam and the perpendicular on the substrate surface) is reflected. The polarization direction of the laser beam emitted by the laser source can be rotated by means of a λ/2 plate arranged in the beam path before the thin-film polarizer, so that the laser beam is p-polarized and s-polarized with equal components before the thin-film polarizer, in order to achieve a division of approximately 50%. But it is also possible to rotate the lambda/2 plate before the thin film polarizer so that the laser beam is subjected to different components of p-polarization and s-polarization before the thin film polarizer to achieve a division different from 50%. The relative intensities of the first and second sub-beams can in principle be set by means of the orientation of the λ/2 plate.
Such an arrangement is schematically illustrated in figure 4. A linearly polarized laser beam 52 emitted by a laser source impinges on a λ/2 plate 54, the λ/2 plate 54 being arranged in the beam path before a beam splitter 56, in which a thin-film polarizer is arranged. The λ/2 plate 54 is oriented such that the relative components of the s-and p-polarizations in the laser beam after the λ/2 plate correspond to the desired relative intensities of the two sub-beams 58, 60 after the beam splitter 56, as described above. Thus the first sub-beam 58 may be, for example, the first laser beam 38 in the arrangement of fig. 3a and 3b, and the second sub-beam 60 may be, for example, the third laser beam 42 shown in fig. 3a and 3 b. The second sub-beam 60 is deflected by the reflecting element 62 such that it extends parallel to the first sub-beam 58. In addition, after the beam splitter 56 in the beam path, in each case a λ/2 plate 64 is provided in the beam path of the first and second partial beam, which corresponds to the λ/2 plate 50 in the beam path of the first and third laser beams 38, 42 shown in fig. 3a and 3 b. That is, the λ/2 plate 64 arranged behind in the beam path of the first sub-beam 58 is used for polarization in the short axis direction. A subsequent λ/2 plate 64 is arranged in the beam path of the second sub-beam 60 for polarization in the long-axis direction.
Second laser beam 40 and fourth laser beam 44 may be provided by beam splitting with an additional arrangement with beam splitter 56 corresponding to the arrangement shown in fig. 4. In this way, the subsequent λ/2 plates 64 are again oriented such that they polarize the third and fourth sub-beams in the short-axis direction and the long-axis direction, respectively. The four laser beams in fig. 3a and 3b may thus alternatively be provided by two laser sources, the laser beams emitted by which may be divided into a first and a second sub-beam 58, 60 or a third and a fourth sub-beam by beam division, respectively.
As already described above, the optical system 30 in fig. 3a and 3b can also be used for superimposing other numbers of laser beams than four, for example two laser beams. Thus, two laser beams may be provided by two laser sources corresponding to four laser beams, or by one laser source, and then the laser beam emitted by the laser source is divided into a first sub-beam and a second sub-beam by beam division by means of an arrangement corresponding to or identical to that shown in fig. 4.
The optical system 30 in fig. 3a and 3b can also be designed to polarize the pulses differently, offset in time from one another, at a predetermined time interval Δ t. This can be achieved by an electronic delay of the trigger signal of the laser source for each laser beam when using separate laser sources. When providing the laser beam by means of sub-beams, a time delay can be achieved by beam bypassing (Strahlumweg). As shown in fig. 4, the second sub-beam 60 follows a path approximately Δ s longer than the first sub-beam 58. The path Δ s may be chosen such that the second sub-beam 60 has a time delay of the predetermined time interval Δ t compared to the first sub-beam 58. The predetermined time interval is preferably 10ns to 20 ns.
By means of the above-described optical system 30, it is thus possible to image at least one laser pulse polarized in the short-axis direction and one laser pulse polarized in the long-axis direction on the illumination line in a uniformly superimposed manner, wherein the pulses polarized in the long-axis direction are delayed by a time interval Δ t with respect to the pulses polarized in the short-axis direction. When four laser beams are superimposed, two laser beams are polarized in the short axis direction and two laser beams are polarized in the long axis direction, wherein the laser beams polarized in the short axis direction are synchronized in time, but the laser beams polarized in the long axis direction are delayed in time by the same time interval Δ t with respect to the laser beams polarized in the short axis direction. By superimposing two (or more) temporally synchronized laser beams from two (or more) laser sources, possible fluctuations in energy density from pulse to pulse can be reduced. Such fluctuations can lead to different crystallization results and inhomogeneities in the crystal structure along the direction of movement ("shot Mura") when the irradiation line is moved from pulse to pulse in the direction of the short axis. It is to be noted here that the combined intensity distribution of two (or more) such superimposed laser beams (laser pulses) can vary in time (from combined intensity distribution to combined intensity distribution) due to the existing temporal jitter (i.e. the temporal deviation of the intensity from the ideal value) and thus cause inhomogeneities in the crystal structure. In particular, this non-uniformity results from the fact that when two (or more) laser sources are combined, each laser source has a time jitter that is uncorrelated with each other. It is therefore advantageous to use a (pulsed) laser source with as little time jitter as possible in the ns range.
The anamorphic homogenizing optics 46 are designed to divide each incident laser beam into a plurality of sub-beams and to superimpose them homogeneously in the long axis direction. I.e. each individual beam produces a uniform line. In the above-described pulsed arrangement with two laser beams, therefore, there are not only laser pulses which are polarized in the short-axis direction and advanced in time and are superimposed and imaged as a uniform line, but also laser pulses which are polarized in the long-axis direction and delayed in time with respect to the first pulse. In an arrangement with four laser beams, therefore, laser pulses each polarized in the short-axis direction and temporally preceding are superimposed and imaged as a uniform line, and laser pulses each polarized in the long-axis direction are temporally delayed relative to the first pulse.
This will be described again in detail with the aid of the disclosed method.
The disclosed method is described in fig. 5 by means of an arrangement with four laser sources, a first laser source 66, a second laser source 68, a third laser source 70 and a fourth laser source 72. The first laser source 66 and the second laser source 68 are each provided for supplying a first laser beam 74 having a first laser pulse 76 and a second laser beam 78 having a second laser pulse 80, wherein the first and second laser pulses 76, 80 are emitted simultaneously by a synchronized trigger signal 82 of the first and second laser sources 66, 68. Third and fourth laser light sources 70, 72 are provided for supplying a third laser beam 84 having a third laser pulse 86 and a fourth laser beam 88 having a fourth laser pulse 90, wherein a trigger signal 92 of third and fourth laser light sources 70, 72 is emitted and propagated in a time-delayed manner with respect to first pulse 76 and second pulse 80, respectively, with a respective electronic delay time Δ t, for example by means of an electronic delay circuit 94. According to the disclosed method, the first laser pulse 76 and the second laser pulse 80 are also linearly polarized in the feed direction, i.e. the x-direction, i.e. the polarization is oriented in the feed direction, and the third laser pulse 86 and the fourth laser pulse 90 are linearly polarized in the long axis direction, i.e. the polarization is oriented in the long axis direction, for example by means of the λ/2 plate 50 described with reference to fig. 3a and 3 b. The first through fourth laser pulses 76, 80, 86, 90 typically have a temporal half-value width (FWHM) in the range of 15ns to 20 ns. Typical times for the time interval Δ t are between 10ns and 20 ns.
The four laser beams 74, 78, 84, 88 with the four laser pulses 76, 80, 86, 90 are shaped into line-shaped laser pulses with a short axis and a long axis, for example by means of the beam shaping device 32 as shown in fig. 3a and 3 b. The laser pulses thus shaped are subsequently imaged on an imaging plane of the semiconductor material as irradiation lines 36, for example by means of an imaging device 34 as shown in fig. 3a and 3 b.
The disclosed method is illustrated in fig. 6 by means of an arrangement with two laser sources. Here, the first laser light source 66 corresponds to the first laser light source in fig. 5, and the second laser light source 70 corresponds to the third laser light source in fig. 5. Accordingly, trigger signal 92 of second laser source 70 is electronically delayed by a time interval Δ t relative to trigger signal 82 of first laser source 66, thereby delaying propagation of second laser pulse 86 by time interval Δ t relative to first laser pulse 76. Furthermore, for example, with the aid of the λ/2 plate 80 described with reference to fig. 3a and 3b, the first laser pulse 76 is linearly polarized in the short-axis direction of the later-shaped laser pulse or line-shaped irradiation line, and the second laser pulse 86 is linearly polarized perpendicular thereto in the long-axis direction. Here, the minor axis direction corresponds to the feed direction in which the subsequently shaped irradiation line 36 is moved relative to the semiconductor material 12 to be processed. Thus, similar to the method shown in fig. 5, the two laser beams 74, 84 with the two laser pulses 76, 86 are shaped into line-shaped laser pulses with a short axis and a long axis, for example by means of the beam shaping device 32 as shown in fig. 3a and 3 b. The laser pulses thus shaped are subsequently imaged as illumination lines 36 on an imaging plane of the semiconductor material 12, for example by means of an imaging device 34 as shown in fig. 3a and 3 b.
The linear geometry of the radiation beam 36 thus shaped is described with reference to fig. 2a to 2 c. The intensity profile of the combinations of the illumination lines thus shaped with respect to time, i.e. the variation of the intensity of the pulses superimposed and temporally delayed from one another as a function of time, is now described with reference to fig. 7. Fig. 7 shows a combined time-intensity curve 96 for the method with two pulsed light beams, as disclosed for example for fig. 6. Fig. 7 also shows the combined intensity of the two laser beams 74, 84 and the pulse intensity of each individual laser beam as a function of time.
The intensity profile denoted by reference numeral 98 in fig. 7 corresponds to the intensity profile of the first laser pulses 76 of the first laser beam 74, the intensity profile denoted by reference numeral 100 corresponds to the intensity profile of the second laser pulses 86 of the second laser beam 84, and the intensity profile denoted by reference numeral 96 corresponds to the combined temporal intensity profile of the first and second pulses 76, 86. First and second laser pulses 76, 86 each have a temporal half-value width (FWHM) between 15ns and 20 ns. As can also be seen in fig. 7, second laser pulse 86 is delayed in time with respect to first laser pulse 76 by a delay time of between about 10ns and about 20 ns. In fig. 7, the delay time is about 20 ns. This results in a pulse profile with a first maximum M1 and a second maximum M2 in the combined time intensity profile 96 and a widened pulse duration 102 with respect to the individual pulse durations, the total pulse duration here being 40ns to 50 ns. The total pulse duration 102 in turn corresponds to a temporal half-width which is associated with the first maximum value ("half-width of the first maximum value"), i.e. the pulse width at which the intensity of the first pulse is half of the maximum value M1.
As also shown in fig. 7, the maximum intensity M1 of first laser pulse 76 is greater than the maximum intensity M2 of temporally delayed second laser pulse 86. In particular, the intensity of the first laser pulse 76 is set relative to the intensity of the second laser pulse 86 such that the ratio M1/M2 of the first maximum M1 to the second maximum M2 of the combined temporal intensity profile 96 lies between 1/1.2 and 1/0.7, i.e. between 0.8 and 1.4. In embodiments where each laser beam is provided by a separate laser source, this is achieved by tuning the intensity of the laser beams relative to each other. In the arrangement schematically shown in fig. 4 for providing two laser beams by dividing the laser beams, the relative intensities can be varied by varying the s-component and p-component of the laser beams before the thin film polarizer 56 by rotating the λ/2 plate 54 accordingly.
As has been detailed above, according to the present disclosure, first laser pulse 76 is linearly polarized in the short axis direction, i.e., the feed direction, and second laser pulse 86 is linearly polarized in the long axis direction.
The present disclosure is based on the recognition that the above-described linear polarization of first and second pulses 76, 86 has a positive effect on the uniformity of semiconductor material layer 12 processed with the laser line. It has been found that a very uniform 50nm to 60nm thick crystalline silicon layer with a regular grain structure can be produced in the polarization of the first pulse in time in the feed direction and in the polarization of the temporally delayed pulse in the long axis direction. The ratio of the first maximum M1 to the second maximum M2 of the combined time intensity profile is here approximately 1: 1, i.e. between 0.8 and 1.4.
In contrast, such a positive effect is not observed for an oppositely polarized, i.e. a first pulse linearly polarized in the long axis direction and a delayed second pulse linearly polarized in the short axis direction.
This recognition is described below with the aid of experimental data:
the semiconductor material to be processed is a 50nm thick thin layer composed of amorphous silicon on a glass substrate as a carrier. The optical device used is a linear beam device emitting light with a wavelength of 343nm with four UV solid-state lasers. The four lasers were operated at a pulse repetition rate of 10 kHz. The pulse length, i.e. the half-width in time, of the transmitted pulse is between 15ns and 20 ns. The energy of the laser pulse is up to 20 mJ. The energy density on the substrate, i.e., on the silicon layer, was 220mJ/cm2. Similar to the method shown in fig. 5, a plurality of first and second laser pulses from first and second of the four lasers are each time synchronized by a synchronized trigger signal of the laser source such that each first laser pulse is emitted simultaneously with a second laser pulse. The plurality of third and fourth laser pulses of the third and fourth lasers are delayed by 10ns to 20ns with respect to the plurality of first and second laser pulses, respectively. The intensities of the four laser beams were set so that the ratio (M1/M2) of the first maximum value to the second maximum value of the combined time intensity profile was 1/1.
The laser pulses of the four laser beams are shaped into laser lines and imaged as illumination lines on amorphous silicon with a corresponding setup as shown in fig. 3a and 3 b. The irradiation line is moved relative to the semiconductor layer at a feed speed of 20mm/s, more precisely in the direction of the short axis of the irradiation line. The length of the irradiation line in the long axis direction was 90mm, and the uniformity was 1.5% (2 σ). The length of the irradiation line in the short axis direction was 67 μm, and the uniformity was 3% (2 σ). The combined time intensity profile has a total pulse length of 45ns (half-value width of the first maximum).
In a first experiment (experiment a), pulses 76, 80 of first and second laser beams 74, 78 were linearly polarized in the short axis direction, and pulses 86, 90 of delayed third and fourth laser beams 84, 88 were polarized in the long axis direction.
In the second experiment (experiment b), the pulses of the first and second laser beams were linearly polarized in the long axis direction, and the pulses of the delayed third and fourth laser beams were polarized in the short axis direction.
Fig. 8a shows an image of a silicon surface taken with a scanning electron microscope for the laser exposure according to experiment a), and fig. 8b shows an image of a silicon surface taken with a scanning electron microscope for the laser exposure according to experiment b). In both images, the feed direction is the x-axis direction (the minor axis direction of the illumination line), i.e. in the vertical direction with respect to fig. 8a and 8 b.
Fig. 8a shows that perpendicular to the feed direction, i.e. in the y-direction, there is a regular grain structure in the long axis direction. In particular, it can be seen that the grains are arranged in approximately equidistant vertically extending rows, i.e. corresponding to the wavelength of the UV laser, with a pitch of approximately 0.35 μm. Or in other words: the grain structure shows a stripe pattern extending in the feed direction, wherein the stripes are arranged at equal intervals and thus give uniformity in the long axis direction. The grain size has high uniformity in the long axis direction (y direction). There is a lower uniformity in the minor axis direction (x-direction) than in the major axis direction.
In contrast, fig. 8b shows that there is no significant uniformity in both the short axis direction (x-direction) and the long axis direction (y-direction). The grain structure appears disordered both in the orientation and in the size of the grains, compared to the grain structure shown in fig. 8 a.
As already described above, the laser crystallization process is based on partial melting of the amorphous silicon layer and subsequent solidification into a crystalline structure starting from unmelted solid silicon on the glass substrate. The time for melting and solidification ranges from 10ns to 100ns, and several hundred μ s is required for the film to subsequently cool to room temperature. A pulse repetition rate of 10kHz corresponds to a period of 100 mus. Since the pulse repetition rate, the feed speed and the width of the irradiation line in the feed direction are determined such that the position of the semiconductor material is exposed a plurality of times during the exposure, i.e. irradiated by a plurality of successive pulses, the period of which pulses corresponding to the pulse repetition rate is smaller than the time required for cooling the film to room temperature, the semiconductor material is repeatedly irradiated with UV light during the crystallization. Furthermore, the exposure by pulsing is relatively long due to the relatively long laser pulsing process of tens of ns. Such multiple exposures require the formation of a more uniform grain structure.
As already mentioned in the introduction, it is also known that the polarization of the laser, especially in combination with the above-mentioned multiple exposures, has a positive effect on the regular polycrystalline silicon grain structure. This results from surface interference effects ("laser induced periodic pattern structure", LIPSS), which result in a modulated intensity distribution. It has thus been pointed out that when light is linearly polarized in the major axis direction, a regular structure is formed in the major axis direction, and accordingly, when light is linearly polarized in the minor axis direction (feed direction), such an effect can be observed in the feed direction.
LIPPS is discussed in many publications, for example, in references (1) to (4) given below. It is assumed here that the modulated intensity distribution is generated by the interaction of the incident light beam with the light beam diffracted on the surface and in the direction of the surface and the resulting periodic distribution of the pulse energy density. The periodic pulse energy density distribution has the form of so-called "ripples", which have an absolute value of λ/(1 ± sin Θ) for a laser having a wavelength λ and an incident angle Θ. A spacing of the order of the wavelength λ is thus given for a perpendicular light incidence (Θ ═ 0 °). The "ripple" extends here in a direction perpendicular to the electric field vector, i.e. perpendicular to the polarization direction of the light beam or light pulse, and has a periodicity in the direction of the electric field vector. The pulse energy density along the "ripple" is minimal or maximal. The periodic pulse energy density results in a periodic temperature distribution over the space on the semiconductor material layer being exposed, where the periodic temperature distribution is similar to the periodic pulse energy density distribution. Thermal diffusion in the semiconductor material layer must also be taken into account for the periodic temperature distribution. It has also been found that the periodic distribution of the pulse energy density varies with the thickness of the layer of semiconductor material due to multiple reflections inside the layer of semiconductor material.
From the above, it can be determined that, from these observations, in order to obtain periodicity or regularity in the long axis direction, the electric field vector must be linearly polarized in the long axis direction, i.e., the light or light pulse must be linearly polarized in the long axis direction.
Accordingly, it has hitherto been assumed that each pulse must contain at least one component polarized in the long axis direction in order to be able to produce a regular grain structure in the long axis direction.
In accordance with the present disclosure, it has now been found particularly advantageous in terms of uniformity of the polycrystalline silicon grain structure to polarize the temporal first pulse 76 or temporal first plurality of pulses 76, 80 in the short axis direction, i.e., the feed direction, and to polarize the delayed second pulse 86 or delayed second plurality of pulses 86, 90 in the long axis direction.
A possible explanation here for promoting a regular grain structure when the second pulse is polarized in the long axis is that the light in this long axis generates an interference modulation (LIPPS) with the laser wavelength and thus supports a structured grain formation in this direction. If the second pulse is linearly polarized in the feed direction, no interferometric modulation is formed in the long axis.
An explanation for why the polarization of the light in the first maximum, i.e. the first pulse, in the short-axis direction (feed direction) is advantageous for the grain structure may be that during this period of irradiation the film is heated more uniformly in the long axis as if the polarization component in the long-axis direction had already been present, since no interferometric modulation takes place there, the first and second pulse components structurally producing part of the liquid phase and promoting the formation of the grain structure.
It was furthermore observed that, when the polarization of the first pulse in the feed direction, i.e. in the short axis direction, and the polarization of the delayed second pulse in the long axis direction were set according to experiment a), the size of the energy density processing window was enlarged to 20 to 25mJ/cm compared to an exposure in which the first pulse and the delayed second pulse had the same polarization distribution in both directions2(210 to 230mJ/cm for energy density treatment window2As said). For this case, the size of the energy density treatment window observed is only about 10mJ/cm2(215 to 225mJ/cm for energy density treatment window2As said).
Finally, it was determined that in "optical retardation" as shown, for example, in fig. 4, favorable results can be achieved compared to "electronic retardation" as shown in fig. 5 and 6. In particular, it was determined that the inhomogeneity in the crystal structure in the direction of movement ("shot Mura") can be reduced in "optical retardation" compared to "electronic retardation".
As mentioned above, when superimposing two (or more) temporally synchronized laser beams from two (or more) different laser sources, fluctuations in the intensity distribution may result due to the presence of temporal jitter between the laser sources.
When observing the combined temporal intensity profile 96 for two laser pulses 76, 86 and forming it for four laser pulses, as shown, for example, in fig. 7, the intensity profile 98 of the first laser pulse 76 is composed of the combined intensity profile 98 of the two first laser pulses 76, 80 and the intensity profile 100 of the second laser pulse 86 is composed of the combined intensity profile 100 of the two second laser pulses 86, 90. The change from the combined intensity profile 98 to the combined intensity profile 98 or from the combined intensity profile 100 to the combined intensity profile 100 can be minimized here by means of a pulsed laser source with as little time jitter as possible (for example in the ns range). Since the two first laser pulses 76, 80 or the two second laser pulses 86, 90 originate from different laser sources both in the case of devices with optical delay and in the case of devices with electronic delay, this "smearing" due to the existing time jitter does not lead to different situations for the electronic delay and the optical delay.
In this way, the combined time-intensity curve 96 in fig. 7 can be obtained by superimposing (combined) intensity curves 98, 100. As shown in fig. 7, the pulse width and delay are selected such that a preceding pulse 98 or preceding combination of pulses 98 is superimposed with a subsequent pulse 100 or subsequent combination of pulses 100. A first maximum M1 and a second maximum M2 are generated, the first maximum M1 being virtually unaffected by the delayed pulse 100 or the delayed pulse combination 100 (since the delayed pulse 100 or the delayed pulse combination 100 contributes little or no to the maximum M1 of the combined intensity profile 96), the position and behavior of the second maximum M2 being significantly affected by the delayed pulse 100 or the delayed pulse combination 100.
The time jitter of the changes that exist between the first pulse 98 and the time-delayed second pulse 100 is therefore reflected in the position and behavior of the change of the second maximum M2, in particular in the position and behavior from the combined intensity profile 96 to the maximum M2 of the combined intensity profile 96. The crystallization process reacts to these intensity variations over the combined time course with deviations in the grain structure, for example deviations in the grain size.
In contrast to the electronic delay, the optical delay is realized by beam splitting and setting the optical delay path (for example in combination with a pulsed laser source with a temporal jitter as small as possible) so that the preceding pulse 98 and the delayed pulse 100 have virtually the same temporal jitter. In this way, fluctuations in the intensity variations can be minimized and a uniform conversion of the amorphous semiconductor layer into the polycrystalline semiconductor layer can be achieved over a large area when pulses 98, 100 or pulse combinations 98, 100 are superimposed.
To achieve a typical 20ns time delay for the second pulse 100 relative to the first pulse 98, an additional optical path of about 6 meters is required. The additional optical path can be realized, for example, by means of a tele spherical telescope which is arranged in the delay path (for example in the beam path of the second partial beam 60 in fig. 4) and has conjugate planes at the entrance and exit of the delay path. In this way, the laser beam (sub-beam 60 in fig. 4) can be imaged in a controlled manner over a large path section. For 1: 1 imaging can for example be provided with a telescope, wherein the focal length of the objective lens is the same as the focal length of the eyepiece. Furthermore, the delay path can be designed such that the delay path (its length) is variable, for example by means of a movably arranged deflection element, such as a mirror. This makes it possible to set the pulse length which is optimal for the crystallization process.
Instead of a time-delayed laser beam or laser pulse by beam splitting and subsequent optical delay, it is also possible according to the present disclosure to employ a UV laser source, wherein two UV laser beams are obtained from an IR source by generating a third harmonic wavelength (343nm) from an IR laser beam (1030 nm). Such a UV laser source generates a second UV laser beam in a second SHG (second harmonic generation)/THG (third harmonic generation) crystal using unconverted IR pulse energy (typically 50%) in the first SHG/THG crystal. The two UV beams are not time-dithered with respect to each other. One of the two UV beams is then optically delayed as described above, for example by means of a tele spherical telescope. So that no beam splitting is necessary in this solution.
It is also shown that in the delayed pulse 100, the smaller the angular distribution of the laser light on the long axis, the more successful the formation of periodic structures on the long axis. It is believed that the reason for this is the dependence of the above explanation on the LIPPS effect of the periodic pulse energy density distribution (λ/(1 ± sin θ) where θ is the angle of incidence to the surface normal) due to the generated interference along the surface, the smaller the angular distribution of the incident light, the less its variation, i.e. the sharper it is. The beam in the delayed pulse is linearly polarized in the long axis direction. The retardation is preferably set for the laser beam imaged close to the optical axis. This means that for beams imaged away from the optical axis, mainly the beam of the preceding pulse with a polarization perpendicular to the long axis.
References (1) to (4):
(1) p.van der wild, "excipient-LASER Annealing: microscopic analysis and a Novel Characterization Technique, SID 2014Digest p194
(2) S.Horita, H.Kaki, K.Nishioka, "Surface modification of an amorphous Si thin film crystallized by a linear polarized Nd: YAG pulse laser beam (Surface modification of amorphous silicon thin film crystallized by a linear polarized Nd: YAG pulse laser beam)", Journal of Applied Physics 102,013501(2007)
(3) H.M van Driel, J.E.Sipe, and J.F.Young, "Laser-Induced Periodic Surface Structure on solid Surface: a common Phenomenon," Phys.Rev.Lett.49,1955-1958(1982) and S.E.Clark and D.C.Emmony, "Ultravitamin Laser-Induced Periodic Surface structures," Phys.Rev.B.40, 2031-2041(1989)
(4) J.F. Young, J.S. Preston, H.M.van Driel, and J.E.Sipe, "Laser-induced periodic surface structure.II.Experimental on Ge, Si, Al, and brass (Laser-induced periodic surface structure, II. experiments on Ge, Si, Al, and brass)", Phys.Rev.B 27, 1155-.

Claims (16)

1. Method for processing a layer of semiconductor material, in particular for producing a crystalline semiconductor layer, having the following steps:
-providing a first laser beam (74) with first laser pulses (76) and a second laser beam (84) with second laser pulses (86),
-shaping the first laser pulse (76) and the second laser pulse (86) into a line-shaped laser pulse having a short axis and a long axis by means of a beam shaping device (32),
-imaging the thus shaped line-shaped laser pulses on the semiconductor material layer (12) by means of an imaging device (34) as an illumination line (36) having a short axis and a long axis,
wherein, the method also comprises the following steps:
-setting the polarization direction of the first laser pulse (76) in the direction of the short axis of the illumination line (36),
-setting the polarization direction of the second laser pulse (86) in the direction of the long axis of the irradiation line (36), and
-time delaying the second laser pulse (86) with respect to the first laser pulse (76) by a predetermined time interval Δ t, the predetermined time interval Δ t being selected such that the illumination line (36) imaged on the layer of semiconductor material (12) has a pulse-shaped combined time-intensity profile (96), the combined time-intensity profile (96) having a first maximum (M1) and a second maximum (M2).
2. The method of claim 1, comprising
-moving the illumination line (36) in a feeding direction relative to the layer of semiconductor material (12), wherein the first laser light pulse (76) is linearly polarized in the feeding direction.
3. Method according to claim 1 or 2, wherein the relative intensities of the first laser pulse (76) and the second laser pulse (86) are selected such that the ratio of the first maximum (M1) to the second maximum (M2) of the combined time-intensity-variation curve (96) lies in the range of 0.8 to 1.4, in particular in the range of 0.9 to 1.2, in particular 1.0.
4. Method according to any of the preceding claims, wherein the combined time intensity profile (96) of the illumination lines (36) has a time half-value width (102) lying between 40ns and 50ns in relation to the first maximum of the combined time intensity profile.
5. The method of any of the preceding claims, comprising
-providing a first laser (66) and a second laser (70) configured to emit said first laser beam (74) and said second laser beam (84), respectively, and-delaying under control said second laser pulse (86) from said first laser pulse (76) by said time interval at.
6. The method of any one of claims 1 to 4, comprising
Providing a first laser, which is designed for providing a laser beam (52) having pulses,
-dividing the laser beam (52) into a first laser beam component (58) and a second laser beam component (60), wherein the first laser beam component (58) forms the first laser beam (74) with the first laser pulses (76) and the second laser beam component (60) forms the second laser beam (84) with the second laser pulses (86).
7. The method according to claim 6, wherein an optical path length of the second laser beam component (60) from a beam spot to the semiconductor material layer (12) is greater than an optical path length of the first laser beam component (58) from the beam spot to the semiconductor material layer (12).
8. The method of any of the above claims, wherein the first laser pulse (76) is one of a plurality of first laser pulses of the first laser beam (74) and the second laser pulse (86) is one of a plurality of second laser pulses of the second laser beam (84), and wherein each of the plurality of laser pulses of the second pulsed laser beam (84) is delayed in time by the predetermined time interval at relative to another of the plurality of laser pulses of the first pulsed laser beam (74).
9. The method of claim 8 as related to any of claims 2 to 7, wherein a feed rate, a pulse repetition rate of the first and second laser beams (74, 84), and a geometric half-value width of the illumination line (36) in a short axis direction are selected such that the position of the semiconductor material layer (12) is exposed by the illumination line (36) a plurality of times.
10. The method of any of the preceding claims, comprising
-providing a third laser beam (78) with third laser pulses (80) and a fourth laser beam (88) with fourth laser pulses (90),
-shaping the first laser pulse (76), the second laser pulse (86), the third laser pulse (80) and the fourth laser pulse (90) into line-shaped laser pulses with a short axis and a long axis by means of a beam shaping device (32),
-imaging the thus shaped line-shaped laser pulses on the semiconductor material layer (12) by means of the imaging device (34) as an illumination line (36) having a short axis and a long axis,
wherein, the method also comprises the following steps:
-setting the polarization direction of the third laser pulse (80) in the direction of the short axis of the illumination line (36),
-setting the polarization direction of the fourth laser pulse (90) in the direction of the long axis of the irradiation line (36), and
-time delaying the fourth laser pulse (90) with respect to the third laser pulse (80) by a predetermined time interval Δ t, the predetermined time interval Δ t being selected such that the illumination line (26) imaged on the layer of semiconductor material (12) has a pulse-shaped combined time-intensity profile (96), the combined time-intensity profile (96) having a first maximum (M1) and a second maximum (M2).
11. An optical system (30) for processing a layer of semiconductor material, in particular for producing a crystalline semiconductor layer, having:
-a beam shaping device (32) arranged for shaping a first laser pulse (76) of the first laser beam (74, 38) and a second laser pulse (86) of the second laser beam (84, 40) into a line-shaped laser pulse having a short axis and a long axis,
an imaging device (34) arranged for imaging the thus shaped line-shaped laser pulses on the semiconductor material layer (12) as an illumination line (36) having a short axis and a long axis,
wherein the optical system (30) further comprises:
-a polarizing arrangement (50) arranged and disposed for aligning a polarization direction of the first laser pulse (76) in a short axis direction of the irradiation line (36), for aligning a polarization direction of the second laser pulse (86) in a long axis direction of the irradiation line (36), and
-a delay device (94) arranged for delaying the second laser pulse (86) with respect to the first laser pulse (76) by a predetermined time interval Δ t selected such that the illumination line (36) imaged on the layer of semiconductor material (12) has a pulse-shaped combined time-intensity profile (96), which combined time-intensity profile (96) has a first maximum (M1) and a second maximum (M2).
12. The optical system (30) according to claim 11, wherein the polarizing means comprises:
a first lambda/2 plate (50) which is arranged in the beam path of the first laser beam (74, 38), in particular is arranged upstream of the beam shaping device (32), and is oriented relative to a first laser pulse (76) incident on the lambda/2 plate (50) in such a way that the first laser pulse (76) is linearly polarized in the direction of the short axis after passing through the lambda/2 plate (50), and
a second lambda/2 plate (50) which is arranged in the beam path of the second laser beam (84, 40), in particular is arranged upstream of the beam shaping device (32), and is oriented relative to a second laser pulse (86) incident on the lambda/2 plate (50) such that the second laser pulse (86) is linearly polarized in the direction of the long axis after passing through the lambda/2 plate (50).
13. The optical system (30) according to claim 11 or 12, wherein the delay device has a delay circuit (94) for setting a trigger signal (92) of the second laser (70) for emitting the second laser beam (84, 40) with the second laser pulse (86) for delaying a time interval Δ t with respect to a trigger signal (82) of the first laser (66), wherein the first laser (66) is arranged for emitting the first laser beam (74) with the first laser pulse (76).
14. The optical system (30) according to claim 11 or 12, wherein the delaying arrangement has a beam detour (Δ s) such that an optical path length of the second laser beam (84, 40) up to an imaging plane of the layer of semiconductor material (12) is larger than an optical path length of the first laser beam (74, 38) up to an imaging plane of the layer of semiconductor material (12).
15. The optical system (30) of any one of claims 11-14,
the beam shaping device (32) is provided for shaping a first laser pulse (76) of the first laser beam (74), a second laser pulse (86) of the second laser beam (84), a third laser pulse (80) of the third laser beam (78), and a fourth laser pulse (90) of the fourth laser beam (88) into a linear laser pulse having a short axis and a long axis,
the imaging device (34) is provided for imaging the linear laser pulses thus shaped on the semiconductor material layer (12) as an irradiation line (36) having a short axis and a long axis,
the polarization device (50) is provided and arranged for aligning the polarization direction of the third laser pulse (80) in the direction of the short axis of the irradiation line (36), for aligning the polarization direction of the fourth laser pulse (90) in the direction of the long axis of the irradiation line (36), and
the delay device (94) is provided for delaying the fourth laser pulse (90) by a predetermined time interval Δ t relative to the third laser pulse (80), which predetermined time interval Δ t is selected such that the irradiation line (36) imaged on the semiconductor material layer (12) has a pulse-shaped combined temporal intensity profile (96), which combined temporal intensity profile (96) has a first maximum (M1) and a second maximum (M2).
16. An apparatus for processing a layer (12) of semiconductor material, in particular for producing a crystalline semiconductor layer, comprises
The optical system (30) of any one of claims 11 to 15,
wherein the device is provided for moving the layer of semiconductor material (12) relative to the irradiation line (36) in a feed direction, wherein the feed direction corresponds to the direction of the short axis of the irradiation line (36).
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