MICROSPOT REFLECTOMETER

Description

TECHNOLOGICAL FIELD

[0001] The present disclosure relates generally to the field of testing a sample and, more specifically, to a reflectometer with optical elements configured to direct light to a sample and a detector.BACKGROUND

[0002] Specular reflectance refers to the reflection of light from a surface, such as a mirror, in which light from an incoming direction is reflected into an outgoing direction. Thus, specular reflectance is a behavior of light which can be measured using optical equipment. Specular reflectance measurement of materials is one way of determining the composition or other chemical, thermal or optical characteristics of the sample material.

[0003] Measurement of specular reflectance has several applications. For example, this technique is used to establish reference standards for other types of reflectance measurements and for calibration of optical measurement devices. In another example, this technique is used in the optical coating industry to develop coatings, for example, mirrors in order to increase the mirror's optical efficiency. In still another example, precise absolute measurement of specular reflectance can be used to measure the thickness or refractive index of a single layer of an optical coating.

[0004] US 2021 / 131786 Al, in accordance with its abstract, states a reflectometer or ellipsometer integrated with a processing tool includes a source module configured to generate a input beam, and a first mirror arranged to receive the input beam. The first mirror is configured to collimate the input beam and direct the input beam toward an aperture plate. The aperture plate has at least two apertures. One of the at least two apertures is arranged to define a measurement beam from a portion of the input beam, and one of the at least two apertures is arranged to define a reference beam from a portion of the input beam. An optical element is arranged within an optical path of the reference beam and outside an optical path of the measurement beam. The optical element is configured to direct the reference beam toward a third mirror. A second mirror is arranged to receive the measurement beam and focus the measurement beam through a window and onto a surface of a sample. The window forms part of a chamber of the processing tool and the sample is disposed within the chamber. At least a portion of the measurement beam is reflected from the surface of the sample as a reflected beam. The second mirror is arranged to receive the reflected beam and direct the reflected beam toward the optical element. The optical element is configured to direct the reflected beam toward the third mirror. The third mirror is arranged to receive the reference beam and the reflected beam and focus the reference beam and the reflected beam onto a collection plane.

[0005] US 6603542 B1, in accordance with its abstract, states a high sensitivity optical inspection system for detecting flaws on a diffractive surface containing surface patterns includes: a first and a second illumination means for illuminating predetermined regions on the diffractive surface to generate a scattered intensity distribution in response to either a flaw or a surface pattern; means for detecting the intensity level of the scattered intensity distribution at a plurality of locations about the diffractive surface; means for establishing a minimum detected intensity level; means, responsive to the minimum detected intensity level, for indicating the absence of a flaw on the illuminated region of the diffractive surface when the minimum detected intensity level is below a threshold intensity level and for indicating the presence of a flaw on the illuminated region of the diffractive surface when the minimum detected intensity level exceeds the threshold intensity level; and means for moving the diffractive surface to generate a scan pattern on the diffractive surface to inspect the entire surface.

[0006] Specular reflectance measurement devices have difficulty in aligning the components and the sample for accurate measurement. Additionally, specular reflectance measurement devices have difficulty focusing the light beam at the sample and at the detector. For example, the light beams tend to disperse along the length of the light beam. Improved techniques for measuring absolute specular reflectance are desirable. For example, devices having enhanced alignment mechanisms and techniques for the components and the sample for improved reflectance measurements are desirable. Devices having focused light paths and spot size on the sample are desirable.SUMMARY

[0007] There is described herein a reflectometer configured to test a sample, in accordance with claim 1. The reflectometer comprises a plurality of light sources with each of the light sources configured to emit a light beam along a light path at a different wavelength. A sample holder is configured to position the sample along the light path. A mirror system is positioned in the light path between the light source and the sample holder and is configured to reflect the light beam from the light source towards the sample. A detector is positioned downstream from the sample holder to receive the light beam that is reflected from the sample. The mirror system comprises a first mirror and a second mirror downstream from the first mirror. The first mirror is configured to be adjustable relative to the plurality of light sources. For each light source, the first mirror is configured to move between a first angular position that forms a first polarization path with a first polarization state and a second angular position that forms a different second polarization path with a different second polarization state. The second mirror is configured to be adjustable to receive the light beam along each of the first polarization path and the second polarization path and direct the light beam along a common path downstream from the second mirror.

[0008] Optionally, a first periscope is positioned along the first polarization path with the first periscope configured to provide the light beam with the first polarization state, and a second periscope is positioned along the second polarization path with the second periscope configured to provide the light beam with the second polarization state.

[0009] Optionally, the first periscope is configured to change the polarization of the light beam to the first polarization state and the second periscope is configured to change the polarization of the light beam to the second polarization state.

[0010] Optionally, the plurality of light sources comprises a first laser that emits the light beam at a first wavelength and a second laser that emits the light beam at a different second wavelength.

[0011] Optionally, each of the plurality of light sources has a unique position such that the light path to the first mirror is different for each of the light sources.

[0012] Optionally, the mirror system comprises: an expanding mirror that receives the light beam from the second mirror; a collimating mirror that receives the light beam from the expanding mirror; and an off-axis parabolic mirror that receives the light beam from the collimating mirror and focuses the light beam to the sample holder.

[0013] Optionally, the mirror system is configured to focus the light beam to a spot size on the sample that is positioned on the sample holder to less than one hundred micrometers or the mirror system is configured to focus the light beam to a spot size less than one hundred micrometers at the position on the sample holder in which the sample is held.

[0014] Optionally, a camera is positioned downstream from the sample holder to image a spot from the sample, and a control unit is configured to receive signals from the camera and adjust the sample holder based on the signals.

[0015] Optionally, the sample holder is adjustable within six degrees of freedom to position the sample.

[0016] Also described herein, and useful for understanding the background to the presently claimed subject-matter, is an unclaimed reflectometer configured to test a sample. The reflectometer comprises a light source that emits a light beam along a light path. A detector is positioned downstream from the light source along the light path. A sample holder is configured to position the sample along the light path with the sample holder positioned along the light path between the light source and the detector. First optical elements are positioned along the light path between the light source and the sample holder. Second optical elements are positioned along the light path between the sample holder and the detector. One of the first optical elements comprises an off-axis parabolic mirror. The light source emits the light beam along a first section of the light path and the off-axis parabolic mirror directs the light beam along a second section of the light path with the first section and the second section being perpendicular.

[0017] Optionally, the first optical elements comprises a collimating mirror configured to direct the light beam to the off-axis parabolic mirror parallel to an optical axis of the off-axis parabolic mirror.

[0018] Optionally, the off-axis parabolic mirror comprises a focal axis that is aligned with a center of the sample holder.

[0019] Optionally, the off-axis parabolic mirror comprises a focal point on a surface of the sample holder.

[0020] Optionally, the light path between the light source and the sample holder is devoid of lenses.

[0021] Optionally, a camera is positioned downstream from the sample holder to image a spot from the sample, and a control unit is configured to receive signals from the camera and adjust the sample holder based on the signals.

[0022] There is also described herein a method of measuring a reflectance of a sample, in accordance with claim 13. The method comprises: projecting a light beam from one of a plurality of light sources, each of the light sources configured to emit a light beam along a light path at a different wavelength; for each of the light sources, adjusting a first mirror to one of a first angular position that directs the light beam to a first polarizing optical element to provide a first polarization to the light beam and a second angular position that directs the light beam to a second polarizing optical element to provide a second polarization to the light beam; adjusting a second mirror to one of a first position and receiving the light beam from the first polarizing optical element and a second position and receiving the light beam from the second polarizing optical element; and directing the light beam from the second mirror downstream along a single light path in both the first position and the second position; thereafter reflecting the light beam from a collimating mirror to an off-axis parabolic mirror; thereafter focusing the light beam from the off-axis parabolic mirror to a spot size that is less than one hundred micrometers on the sample that is mounted on a sample holder ; and thereafter reflecting the light beam from the sample through one or more downstream optical elements to a detector.

[0023] Optionally, the method further comprises reflecting the light beam from the collimating mirror parallel to an optical axis of the off-axis parabolic mirror.

[0024] Optionally, the plurality of light sources comprises different lasers, and projecting the light beam from one of the plurality of light sources comprises projecting the light beam from one of a first laser at a first wavelength and a second laser at a second wavelength.

[0025] The features, functions and advantages that have been discussed can be achieved independently in various examples or may be combined in yet other examples, further details of which can be seen with reference to the following description and the drawings.BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 is a schematic diagram of a reflectometer. Figure 2 is a schematic diagram of a reflectometer. Figure 3 is a schematic diagram of a portion of a light path within a reflectometer. Figure 4 is a schematic diagram of a portion of a light path within a reflectometer. Figure 5 is a perspective view of a sample holder that is supporting a sample. Figure 6 is a schematic diagram of a reflectometer. Figure 7 is a schematic diagram of a portion of a light path within a reflectometer. Figure 8 is a schematic diagram of a control unit. Figure 9 is a flowchart diagram of a method of measuring a reflectance of a sample. DETAILED DESCRIPTION

[0027] The application is directed to a reflectometer configured to test a sample. The reflectometer includes a light source that emits a light beam to the sample that is placed on a sample holder. The reflectometer focuses the light beam to a reduced spot size on the sample. The reflectometer is configured to view structure / defects in the sample using a detector that is downstream from the sample.

[0028] Figure 1 illustrates a schematic diagram of a reflectometer 15 that is configured to test a sample 100. The reflectometer 15 includes a light source 20 that emits a light beam 70 along a light path. A sample holder 50 positions the sample 100 along the light path. A detector 80 positioned downstream receives the reflectance from the sample 100. One or more optical elements 30 are positioned along the light path upstream from the sample 100. One or more second optical elements 60 are positioned along the light path downstream from the sample 100.

[0029] The reflectometer 15 is configured to focus the light beam 70 onto the sample 100. In some examples, the reflectometer 15 focuses the light beam on the sample 100 to a spot size that is less than one hundred micrometers (100µm).

[0030] The light source 20 is configured to emit the light beam. The light source 20 can be configured to emit a light within a variety of wavelengths. In some examples, the light source 20 is a laser that outputs a light beam within a range of between 6.5µm - 1550nm. In some examples, the light source 20 is a quantum cascade laser (QCL) tunable to emit a light beam within a range of between 7µm - 12µm. In some examples, the light source is a fixed wavelength laser. The light source 20 can include a variety of other configurations including but not limited to various lasers, light emitting diodes, broadband sources, light bulbs, and natural sun light.

[0031] The upstream optical elements 30 are positioned upstream from the sample 100 between the light source 20 and the sample holder 50. The downstream optical elements 60 are positioned downstream from the sample 100 between the sample holder 50 and the detector 80. The optical elements 30, 60 can include a variety of different elements including mirrors and lenses to shape and direct the light beam 70.

[0032] Figure 2 illustrates an example of a reflectometer 15 that includes a light source 20, upstream optical elements 30, sample holder 50 to hold a sample 100, downstream optical elements 60, and detector 80. In this and other examples, the upstream optical elements 30 include a mirror system 39 that includes reflective members that direct the light towards the sample 100. The reflective members reflect the light beam 70 without the light beam 70 passing through the optical elements 30. In some examples, the mirror system 39 is devoid of lenses.

[0033] As illustrated in Figure 2, the mirror system 39 includes an expanding mirror 31 that reflects the light beam 70 emitted from the light source 20. The expanding mirror 31 causes the light rays of the light beam 70 to angle outward away from an optical axis of the light beam 70 as the light beam 70 is directed from the first mirror 31 to the second mirror 32.

[0034] In some examples, the second mirror 32 is a collimating mirror that causes the light rays of the light beam 70 to become more aligned as they move along the light path towards the third mirror 33. In some examples, the second mirror 32 causes the reflected rays to be parallel to an optical axis of the light beam 70 between the second mirror 32 and the third mirror 33.

[0035] The third mirror 33 is positioned downstream from and receives the reflected light beam 70 from the second mirror 32. In some examples, the third mirror 33 is an off-axis parabolic (OAP) mirror. As illustrated in Figure 3, the OAP mirror 33 has an optical axis 34 and a focal axis 35. The focal axis 35 passes between a center C of the OAP mirror 33 and a focal point 36 at the sample holder 50.

[0036] In some examples, the surface of the OAP mirror 33 is a section of a parent parabola. The optical axis 34 of the OAP mirror 33 is parallel to and offset from an optical axis of the parent parabola. The focal point 36 of the OAP mirror 33 is positioned on the optical axis of the parent parabola.

[0037] During use, the collimated light from the second mirror 32 is directed parallel to the optical axis 34 of the OAP mirror 33. This parallel arrangement provides for the OAP mirror 33 to reflect and focus the light at the focal point 36 which is offset from the optical axis 34. In some examples, the focal axis 35 is perpendicular to the optical axis 34. In some examples, the focal point 36 is positioned outward from a surface of the sample holder 50. The sample 100 is mounted on the surface of the sample holder 50 and positioned at the focal point 36 of the OAP mirror 33. In other examples, the focal point 36 is positioned on the surface of the sample holder 50. In the various examples, the focusing of the OAP mirror 33 results in a spot size less than 100µm at the face of the sample 100.

[0038] Figure 4 illustrates a schematic diagram of the light path between the light source 20 and the sample 100 at the sample holder 50. The light path includes a first section 75a that includes an optical axis 37 between the light source 20 and the first mirror 31. A second section 75b extends between the first mirror 31 and the second mirror 32. A third section 75c includes the optical axis 34 that extends between the second mirror 32 and the third mirror 33. A fourth section 75d includes the focal axis 35 that extends between the third mirror 33 and the sample 100. In some examples, the optical axis 37 of the first section 75a and optical axis 34 of the third section 75c are parallel. In some examples, the focal axis 35 is perpendicular to the optical axis 37. In some examples, the focal axis 35 is perpendicular to the optical axis 34.

[0039] As illustrated in Figure 2, the OAP mirror 33 directs the light away from the detector 80. This configuration provides for a smaller overall size of the reflectometer 15. In some examples, a distance Q between the light source 20 and a camera 63 is 200mm.

[0040] The sample holder 50 is configured to support the sample 100. The sample holder 50 includes a platform 51 on which the sample 100 is positioned during testing. In some examples, the platform 51 includes attachment members, such as but not limited to mechanical fasteners to secure the sample 100. The sample holder 50 is adjustable to move the platform 51 to provide for manipulating and aligning the focus spot. In some examples, the sample holder 50 provides for six degrees of freedom of movement to provide for adjustment for alignment of the focus spot.

[0041] Figure 5 illustrates a sample holder 50 for positioning a sample 100. The sample holder 50 includes a platform 51 for positioning the sample 100. Legs 53 that extends upward from a base 52 support the platform 51. The legs 53 are adjustable to manipulate and position the platform 51. In some examples, the legs 53 are adjustable to provide for six degrees of freedom of movement of the platform 51 and thus the attached sample 100. One or more motors 54 provide for adjusting the legs 53.

[0042] As illustrated in Figure 2, a camera 63 and mirror 62 provide for aligning the sample 100. The mirror 62 is a flip mirror that is moveable between a first position within the light path and a second position out of the light path. For alignment, the mirror 62 is flipped into the light path to direct light to the camera 63 which is otherwise located away from the light path. When the sample 100 is aligned, the mirror 62 is flipped out of the way to allow the light to be detected by the detector 80. The camera 63 detects the alignment of the sample and the focusing of the light on the sample 100. A control unit 90 receives signals from the camera 63 and detects the focus and the necessary changes to the sample holder 50. In some examples, the focusing is a completely automated process through the control unit 90. In other examples, one or more aspects of the system are manually operated (e.g., adjustment of the sample holder 50).

[0043] The downstream optical elements 60 include a collection lens 61. The collection lens 61 focuses the light beam towards the detector 80. The detector 80 captures the light from the sample 100. The detector 80 can include various different optical elements. The detector 80 can be configured to detect various different types of light depending upon the light source 20. Further, the detector 80 can detect light at various different polarizations including, but not limited to, vertical polarization, horizontal polarization, right-hand (RH) polarization, and / or left-hand (LH) polarization. In some examples as illustrated in Figure 2, the detector 80 employs an integration sphere 81 with an imaging lens. In other examples, the detector 80 does not include an integration sphere 81.

[0044] In some examples as illustrated in Figure 2, the reflectometer 15 includes a single light source 20. In other examples as illustrated in Figure 6, the reflectometer 15 includes two or more light sources 20a-20n. The different light sources 20a-20n emit light at different wavelengths or ranges of wavelengths. In some examples, the reflectometer 15 includes four light sources 20 that each emit light at a different wavelength or range of wavelengths. In some examples, the light sources 20 include: a first light source 20a that emits light within a range of wavelengths between 6.5µm - 13.3 µm; a second light source 20b that emits light within a range of wavelengths between 3.52µm - 5.11µm; a third light source 20c that emits light at a wavelength of 1550nm; and a fourth light source 20d that emits light at a wavelength of 1064nm. In some examples, one or more of the light sources are tunable to adjust the wavelengths by 0.01µm.

[0045] As illustrated in Figure 6, a polarization control area 40 is positioned downstream from the light sources 20 and provides for selectively controlling the polarization of the light. The polarization control area 40 includes one or more optical elements 41 to control the polarization.

[0046] Figure 7 illustrates an example that includes a pair of light sources 20a, 20b and a pair of mirrors 43, 44. In some examples, mirrors 43, 44 are each galvanometer mirrors. In other examples, one or both mirrors 43, 44 are different mirror types. The first mirror 43 is adjustable to move between different angular positions to receive light from either of the light sources 20a, 20b. The first mirror 43 directs the light to either a first optical element 45 or a second optical element 46. In some examples, the different optical elements 45, 46 provide for different polarizations. In some examples, optical element 45 provides for S-polarization and optical element 46 provides for P-polarization. In some examples, the optical elements 45, 46 are periscopes that uses one or more prisms, lens, or mirrors to reflect the light.

[0047] The second mirror 44 is configured to direct the light downstream to the first mirror 31. The second mirror 44 is positionable between a first position to receive light from optical element 45 and direct it to the first mirror 31, or to receive light from optical element 46 and direct it to the first mirror 31. The second mirror 44 functions to recombine the light paths such that both travel down the same light path to the first mirror 31. In some examples with different polarizations, the second mirror 44 recombines the light paths such that both S and P polarization states travel down the same light path to the first mirror 31.

[0048] In some examples, the reflectometer 15 is computer controlled. As illustrated in Figure 8, the reflectometer 15 includes a control unit 90 that oversees the operation. The control unit 90 includes processing circuitry 91 that operates according to program instructions 93 stored in memory circuitry 92. The processing circuitry 91 includes one or more circuits, microcontrollers, microprocessors, hardware, or a combination thereof. The processing circuitry 91 can include various amounts of computing power to provide for the needed functionality.

[0049] Memory circuitry 92 includes a non-transitory computer readable storage medium storing program instructions 93, such as a computer program product, that configures the processing circuitry 91 to implement one or more of the techniques discussed herein. Memory circuitry 92 can include various memory devices such as, for example, read-only memory, and flash memory. Memory circuitry 92 can be a separate component as illustrated in Figure 8 or can be incorporated with the processing circuitry 91. Alternatively, the processing circuitry 91 can omit the memory circuitry 92, e.g., according to at least some examples in which the processing circuitry 91 is dedicated and non-programmable.

[0050] Interface circuitry 94 provides for sending and / or receiving signals from one or more of the components of the reflectometer 15. Components include but are not limited to motors that adjust the positioning of one or more of the optical elements 30, 60, motors 54 to adjust the sample holder 50, camera 63, and detector 80. The interface circuitry 94 can provide for one-way communications or two-way communications that are both to and from the components.

[0051] Communication circuitry 95 provides for communications to and from the control unit 90 with a remote node (e.g., operator equipment, server, database). Communications circuitry 95 provides for sending and receiving data with one or more remote nodes.

[0052] A user interface 96 provides for a user to control one or more aspects of the reflectometer 15 during operation. The user interface 96 includes one or more input devices 98 such as but not limited to a keypad, touchpad, roller ball, and joystick. The user interface 96 also includes one or more displays 97 for displaying information regarding the testing and / or for an operator to enter commands to the processing circuitry 91.

[0053] In some examples, the control unit 90 controls the full operation of the reflectometer 15. Additionally or alternatively, one or more of the components can be controlled by a user. In some examples, the user is able to adjust the sample holder 50 according to output indicated on the display 97. In another examples, the user inputs commands to control the type of light source 20 used during the testing.

[0054] Figure 9 illustrates a method of measuring a reflectance of a sample 100. A light beam is projected from a light source 20 (block 110). The light beam is reflected from a collimating mirror 32 to an off-axis parabolic mirror 33 (block 112). The light beam is focused from the off-axis parabolic mirror 33 to a spot size that is less than one hundred micrometers on the sample 100 that is mounted on a sample holder 50 (block 114). The method includes reflecting the light beam from the sample through one or more downstream optical elements 60 to a detector 80 (block 116).

[0055] The examples may be carried out in other ways than those specifically set forth herein without departing from essential characteristics of the claims. The present examples are to be considered in all respects as illustrative and not restrictive, and all changes coming within the meaning of the appended claims are intended to be embraced therein.

Claims

1. A reflectometer configured to test a sample, the reflectometer comprising: a plurality of light sources (20), each of the light sources (20) configured to emit a light beam along a light path (70) at a different wavelength; a sample holder (50) configured to position the sample along the light path (70); a mirror system (39) positioned in the light path between the light source (20) and the sample holder (50), the mirror system (39) configured to reflect the light beam from the light source (20) towards the sample (100); and a detector (80) positioned downstream from the sample holder (50) to receive the light beam that is reflected from the sample; the mirror system (39) comprising: a first mirror (43); a second mirror (44) downstream from the first mirror (43); the first mirror (43) configured to be adjustable relative to the plurality of light sources (20), for each light source (20) the first mirror (44) configured to move between a first angular position that forms a first polarization path with a first polarization state and a second angular position that forms a second polarization path that is different and with a different second polarization state; and the second mirror (44) configured to be adjustable to receive the light beam along each of the first polarization path and the second polarization path and direct the light beam along a common path downstream from the second mirror (44).

2. The reflectometer of claim 1, further comprising: a first periscope positioned along the first polarization path, the first periscope configured to provide the light beam with the first polarization state; and a second periscope positioned along the second polarization path, the second periscope configured to provide the light beam with the second polarization state.

3. The reflectometer of claim 2, wherein: the first periscope is configured to change a polarization of the light beam to the first polarization state; and the second periscope is configured to change the polarization of the light beam to the second polarization state.

4. The reflectometer of any preceding claim, wherein the plurality of light sources comprise a first laser that emits the light beam at a first wavelength and a second laser that emits the light beam at a different second wavelength.

5. The reflectometer of any preceding claim, wherein each of the plurality of light sources (20) has a unique position with the light path to the first mirror (43) being different for each of the light sources.

6. The reflectometer of any preceding claim, wherein the mirror system (39) comprises: an expanding mirror (31) that receives the light beam from the second mirror (44); a collimating mirror (32) that receives the light beam from the expanding mirror (31); and an off-axis parabolic mirror (33) that receives the light beam from the collimating mirror (32) and focuses the light beam to the sample holder (50).

7. The reflectometer of any preceding claim, wherein the mirror system (30) is configured to focus the light beam to a spot size on the sample (100) that is positioned on the sample holder (50) to less than one hundred micrometers.

8. The reflectometer of any preceding claim, further comprising: a camera (63) positioned downstream from the sample holder (50) to image a spot from the sample (100); and a control unit (90) configured to receive signals from the camera (63) and adjust the sample holder (50) based on the signals.

9. The reflectometer of claim 8, wherein the sample holder (50) is adjustable within six degrees of freedom to position the sample (100).

10. The reflectometer of any preceding claim, wherein the light path (70) between the plurality of light sources (20) and the sample holder (50) is devoid of lenses.

11. The reflectometer of any preceding claim, wherein the sample holder (50) comprises: a platform (51) for positioning the sample (100); a base (52); adjustable legs (53) that extend upward from the base (52), wherein the legs (53) support the platform (51); and one or more motors (54) for adjusting the legs (53).

12. The reflectometer of any preceding claim, wherein the first mirror (43) and the second mirror (44) are each galvanometer mirrors.

13. A method of measuring a reflectance of a sample, the method comprising: projecting a light beam from one of a plurality of light sources (20), each of the light sources configured to emit a light beam along a light path (70) at a different wavelength; for each of the light sources, adjusting a first mirror (43) to one of a first angular position that directs the light beam to a first polarizing optical element (45) to provide a first polarization to the light beam and a second angular position that directs the light beam to a second polarizing optical element (46) to provide a second polarization to the light beam; adjusting a second mirror (44) to one of a first position and receiving the light beam from the first polarizing optical element (45) and a second position and receiving the light beam from the second polarizing optical element (46); and directing the light beam from the second mirror (44) downstream along a single light path in both the first position and the second position; thereafter reflecting the light beam from a collimating mirror (32) to an off-axis parabolic mirror (33); thereafter focusing the light beam from the off-axis parabolic mirror (33) to a spot size that is less than one hundred micrometers on the sample that is mounted on a sample holder (50); and thereafter reflecting the light beam from the sample through one or more downstream optical elements (60) to a detector (80).

14. The method of claim 13, further comprising reflecting the light beam from the collimating mirror (32) parallel to an optical axis (34) of the off-axis parabolic mirror (33).

15. The method of claim 13 or claim 14, wherein the plurality of light sources comprises different lasers, and projecting the light beam from one of the plurality of light sources comprises projecting the light beam from one of a first laser at a first wavelength and a second laser at a second wavelength.