KR100899321B1 - Laser thermal annealing of lightly doped silicon substrates - Google Patents

Abstract

The present invention relates to an apparatus and method for performing laser thermal annealing (LTA) of a substrate at room temperature using an anneal radiation beam that is not substantially absorbed by the substrate. The method takes advantage of the fact that for some substrates, such as undoped silicon substrates, the absorption of long wavelength radiation (greater than 1 micron) is a strong function of temperature. The method includes heating the substrate to a significant critical temperature at which absorption of the long wavelength anneal radiation is followed by irradiating the substrate with the anneal radiation to produce a temperature at which the substrate can be annealed.

Laser Thermal Annealing, Annealing Optical System, Heating Device

Description

LASER THERMAL ANNEALING OF LIGHTLY DOPED SILICON SUBSTRATES}

Cross Reference to Related Application

This application is related to US Patent Application No. 10 / 287,864, filed November 6, 2002.

Background of the Invention

FIELD OF THE INVENTION

FIELD OF THE INVENTION The present invention relates to laser thermal annealing and, more particularly, to an apparatus and method for performing laser thermal annealing of a substrate that does not efficiently absorb annealing radiation beams at ambient temperature.

Description of the prior art

Laser thermal annealing or LTA (or referred to as “laser thermal processing”) is a technique used to rapidly raise and lower the temperature of the surface of a substrate to cause a change in properties. One embodiment may include annealing and / or activating dopants of a source, drain or gate region of a transistor used to form an integrated device or circuit. In addition, LTAs can cause chemical reactions to form silicide regions in integrated devices or circuits, to reduce poly-silicon runner resistances, or to form or remove material from substrates (or wafers). Can be used for

LTA offers the possibility of accelerating the annealing cycle by a factor of 1000 compared to conventional annealing techniques, thereby substantially eliminating the diffusion of dopant impurities during the annealing or activation cycle used for the silicon wafer. The result is a steeper dopant profile, in some cases, a higher level of activation. This results in a higher performance (eg faster) integrated circuit.

US Patent Application No. 10 / 287,864 discloses performing LTA of a doped silicon substrate using CO ₂ laser radiation. The laser radiation is focused in narrow lines and is scanned at a constant velocity in a raster pattern across the substrate. However, this approach only works well with relatively high concentration doped substrates (i.e. dopant concentrations of about 3x10 ¹⁷ atoms / cm ³ or more) whose absorption length of laser radiation in doped silicon is less than or approximately equal to the heat diffusion length. . Conversely, for lightly doped substrates (ie dopant concentrations of about 1 × 10 ¹⁶ atoms / cm ³ or less), the CO ₂ laser radiation passes through the substrate without adding significant energy to the substrate.

Therefore, there is a need for a method of efficiently performing LTA of a lightly doped silicon substrate using radiation that passes through the substrate in another way without heating, such as CO ₂ laser radiation having a wavelength of 10.6 μm.

Summary of the Invention

A first aspect of the invention is an apparatus for performing laser thermal annealing of a substrate having one surface. The apparatus includes a laser capable of producing continuous annealing radiation having a wavelength at room temperature that is not substantially absorbed by the substrate. The apparatus also includes an anneal optical system for receiving the anneal radiation and forming an anneal radiation beam that forms a first image on the substrate surface, the first image being scanned across the substrate surface. The apparatus further includes a heating device that heats at least a portion of the substrate to a critical temperature such that the anneal radiation beam incident into the heated portion is substantially absorbed by the surface of the substrate during scanning. In one exemplary embodiment, heating of a portion of the substrate may be done using a short-wavelength laser diode beam just before the long-wave anneal beam.

A second aspect of the invention is a method of laser thermal annealing a substrate. The method comprises, at room temperature, providing an anneal radiation beam from a laser having a wavelength that is not substantially absorbed by the substrate, and heating at least a portion of the substrate to a threshold temperature so as to provide an annealing radiation beam at the portion where the anneal radiation beam is heated. And substantially absorbed in the vicinity of the surface. The method also includes initiating a self-sustaining annealing condition by heating a portion of the surface of the substrate, just prior to scanning the annealing radiation beam across the substrate.

Brief description of the drawings

1A is a cross-sectional view of one exemplary embodiment of an LTA device of the present invention that includes an LTA optical system and a silicon substrate to be processed by the system, the LTA device being a heating chuck that supports and preheats the substrate, and the rest of the device; An optional heat shield surrounding the chuck to reduce radiation coupling and promote efficient substrate heating.

FIG. 1B is a cross-sectional view of one embodiment of an LTA device of the present invention similar to that shown in FIG. 1A including a heating enclosure surrounding the substrate to preheat the substrate.

FIG. 1C is a cross-sectional view of one embodiment of an LTA apparatus of the present invention similar to that shown in FIG. 1A, wherein the heating chuck and optional heat shield are replaced with an optical heating system that preheats at least a portion of the substrate using a preheated radiation beam. do.

FIG. 2 shows the absorption path length L _A (μm) versus the substrate temperature T _S for the 10.6 μm wavelength annealing radiation beam in an undoped silicon substrate. Plot of diffusion length L _D versus substrate temperature T _S (° C.) associated with a radiation beam having a dwell time of 200 μs.

3 is a computer diagram of a substrate temperature profile as a function of depth (μm) and annealing radiation beam position (μm) showing the “hot spots” formed in a substrate by an anneal radiation beam associated with self-maintaining annealing conditions. It is a simulation.

4A is a schematic diagram illustrating one exemplary embodiment of the relative intensity and beam profile of a preheating and annealing radiation beam as a function of position on a substrate surface.

4B is an enlarged cross-sectional view of the substrate showing how heat from the preheated radiation beam imaged in front of the anneal radiation beam facilitates absorption of the anneal beam on the substrate to achieve self-maintaining annealing conditions.

5 shows the maximum substrate temperature T _MAX (° C.) versus the incident power P _I (W / cm) of the anneal radiation beam generated by irradiating a heavily doped silicon substrate with an anneal radiation beam having a wavelength of 10.6 μm. A plot is shown.

FIG. 6 shows a plot of the maximum substrate temperature T _MAX (° C.) as a function of initial substrate temperature T _I for the different incident power P _I of the anneal radiation beam, for an undoped substrate obtained by finite element simulation. One drawing.

FIG. 7 is a plot of the absorption length L _A (μm) of a 780 nm preheated radiation beam in silicon as a function of substrate temperature T _S (° C.).

8A is a cross-sectional view of one embodiment of the optical relay system of FIG. 1C as viewed from the Y-Z plane.

FIG. 8B is a cross-sectional view of an embodiment of the optical relay system of FIGS. 1C and 8A, as viewed from the X-Z plane.

9A is an enlarged cross-sectional view in the X-Z plane of the heating radiation source and the cylindrical lens array.

9B is an enlarged cross-sectional view in the Y-Z plane of the heating radiation source and the cylindrical lens array.

10A is an enlarged schematic view of the preheat radiation source, the relay lens and the preheat radiation beam at a normal angle of incidence to the substrate, and polarizers arranged in the preheat radiation beam to reduce the amount of preheat radiation reflected from the substrate and returned to the preheat radiation source. And a quarter wave plate.

10B is an enlarged schematic diagram of the preheat radiation beam at a normal angle of incidence to the preheat radiation source, the relay lens and the substrate, and polarizers arranged in the preheat radiation beam to reduce the amount of preheat radiation scattered from the substrate and returned to the preheat radiation source. And a Faraday rotator

The various components shown in the drawings are not necessarily drawn to scale, but merely contemplated. Certain parts of the component may be exaggerated, while others may be reduced. The drawings are intended to illustrate various implementations of the invention that can be understood and appropriately carried out by those skilled in the art.

Detailed description of the invention

TECHNICAL FIELD The present invention relates to laser thermal annealing (LTA) of substrates, and more particularly, to apparatus and methods for performing LTA of lightly doped silicon wafers (substrates). Here, "low concentration doping" means a dopant concentration of 10 ¹⁶ atoms / cm ³ or less. Dopant concentration in the substrate may be associated with a vertical substrate product to achieve the desired resistance level and substrate type (N-type or P-type).

In the following description, a generalized embodiment of the LTA apparatus of the present invention is described with the description of the "self-maintaining annealing conditions" required to be produced by the present invention. This is understood by various exemplary embodiments of the present invention. In addition, the present invention is further described in connection with a number of different substrate temperature plots showing the important properties of absorption of radiation by a silicon substrate. Also, a method of determining an appropriate power level of a preheat radiation beam is described according to an example of a heating lens used in one exemplary embodiment of heating a substrate with a preheat radiation beam. In addition, preferred scanning and orientation of the preheating and annealing radiation beams are described in detail.

I. Generalized LTA  Device

1A is a cross-sectional view of one embodiment of the LTA apparatus 8 of the present invention, with the substrate 10 to be annealed. Substrate 10 is a " undoped " or, strictly speaking, a lighter doped body (bulk) region (typically less than very small junction regions or devices that typically contain very high doping levels only in extremely shallow regions). 16) and an upper surface 12. The reference letter N represents a vertical line with respect to the upper surface 12 of the substrate. In one exemplary embodiment, the substrate 10 is a silicon wafer.

The LTA apparatus 8 comprises an LTA optical system 25 having an annealing radiation source 26 and an LTA lens 27 arranged along the optical axis A1. Lens 27 receives a continuous (ie non-pulse) annealing radiation 18 from annealing radiation source 26 and forms a continuous image 30 (eg, a line image) on substrate surface 12. Produces an annealing radiation beam 20. The annealing radiation beam 20 is incident on the upper surface 12 at the incident angle θ ₂₀ measured with respect to the surface vertical line N and the optical axis A1.

Arrow 22 represents one embodiment of the direction of movement of the anneal radiation beam 20 with respect to the substrate surface 12. In turn by a movable stage MS operatively connected to a stage driver 29 which causes the stage (and, here, the substrate) to move at a selected speed and direction relative to the annealing radiation beam 20 or some other reference. The substrate 10 is supported by the supported chuck 28. The scanning movement of the movable stage MS is shown by the arrow 22 '. In one exemplary embodiment, the stage MS can move in at least two dimensions.

In one exemplary embodiment, the LTA apparatus 8 includes a reflected radiation monitor M1 and a temperature monitor M2. As indicated by the radiation 20R, the reflected radiation monitor M1 is arranged to receive the radiation reflected from the substrate surface 12. The temperature monitor M2 is arranged to measure the temperature of the substrate surface 12, and in one exemplary embodiment, the substrate is at a vertical angle of incidence at or near where the image 30 is formed by the anneal radiation beam 20. It is arranged along the surface vertical line (N) to examine the. The monitors M1 and M2 are controlled by a controller (directly) to provide feedback control based on the measurement of the amount of reflected radiation 20R and / or the measurement temperature of the substrate 12, as described in more detail below. (Described below).

In one exemplary embodiment, LTA apparatus 8 is included in annealing radiation source 26, stage driver 29, monitors M1 and M2, as well as lens 27 used as an incident power monitor. It further includes a controller 32 operatively connected to the optional monitor M3. The controller 32 is, for example, a microprocessor or microcontroller coupled to a memory, a programmable logic array (PLA), a field-programmable logic array (FPLA), a programming array logic (PAL) or other control device. (Not shown). The controller 32 has two modes, namely: 1) an open-loop that maintains a constant power delivered to the substrate 10 by the annealing radiation beam 20 with a constant scan rate through the stage driver 29; And 2) a closed-loop that maintains a constant maximum temperature of the substrate surface 12 or a constant power absorbed by the substrate. The maximum substrate temperature varies directly with the absorbed power and inversely with the square root of the scan rate.

In one exemplary embodiment, closed loop control is used to maintain a constant ratio of absorbed power in the anneal radiation beam 20 incident on the substrate relative to the square root of the scan rate, ie, P ₂₀ is the anneal radiation. The amount of power in beam 20, and when P ₃₀ is the reflected power, the absorbed power is P _a = P ₂₀ − P ₃₀ . When V is the scan rate of the substrate 10 with respect to the anneal radiation beam, the ratio P _a / V ^1/2 is kept constant in order to maintain a constant temperature indirectly.

For closed loop operation based on direct maximum temperature measurement, the controller 32 receives a signal (e.g., an electrical signal), such as the maximum substrate temperature, from the temperature monitor M2 via the signal S2, and the constant maximum Control the incident power or scan rate to maintain the substrate temperature. The absorbed power (P _a), the input power (P _I) in, generated by the reflected radiation monitor (M1) of a through signal (S4) obtained by sampling a part of the annealing radiation beam annealing radiation beam 20 Obtained by subtracting the power P ₃₀ in the annealing radiation beam 20R reflected through the received signal S1.

In addition, the controller 32 will calculate the parameters and input parameters (eg, the desired absorbed power level and dwell time) based on the received signal. In addition, the controller 32 is coupled to receive the external signal S3 from the actuator or from a master controller (not shown) that is part of a larger assembly or processing tool. This parameter indicates a predetermined dose (amount) or desired maximum substrate temperature of the anneal radiation 20 to be supplied for processing the substrate. The parameter signal may also indicate the intensity, scan rate, scan speed, and / or number of scans to be used to deliver a predetermined dose of anneal radiation 20 to the substrate 10.

In one exemplary embodiment, the anneal radiation source 26 is a CO ₂ laser, so the anneal radiation beam 20 has a wavelength of 10.6 μm. Typically, however, the anneal radiation source 26 is any continuous radiation source that emits radiation having a wavelength at room temperature that is substantially not absorbed by the substrate, but the substrate, or a sufficient portion of the substrate, is higher. When present at a temperature, it is substantially absorbed by the same substrate. In a preferred embodiment, the anneal radiation source 26 is a laser.

The LTA apparatus 8 utilizes the absorption of the annealing radiation beam 20 near the top of the substrate to efficiently raise the temperature of the top of the substrate while maintaining the temperature of the body of the substrate which is substantially unchanged. That is, when the substrate is a semiconductor wafer, the present invention is directed to increasing the temperature of the wafer on or near the surface on which the device (e.g., transistor) is formed, rather than heating the wafer body.

However, at ambient temperatures, lightly doped and undoped substrates are difficult to anneal because the long wavelength radiation beam passes directly through the substrate without being heated enough to detect the top surface. On the other hand, the highly doped substrate is not difficult to anneal since the incident anneal radiation is first absorbed by the material about 100 microns and raises its temperature to the desired annealing temperature.

The body (bulk) 16 of the substrate 10 that has not absorbed radiation enough to be detectable from the beam and rapidly heats the top region when the anneal radiation beam 20 is no longer applied to the substrate. . The present invention takes advantage of the fact that the absorption of radiation in lightly doped silicon at a constant infrared wavelength, such as a CO ₂ laser wavelength of 10.6 μm, is strongly dependent on the substrate temperature. When significant absorption of the anneal radiation beam 20 occurs, the substrate surface temperature increases to cause stronger absorption, which in turn results in stronger heating of the substrate surface and the like.

II. Self-maintenance Annealing  Condition

2 is a plot of substrate temperature T _S (° C.) versus absorption length L _A (μm) (vertical axis) versus 10.6 μm wavelength radiation in a silicon substrate. Also included as a function of the substrate temperature T _S are the points for the diffusion length L _D (μm) for 200 μs dwell time. The absorption length L _A is the thickness that reduces the intensity of the anneal radiation beam 20 by 1 / e. The thermal diffusion length L _D is the depth at which the instantaneous surface temperature rise propagates into the material after a constant dwell time ���. L _A and L _D have values of approximately the same ˜60 μm at temperatures T _S to 600 ° C.

The strong fluctuations in the absorption path length L _A with the substrate temperature T _S are two possible steady-state conditions, namely: (1) the annealing radiation beam 20 is substantially absorbed and the substrate Or (2) the annealing radiation beam when the anneal radiation beam 20 is substantially absorbed near the substrate surface 12 so that it moves over (i.e., is scanned on) the substrate surface. Create a substrate surface corresponding to image 30 moving along 20) and a condition causing a "hot spot" just below the substrate surface.

3 is a computer simulation of the substrate temperature (° C.) profile as a function of depth (μm) and annealing radiation beam position (μm). The temperature profile is a hot spot (indicated by 31) that moves within the substrate and across the substrate surface 12. The moving hot spot 31 is used to preheat the area of the substrate 10 in front of the advance image 30 by thermal diffusion (see FIG. 4B to be described later). Substrate preheating associated with propagation of the hot spot 31 allows the radiation of the annealed radiation beam 20 to be efficiently absorbed near the upper surface 12 when the beam is scanned across the substrate surface front. Steady state condition (2) is sought to be produced using the apparatus (8) of the present invention and the appended method, here referred to as "self-maintaining annealing condition".

A general method of generating a self-maintaining annealing condition according to the present invention is that the substrate 10 (or the substrate 10) is adapted such that the annealing radiation beam 20 is substantially absorbed by the substrate, i. Selecting an area or portion of the substrate) to a critical temperature T _C (eg, 350 ° C. or higher, if described in greater detail).

The exact T _C value depends on the temperature distribution in the substrate, the dopant concentration of the substrate, and the annealing radiation beam intensity. Thus, in an exemplary embodiment, the threshold value T _C is determined by experiment. This may include, for example, measuring the highest temperature produced by the annealing radiation beam of the test substrate having one of various initial or constant initial temperature conditions and various annealing and preheating radiation beam intensities. Preheating of the substrate to result in self-maintaining annealing conditions can be achieved in a number of ways. For an LTA apparatus 8 comprising a heating device for heating the substrate 10 to carry out a method of creating a self-maintaining annealing condition in a lightly doped silicon substrate 10 for the purpose of performing the LTA. Some exemplary embodiments are described below.

III. Heating Chuck Embodiment with Selective Heat Shield

Referring again to FIG. 1A, in one exemplary embodiment, the chuck 28 includes a heating element 50 that is thermally conductive and connected to a power source 52 that is controlled by the controller 32 and connected in turn. . The thermal insulation layer 53 surrounds the bottom and the end of the chuck 28 to limit the unwanted heating of the stage and heat loss from the chuck.

In operation, the controller 32 activates a power source 52 that in turn supplies power to the heating element 50. In response, the heating element 50 generates heat 56. In one exemplary embodiment, the amount of heat generated 56 is controlled by the temperature sensor 57 of the chuck and is operatively connected to a power source 52 (or otherwise to the controller 32) so that the chuck temperature is It is limited to a constant, predetermined, maximum value. When the substrate is loaded onto the chuck, the temperature of the substrate rapidly reaches the same temperature as the chuck. Usually, the chuck temperature T _CH is about 400 ° C.

Also in another exemplary embodiment, the apparatus 8 optionally includes a heat shield 62 supported over the substrate 12 to reflect the heat 56 back to the substrate. This results in less heating of the device components on opposite sides of the more uniform substrate heating and shielding. In one exemplary embodiment, the heat shield 62 is a glass plate coated with gold. The heat shield 62 includes an opening 64 that allows the anneal radiation beam 20 to reach the surface 12 of the substrate 10.

Ⅳ. Heated Enclosure  ( enclosure ) Embodiment

Referring to FIG. 1B, in another exemplary embodiment, the apparatus 8 includes an interior region 82 large enough to enclose both the substrate 10 and the chuck 28 or the substrate, the chuck and the stage MS. With a heated enclosure 80 (eg, an oven). Enclosure 80 includes additional heating elements 50 (preferably other than those included in chuck 28) that are connected to power source 52. The power source 52 is connected to the controller 32. In one exemplary embodiment, the enclosure 80 includes a window or opening 84 that causes the anneal radiation beam 20 to reach the surface 12 of the substrate 10. The thermal insulation layer 53 described above in connection with FIG. 1A is preferably provided at the end and bottom of the chuck to limit unwanted heat loss from the chuck to the stage.

In operation, the controller 32 activates a power source 52 that in turn supplies power to the heating element 50. In response, the heating element 50 generates heat 56 and, as a result, raises the temperature of the chuck and the substrate to approximate a maximum critical temperature T _C of about 400 ° C. Preferably, enclosure 80 is thermally insulated such that heat 56 remains trapped within inner region 82, thereby promoting efficient and uniform heating of the substrate.

Ⅴ. Preheated Radiation Beam Embodiment

Referring to FIG. 1C, in another exemplary embodiment, the apparatus 8 includes a preheating optical relay system 140 having a preheating radiation source 142 and a relay lens 143 arranged along the optical axis A2. Include. The preheat radiation source is to emit radiation 147 applied to the relay lens 145 into the preheat radiation beam 150 used to heat the substrate just before it is heated by the anneal radiation beam. The radiation 147 has a wavelength that is easily (substantially) absorbed by 100 μm or less of silicon. In one exemplary embodiment, the preheated radiation source 142 is a laser diode array that emits preheated radiation 147 having a wavelength of 0.8 μm (800 nm) or 0.78 μm (780 nm). An example of the relay lens 143 is described below. The preheated radiation source 142 and the relay lens 143 are operatively connected to the controller 43 together with the monitors M1 and M2 shown in FIG. 1A, although not shown in FIG. 1C for ease of explanation.

In operation, the preheated radiation source 142 emits radiation 147 received by the relay lens 143. The relay lens 143 creates a preheated radiation beam 150 that forms an image 160 (eg, a line image) on the substrate surface 12. The preheated radiation beam 150 is incident on the substrate surface 12 at an incident angle θ ₁₅₀ measured with respect to the substrate surface vertical line N.

In one exemplary embodiment, as shown in FIG. 1C, the image 30 formed by the anneal radiation beam 20 and the image 160 formed by the preheat radiation beam 150 are on the substrate surface 12. Located side by side. Thus, the preheat radiation beam 150 operates to locally preheat a portion or area of the substrate immediately in front of the portion irradiated by the anneal radiation beam 20. Arrow 22 ′, in the exemplary embodiment, moves relative to the fixed radiation beams 20 and 150 (or, equivalently, the fixed images 30 and 160) to achieve scanning of these beams (or images). The movement of the substrate 10 (eg, via the moving chuck 28; see FIG. 1) will be described.

In another exemplary embodiment, the preheated radiation beam 150 and the annealed radiation beam 20 are, for example, 1 / e ² intensity contours of each beam intensity profile, as shown in FIG. 4A. Partially overlapped

4B is an enlarged cross-sectional view of one exemplary embodiment of a substrate irradiated by beams 20 and 150. 4B shows how heat 166 from the preheated radiation beam 150 imaged in front of the anneal radiation beam 20 facilitates absorption of the anneal beam near the top surface of the substrate. Heat 166 from the preheated radiation beam 150 is diffused to the substrate 10 in front of the anneal radiation beam 20. When the beam of radiation moves with respect to the substrate, as indicated by arrow 22 ', the annealed radiation beam 20 passes through a preheated area (ie, substrate portion) by the preheated radiation beam 150. This process is used to raise the temperature of the substrate above the threshold temperature T _C at and near the substrate surface. This allows the anneal radiation beam 20 to be efficiently absorbed into the substrate, as indicated by the absorbed anneal radiation beam 20 '(dashed line). The relatively fast absorption of the anneal radiation beam 20 ′ in the substrate 10 near the substrate surface 12 results in annealing temperature T _A at a maximum at the trailing edge of the anneal radiation beam. For example, to rise rapidly up to about 1600 K). This induces annealing of selected regions formed in the substrate, for example by activating dopants implanted into the top surface of the substrate.

Ⅵ. Substrate Temperature Plot

5 is a plot of the maximum substrate temperature T _MAX (° C.) produced by irradiating a heavily doped silicon substrate with 10.6 μm radiation as a function of the incident power P _I (W / cm) of radiation. To derive this data, a two-dimensional, finite element simulation program was used. The simulation showed an infinitely long anneal radiation beam. Therefore, the beam power is measured in Watts / cm rather than Watts / cm ² . In addition, the simulation showing the annealed radiation beam 20 had a Gaussian beam profile with a full-width half-maximum (FWHM) of 120 μm and was scanned across the substrate top surface 12 at a rate of 600 mm / s, yielding 200 Hz. Dwell time of was generated. Here, the “dwell time” is the length of the time image 30 formed by the anneal radiation beam 20 present over a particular point on the substrate surface 12. In this case, the plot shows an approximately linear relationship between the input power (P _I) and the maximum substrate temperature (T _MAX). Since the two-dimensional model indicated that the anneal radiation beam 20 was infinitely long, there was no additional energy loss at the end of the line image 30. The finite beam length did not cause some additional heat loss at the end of the beam, thereby causing a lower maximum temperature for a given incident power level P _I.

FIG. 5 shows that for an absorbed (ie heavily doped) substrate, an incident power P ₁ of about 500 W / cm leads the maximum substrate surface temperature T _MAX from ambient to 427 ° C. for a particular set of conditions. Indicates what is required to do. This may be compared to about 1150 W / cm to take the temperature up to the melting point of silicon at 1410 ° C. for the same set of conditions.

The relationship shown in FIG. 5 is a good approximation for the preheat radiation beam 150 having the same width and dwell time as the annealing radiation beam 20. Heat diffusion is the main mechanism for distributing heat in the substrate in both cases. The peak substrate temperature T _{MAX of} 400 ° C. is equal to the uniform substrate temperature T _S of 400 ° C., since the previous temperature distribution descends around the substrate at a distance approximately equal to the heat diffusion length L _D. Absorption of the annealing radiation beam 20 hardly occurs.

FIG. 6 shows the maximum substrate temperature T _MAX as a function of the initial substrate temperature T _I for two different incident powers P _I of the anneal radiation beam 20 having a wavelength of 10.6 μm for the undoped silicon substrate. (° C). It is also derived from a two-dimensional finite element model. For about 327 ° C. or less, the angle of incidence has little effect, and the maximum temperature T _MAX is almost equal to the initial substrate temperature T _I. That is, the annealed radiation beam 20 passes through the substrate 10 and does not heat to such an extent that the substrate can be sensed. However, at an initial substrate temperature (T _I ) between 377 ° C. and 477 ° C., depending on the amount of incident power P _I of the anneal radiation beam, a detectable absorption of the anneal radiation beam 20 occurs. . The result is a sharp increase in maximum substrate temperature T _MAX . When high-absorption, high temperature transitions occur, additional irradiation by the anneal radiation beam 20 increases the maximum temperature T _MAX linearly.

It was found that the units of power used in the plots of FIGS. 5 and 6 are watts per centimeter (W / cm). This power represents power per unit length of the scanning image 30 (e.g., line image) contained between half power points. Thus, the power of 1150 W / cm of the image 30 having a width of 120 μm corresponds to an average intensity of 95,833 W / cm ² .

In order to create a self-maintaining annealing condition, the temperature that must be generated by the preheated radiation beam 142 to heat the substrate to the threshold temperature T _C can be estimated from the information in the plot of FIG. 6. The plot in FIG. 6 indicates that when the substrate reaches a uniform temperature (T _I ) of approximately 427 ° C., there is a steep increase in substrate temperature (T _MAX ) indicating the onset of self-maintaining annealing conditions. If a laser diode source is used to provide the necessary preheating, considerably higher temperatures will be expected since the diode source produces a nonuniform temperature distribution that falls around at about one heat spreading length.

7 is a plot of the absorption length L _A (μm) of 780 nm radiation in undoped silicon as a function of substrate temperature T _S (° C.). Absorption characteristics at 800 nm are very similar to those at 780 nm. As can be seen from the plot, even at room temperature, the absorption length (L _A ) is about 10 μm, short enough to ensure a temperature distribution preferentially determined by efficient heating of the substrate surface area and thermal diffusion over a time scale of at least 200 Hz. to be.

CO ₂ laser beam (such as annealing radiation beam 20) in an undoped silicon substrate having a non-uniform temperature distribution, such as produced by a laser diode source (used to produce the preheated radiation beam 150). In order to obtain an efficient absorption of, a temperature corresponding to an absorption length of about 100 μm is estimated. This is achieved with a peak substrate temperature (T _MAX ) of about 550 ° C. Referring again to FIG. 5, the maximum substrate temperature T _MAX of 550 ° C. requires a preheated radiation beam 150 to have a power of about 600 W / cm (50,000 W / cm ² ).

Iii. Determine the preheated radiation beam power

In operation, it is a simple matter to determine the minimum power of the preheated radiation beam 150 necessary to achieve efficient coupling of the annealed radiation beam 20 to the substrate 10. In one exemplary embodiment, using an anneal radiation beam 20 set to a power level sufficient to anneal the absorbing substrate, at room temperature, the substrate that is not substantially absorbed at the wavelength of the anneal radiation beam 20 is a preheated radiation beam. 150 and the annealing radiation beam 20 are irradiated. The power level of the preheat radiation beam 150 is increased until the annealing temperature is detected at the substrate. This may be achieved, for example, by measuring the substrate temperature with the temperature monitor M2 shown in FIG. 1A.

The transition from slight or non-coupling of the substrate to the annealing radiation beam to efficient coupling at the substrate surface is usually very steep. If the substrate temperature T _S is too low, neither the transition to the annealing temperature nor the steep transition to the melting point of the substrate will occur. In addition, when the substrate temperature rises, there is a narrow range of annealing power levels that allow stable operation below the melting temperature. An additional increase in substrate temperature increases the range of annealing power levels and the corresponding range of annealing temperatures. Thus, it is not possible to initiate an absorption transition of the annealing radiation beam 20 in the substrate or otherwise define sharply the power level of the preheating radiation beam 150 causing an annealing temperature in the substrate. However, there is a minimum practical power level at which the desired range of annealing temperatures cannot be reliably achieved. In one exemplary embodiment, the preheated radiation beam 150 is at a power level slightly above this minimum level required to ensure that the anneal radiation beam is efficiently absorbed by the substrate and a large range of annealing temperatures is readily utilized. Is set.

In one exemplary embodiment, the self-the amount of power of the preheating radiation beam 150 needs to initiate a holding annealing conditions (P _I), is the amount required to produce the maximum substrate temperature (T _MAX) of 550 ℃. Assuming a 200 watt dwell time, the graph of FIG. 5 shows that this corresponds to an incident power of about 600 W / cm. However, obtaining an intensity of 600 W / cm of the preheated radiation beam 150 producing an image 160 having a width comparable to that of the annealed radiation beam image 30 is not as easy as initially shown. In one exemplary embodiment, the preheated radiation beam 150 preferably has an angle of incidence θ ₁₅₀ at or near the Brewster angle for silicon that is about 75 ° C. This angle minimizes reflected radiation and equalizes the energy absorbed by the substrate for the type of structure that will be present on the substrate. At an angle of incidence θ ₁₅₀ of about 75 ° C., the preheated radiation beam 150 is damaged at the substrate surface 12, covering an increased area by a factor of about 4, and the intensity decreases proportionally.

The total power in the preheat radiation beam 150 can be increased, for example, by making the preheat source larger, for example by adding an additional row of laser diodes. However, this proportionally increases the width of the preheated radiation beam 150. Increased preheat radiation beam width increases the dwell time and heat spreading depth, and also increases the power required to achieve the desired maximum temperature. Thus, the relay lens 143 needs to be designed because it is possible to provide a preheating radiation beam 150 with sufficient intensity to heat the substrate to within the critical temperature range using the available preheating radiation source 142. An example of such a relay according to the present invention is described below.

Iii. Example Embodiments of Optical Relay Systems

8A and 8B are respective cross-sectional views of one exemplary embodiment of the optical relay system 140 and the substrate 10. 8A is a view in the Y-Z plane, and FIG. 8B is a view in the X-Z plane. 8A and 8B, the relay is divided into two parts for fixing on the page, and the lens element having the surfaces S13 and S14 is shown in two parts.

In an exemplary embodiment, the preheated radiation source 142 is 95054 Coherent Semiconductor Group of 5100 Patrick Henry Drive, Santa Clara, CA. And a two-dimensional laser diode array such as a LightStack ^{™ 7} × 1 / L PV array available from. The LightStack ^™ array includes seven rows of water-cooled laser diodes, each 10 mm long and stacked at 1.9 mm intervals. Each diode's row can emit 80 watts of optical power. The relay lens 143 has a target plane OP (in which the preheated radiation beam 142 is arranged), an image plane IP (in which the substrate 10 is arranged), and an optical axis connecting the image plane and the target plane ( A2).

In one exemplary embodiment and as described above, the relay lens 143 is configured to generate a preheated radiation beam 150 that forms an image 160 (eg, a line image) scanned across the substrate 10. Is designed. Scanning of image 160 may be accomplished in any of a number of ways, such as moving chuck 28 (via movable stage MS) relative to relay lens 143 (FIG. 1). Locally irradiating the substrate 10 with the image 160 is immediately easier to achieve the high beam intensity needed to heat the substrate over a relatively small image area, thereby immediately irradiating the entire substrate. It is preferable. Thus, the local preheating provided by the relay lens 143 should have concurrency with irradiating the substrate with the anneal radiation beam 20.

Since the radiating characteristics of the laser diode are anisotropic and the spacing between adjacent diodes is very different in the X and Y planes, the relay lens 143 is used to produce an anamorphic (< / RTI > need to be anamorphic). In addition, a relatively high number of openings are required in the image plane IP in order to achieve the required intensity in the image 160 in the substrate 10.

Thus, referring again to FIGS. 9A and 9B, the relay lens 143 sequentially configures the laser diode 198 constituting the preheat radiation source 142 from the preheat radiation source 142 and along the optical axis A2. And a cylindrical lens array 200 having lenslets 201 corresponding to the number of rows of. The cylindrical lens array 200 has a power in the XZ plane, with the radiation angle 10 ° in the YZ plane (FIG. 9B), with each preheated radiation emitted from the radiation source 142 in the XZ plane. It operates to parallel the beam 147 (FIG. 9A). The combination of the diode array and the cylindrical lens array is used as an input to the anamorphic relay, which combines to reimage the cylindrical lens array on the substrate.

Table 1 describes lens design data for one exemplary embodiment of relay lens 143, as shown in FIGS. 8A and 8B.

Referring again to FIGS. 8A and 8B, the relay lens 143 is composed of two imaging sub-relays R-1 and R-2 in series with a common intermediate image plane IM. Sub-relay R1 is an anamorphic relay using mainly cylindrical lens elements with substantially different power in the YZ and XZ planes, while sub relay R-2 uses a spherical element and has a reversed ratio of 1: 6. It is a conventional relay having a demagnification ratio. The anamorphic relay R-1 has a magnification ratio of 1: 1 in the Y-Z plane and a reverse magnification ratio of 1:10 in the X-Z plane. The relay lens 143 is telecentric in the object plane (OP) and the image focal plane (IP).

The telecentricity in both the object plane (OP) and the image plane (IP) is the spherical field lens 202 (surfaces s1 and s2) and the cylindrical lens 204 arranged immediately adjacent to the preheated radiation source 142. Surface s3 and s4). The cylindrical lens 204 has power only in the Y-Z plane and forms a pupil image in the Y-Z plane at s5. Next, there are two cylindrical lenses 206 and 208 (surfaces s6 to s9) with power in the Y-Z plane that re-images the diode array 1: 1 to the intermediate image plane. Surface s10 identifies the pupil plane in the X-Z plane. They also follow a pair of cylindrical lenses 210 and 212 (surfaces s11 to s14) having power in the X-Z plane for reimaging the diode array in the intermediate image plane at a reverse magnification ratio of 10: 1. The intermediate image is reimaged on the final image plane by a group of spherical lenses 214 to 222 (surfaces s15 to s24) that form a sub relay at a reverse magnification ratio of 6: 1. Thus, the relay has an overall inverse magnification of 6: 1 in the plane including the row of diodes and 60: 1 in the plane perpendicular to each load of the diode.

The 6: 1 inverse magnification ratio in the YZ plane has a size of 10 mm of the non-parallel (slow axis) of the preheat radiation source 142 from 1.10 mm in the image plane (IP) from 10 mm in the object plane (OP). Reduce to mm Also in the same plane, the 10 ° cone angle of the radiation emitted from the preheating radiation source 142 in the object plane OP is increased to 60 ° in the image plane IP.

The inverse magnification of the X-Z plane is 60: 1. Thus, the 11.4 mm dimension of the laser diode array (measured in the X-direction across the seven rows of the diode) constituting the efficient source 220 in the object plane OP is reduced to 0.19 mm in the image plane IP. do. In addition, the spread of the 1 ° FWHM angle from the efficient source 220 to the parallel beam is increased to a 60 ° cone angle in the image plane IP.

The overall efficiency of transferring the preheated radiation 147 from the radiation source 142 in the object plane OP to the substrate 10 in the image plane IP includes 50% (reflection loss in the substrate surface 12). Is assumed, the relay lens 143 of FIGS. 8A and 8B can bring 280 W into the image 160. For an exemplary image 160 dimension of 1.6 mm by 0.19 mm, this achieves a power density of 921 W / mm ² . At normal incidence (θ ₁₅₀ = 0 °), this power density is about 500 ° C., assuming a temperature of room temperature (ie, ˜20 ° C.) silicon substrate 10 with a dwell time of about 0.2 ms, a temperature near 520 ° C. Raise to. This is equivalent to that generated with a diode array image 160 that is at or above the critical, uniform temperature (T _C ) of 400 ° C. required to initiate the self-maintaining annealing condition and is located directly in front of the annealing laser image 30. As well as within the exact range for non-uniform temperature distribution. In this case, the preheated radiation beam 150 precedes the annealing radiation beam 20 (ie, scanned immediately in front). In this way, the maximum temperature T _MAX produced by the preheat radiation beam is achieved only before the anneal radiation beam 20 irradiating the same preheated portion of the substrate. In one exemplary embodiment, the preheat radiation beam always precedes the anneal radiation beam because the relative position of the preheat and anneal radiation beam is reversed every time the scan direction is reversed.

Iii. Radiation beam scanning and orientation

As described above, in one exemplary embodiment, the image 160 formed by the preheated radiation beam is scanned across the substrate 10. Also in this regard, since the image 30 formed by the anneal radiation beam 20 is scanned across the substrate, it is incident on the portion of the substrate preheated by the preheat radiation beam.

In an exemplary embodiment, the scanning is performed by moving the substrate in a spiral, raster or boustrophedonic pattern. In the boustrepidonic scanning pattern, the scan direction is reversed and the cross-scan position is increased after every scan. In this case, as described above, it is necessary to change the relative positions of the preheat radiation beam 150 and the anneal radiation beam 20 between each scan. In one exemplary embodiment, this is accomplished by shifting the position of the entire relay lens 143. When the annealed radiation beam 20 is about 120 μm wide (FWHM) and the preheated radiation beam 250 is about 190 μm wide (top-hat profile), the relay lens 143 is beam centered. The length between them should be moved by about 2 times or by about 393 μm in a direction parallel to the scan direction. This is achieved, for example, via a signal from a controller 32 operatively connected to the preheat relay lens 143 to achieve movement of the relay lens (FIG. 1C). In a similar manner, the controller controls the focus of the preheated radiation beam 150 by adjusting the focus, tip, and tilt parameters of the substrate prior to scanning.

As described in the above-mentioned US patent application Ser. No. 10 / 287,864, an annealing radiation beam 20 that is incident on the substrate 10 at an angle of incidence, at or near Brewster's angle, and P-polarized It is preferable to have. This is because, during annealing, the film stack to be impinged on the substrate has a low reflectance and a small variation in reflectance depending on these conditions.

In one exemplary embodiment, the preheated radiation beam 150 is arranged to strike the substrate at an angle of incidence θ ₁₅₀ , at or near Brewster angle, in a manner similar to that of the anneal radiation beam 20. Typically, this angle reduces the variation in reflectivity between different film stacks to be found in the substrate prior to the activation (annealing) step. However, this beam orientation (angle) works very well at the annealing wavelength but is not efficient at the wavelength used during preheating. The rough equivalent between the thickness of the film and the preheat radiation beam wavelength used to make the semiconductor structures (eg, devices 14 such as transistors) causes a greater variation in substrate reflectivity at all angles of incidence. In addition, the angle of incidence (θ ₁₅₀ ) at or near Brewster's angle (θ ₁₅₀ ) spreads the image 160 over an area three or four times larger than at a vertical angle of incidence (ie, θ ₁₅₀ = 0 °), Lower the density to the corresponding amount. When the scan rate is kept unchanged, the maximum temperature is also reduced because it is usually set by the annealing radiation beam contour.

One of the problems with operation at or near normal incidence is that the reflectance of the radiation can be very high and can cause serious damage when returning to the radiation source (eg diode array). 10A and 10B are schematic diagrams illustrating exemplary embodiments of the preheat relay optical system 140 to reduce the amount of preheat radiation reflected or scattered back to the preheat radiation source 142 (FIG. 1C). Referring to FIG. 10A, in a preferred embodiment, the preheated radiation beam 150 has a vertical angle of incidence of θ ₁₅₀ = 0 °. The angle of normal incidence results in a large amount of preheated radiation beam 150 that is reflected from the substrate (reflected preheated radiation is represented by 150R) and transmitted back towards preheated radiation source 142. If the reflected preheat radiation 150R returns to the preheat radiation source 142, it may fail to promote the time of the source. When the emitted preheat radiation 147 is polarized (as with a laser diode), in one exemplary embodiment, the amount of reflected preheat radiation 150R back to the preheat radiation source is the polarization direction of the preheat radiation beam. Is arranged by arranging the polarizer aligned and the quarter wave plate 143WP positioned between the polarizer and the substrate. The quarter wave plate transmits radiation traveling from the polarizer to the substrate into circularly polarized radiation at the substrate. Any radiation returning from the substrate is converted back to linearly polarized radiation after passing through a quarter wave plate. However, the polarization direction of the returned radiation is orthogonal to the original direction. Thus, the returned beam is not transmitted by the polarizer and does not reach the laser diode array.

Next, referring to FIG. 10B, the incidence angle θ ₁₅₀ is the preheat radiation source even when the de-vertical incidence angle is selected such that the reflected (semi-reflective) preheat radiation 150 cannot return to the preheat radiation source. Scattered (or non-reflective) preheated radiation 150S returned to may indicate a problem. Even small amounts of radiation returned to some types of preheated radiation sources (such as lasers) can cause instability of operation. It is also desirable to use P-polarized preheated radiation when operating outside the normal angle of incidence to increase the proportion of radiation absorbed in the substrate and to reduce variations in absorption caused by the various structures on the substrate.

Thus, in one exemplary embodiment, the amount of preheat radiation 150S returned to the preheat radiation source 142 is reduced by adding Faraday rotor 143F under polarizer 143P and relay lens 143. Faraday rotator 143F is located between polarizer 143P and substrate 10. In operation, the Faraday rotor rotates the polarization of the preheat radiation beam 150 at 90 ° after the two have passed through the rotor, and the polarizer rotates the polarization rotating preheat radiation 150S from returning to the preheat radiation source 142. ) Is blocked. Also, operating the optical relay system 140 such that the preheat radiation beam 150 deviates from the normal angle of incidence facilitates the measurement of the power of the reflected preheat radiation beam 150R, which is a useful diagnosis.

Power measurements of the incident preheat radiation beam 150 and reflected preheat radiation 150R may be used to calculate the power absorbed by the substrate 10. This is then used to estimate the maximum temperature generated by the preheat radiation beam 150. By maintaining the power absorbed in the preheat radiation beam 150 above a certain minimum threshold, sufficient preheating is ensured to cause strong absorption of the anneal radiation beam 20 by the substrate.

In the foregoing detailed description, for ease of understanding, various features are grouped together in various exemplary embodiments. Numerous features and advantages of the invention will be apparent from the description, and therefore, by the appended claims, are intended to include all of the features and advantages of the described apparatus that follow the true spirit and scope of the invention. In addition, it is not desirable to limit the invention to the precise construction and operation described herein, since various modifications and variations can readily occur to those skilled in the art. Accordingly, other embodiments are within the scope of the appended claims.

Table 1

Claims (35)

An apparatus for performing laser thermal annealing of a substrate having a surface, the apparatus comprising: At room temperature, a laser capable of producing continuous annealing radiation having a wavelength that is not absorbed by the substrate; An anneal optical system for receiving said anneal radiation and forming an anneal radiation beam for forming a first image on said substrate surface, said first image being scanned across said substrate surface; And A heating device, heating at least a portion of the substrate to a threshold temperature such that, during scanning, the annealing radiation beam incident on the portion is absorbed near the surface of the substrate at the portion, The heating device, A preheat radiation source configured to emit preheat radiation at a wavelength absorbed by the substrate at room temperature; And Receiving the preheat radiation from the preheat radiation source and preheating a second image across the substrate surface to preheat a portion of the substrate that is in front of the scanned first image or partially overlaps the scanned first image. And a relay system for forming a preheated radiation beam for forming and scanning. The method of claim 1, And the wavelength of the annealing radiation beam is greater than 1 μm. The method of claim 1, And the substrate is supported by a movable stage that achieves the scanning by moving the substrate with respect to the annealing and preheating radiation beams. The method of claim 1, And the annealing radiation has a wavelength of 10.6 μm. The method of claim 1, And the substrate is undoped or lightly doped silicon. The method of claim 1, And the first image formed by the anneal radiation is a line image. The method of claim 1, And the heating device comprises a heated chuck supporting the substrate and causing the substrate to heat to the critical temperature. The method of claim 7, wherein And the heating device further comprises a heat shield for reflecting heat released from the substrate back to the substrate. The method of claim 1, And the heating device comprises a heated enclosure surrounding the substrate and causing the substrate to heat to the critical temperature. The method of claim 1, And the relay system comprises a relay lens. The method of claim 10, And the second image is a line image. The method of claim 10, And the preheated radiation beam has a wavelength of 780 nm or 800 nm. The method of claim 10, The relay lens, The preheated radiation beam is arranged to be incident on the substrate at a vertical angle of incidence, Further comprising a polarizer and a quarter wave plate, And the polarizer and the quarter wavelength plate are arranged in the preheat radiation beam to reduce the amount of preheat radiation reflected from the substrate from reaching the preheat radiation source. The method of claim 10, And a polarizer and a Faraday rotor are arranged in the preheat radiation beam to prevent preheat radiation from returning to the preheat radiation source. The method of claim 10, The preheated radiation source is an array of laser diodes, And the relay lens is anamorphic and forms on the substrate the second image, such as a line image of the array of laser diodes. The method of claim 10, And the relay lens comprises elements adjustable to maintain the second image in a focused state. The method of claim 1, A chuck supporting the substrate; A movable stage for supporting the chuck; And And a stage driver operatively connected to the movable stage for selectively moving the stage to selectively move the substrate to achieve the scanning. A method of laser thermal annealing of a substrate at room temperature, wherein the substrate does not absorb the radiation wavelength of the laser beam to the extent that it satisfies the self-maintaining annealing conditions, At least a portion of the substrate is heated to a threshold temperature such that when the laser beam is irradiated by the laser beam, the laser beam is in the heated portion to such an extent that the substrate satisfies a self-maintaining annealing condition in the heated portion. Absorbable in the vicinity of the surface of the substrate; And Initiating a self-maintaining annealing condition by scanning the laser beam over the heated portion of the substrate. The method of claim 18, And the substrate is an undoped or lightly doped silicon substrate. The method of claim 18, And said heating step comprises providing heat to said substrate through a heated chuck supporting said substrate. The method of claim 20, Reflecting heat radiated from the substrate back to the substrate. The method of claim 18, And said heating step comprises providing heat to said substrate through a heated enclosure surrounding said substrate. The method of claim 18, And the heating step comprises irradiating the portion of the substrate with a preheated radiation beam having a wavelength absorbed by the substrate at room temperature. The method of claim 23, The wavelength of the preheat radiation beam is 780 nm or 800 nm. The method of claim 23, Generating the preheated radiation beam using an array of diode lasers. The method of claim 23, The laser beam forms a first image on the substrate, The preheated radiation beam forms a second image on the substrate, The laser thermal annealing method includes scanning the first and second images across the substrate with the second image in front of the first image or partially overlapping the first image. Thermal annealing method. The method of claim 26, During scanning, maintaining the second image in front of the first image or partially overlapping the first image. The method of claim 18, And the laser beam has a wavelength in excess of 1 μm. The method of claim 28, The laser beam has a wavelength of 10.6 μm. The method of claim 23, i) irradiating a set of test substrates with a set of preheated radiation beams and a corresponding set of laser beams having a threshold temperature generated by said preheated radiation beam to enable self-maintaining annealing conditions, and ii) a selected intensity. Experimentally determining at least one of the minimum power required for the preheat radiation beam to thereby enable the self-maintaining annealing condition. The method of claim 18, And the critical temperature is at least 360 ° C. The method of claim 26, The scanning is performed in a pattern selected from the group of patterns comprising raster, boustrophedonic, and spiral. The method of claim 26, Scanning is performed by selectively moving a movable substrate stage supporting a chuck which in turn supports the substrate. The method of claim 23, The preheat radiation beam has a power level, The power level is, a) scanning the substrate with a laser beam having sufficient power to anneal the absorbing substrate; b) raising the power level of the preheated radiation beam until the annealing temperature is reached on the substrate surface in step a) and repeating step a); And c) setting the power level to a minimum causing said annealing temperature to be reached. A method of laser thermal annealing of a substrate at room temperature, wherein the substrate does not absorb the radiation wavelength of the laser beam to the extent that it satisfies the self-maintaining annealing conditions, Irradiate a portion of the substrate with a scanning preheat radiation beam having a preheat beam wavelength to heat at least a portion of the substrate to raise it to a critical temperature, thereby heating the portion of the substrate when irradiated by the laser beam. Allowing the laser beam to be absorbed near the surface of the substrate at the heated portion to ensure that the substrate satisfies a self-maintaining annealing condition at the substrate; And Initiating the self-maintaining annealing condition by scanning the laser beam behind or partially overlapping the preheat radiation beam.

Images