KR20080090225A - Apparatus and method for measuring surface shape using polarization split interferometer. - Google Patents

Abstract

The present invention relates to an optical interferometer and a measuring method thereof for use in precisely measuring a three-dimensional surface shape of a measurement sample. According to the present invention, two optical paths of polarized separated reference reflected light are configured, and reflectances of the first reference reflected light and the second reference reflected light are different from each other, and then the interference signal having higher visibility is selected at each measurement point in the measurement field. By using in the calculation, it is possible to more accurately measure the three-dimensional surface shape of a sample having heterogeneous reflectance such as a high reflectance pattern on a low reflectance substrate.

Description

Surface profile measuring apparatus and method using separated polarization interferometer

1 is a conceptual diagram of a measuring apparatus using a conventional white light scanning interferometer and an optical phase shift interferometer.

Figure 2 is a conceptual view of the surface shape measurement apparatus to which the polarization separation reference plane interferometer of the present invention.

3 is an interference signal detection effect by optimized optical interference visibility using monochromatic light as an example.

Figure 4a is a conceptual diagram of the method for measuring the surface shape of the hetero reflectance measurement sample of the present invention.

Figure 4b is a first embodiment of the interference signal detection method for measuring the shape of the heteroreflective sample of the present invention.

Figure 4c is a second embodiment of the interference signal detection method for measuring the shape of the heteroreflective sample of the present invention.

The present invention relates to a measuring device for precisely measuring the fine shape formed on the surface of the workpiece using an optical interference measuring method, and the present measuring method is useful for non-destructive and non-contact precise measurement of the fine shape of various measurement samples. 3D measurement method

Recently, the process technology and processing technology have become more precise throughout the industry, such as semiconductor, flat panel display, MEMS, optical components, ultra-precision processing, and moreover, it has entered the stage of demanding ultra-precision manufacturing technology in nano units. In addition, the precision manufacturing process is also changing from a simple two-dimensional pattern to a complex three-dimensional shape, accordingly, the importance of the three-dimensional fine shape measurement technology is also required.

Optical measurement methods for measuring microstructures formed on the surface of workpieces include phase shifting interferometry, white light scanning interferometry, moire measurement, confocal scanning microscopy. microscope, etc. These measurement techniques are mainly to check the three-dimensional shape of the entire field of view by measuring the height of the three-dimensional structure formed on the surface along with the geometrical shape on the two-dimensional plane, mainly with the illumination system, optical system and camera It is based on the optical vision system and image processing technology composed of representative light detectors.

Among the optical micro-shape measuring methods, the white light scanning interferometer and the optical phase shift interferometer are precision non-contact three-dimensional measuring methods widely applied in inspection of minute shapes such as semiconductor patterns, display panel patterns, circuit patterns of printed circuit boards, and MEMS devices.

Fig. 1 shows a representative configuration of a conventional white light scanning interferometer and a light phase shift interferometer.

Although the white light scanning interferometer and the optical phase shift interferometer are based on different measurement principles, a common feature is that they are measured using optical interference signals, the former using a broad wavelength light source and the latter using a narrow band light source. Except for this, it can be implemented in the same optical system, so the two measurement methods are used together in the commercialized measurement system.

An optical interference signal is a light and dark form of a physical phenomenon caused by the optical path difference between two light beams when light simultaneously starting from an arbitrary reference point is combined after moving through different optical paths. Say what is expressed.

Light emitted from the light source 100 becomes parallel light through the condensing lens 101 and is incident on the optical interference objective lens 103 by the beam splitter 102.

The light incident on the optical interference objective lens 103 is separated into two lights by the beam splitter 105 in the optical interference objective lens, and one of the two lights is fixed to the reference plane 104 processed with high accuracy as reference light. The incident and other light is irradiated to the measurement object 106 to be measured as the measurement light. The reflected light after incident on the reference plane and the measurement plane meet in the same optical path to cause interference, and the interference light signal reaches the photodetector 108 via the imaging lens 107.

Since the reference plane uses a mirror plane that can be considered as an optical plane, the interference signal of the image acquired through the photodetector includes the height information relative to the reference plane. The period of the interference signal detected by the photodetector (interference signal interval) has a constant relationship with the wavelength of the light source to be used, and in general, the period of the interference signal corresponds to the half wavelength of the used light wavelength.

The white light interferometer and the optical phase shift interferometer are a measurement method that can measure the three-dimensional shape of the measurement surface by analyzing the principle of the basic interference signal using digital image signal processing technology.

White light scanning interferometers use a short coherent length of broadband wavelength light. The interference length is a characteristic of a light source used, which means an optical path difference in which an interference signal is generated, and is expressed as a physical distance difference between a reference light and a measurement light. A light source such as a laser has a interference length of several km, and thus an interference signal can be easily obtained under any circumstances. However, a broadband light such as white light is roughly because interference signals caused by light of various wavelengths interact with each other. Interference signals occur only within a distance difference of several um.

All pixels in the image of a photodetector, such as a camera, while the optical measurement system performs a vertical scan and moves at a small interval of several tens of nm in the direction of the optical axis to obtain an interference signal having the relative height information of the measurement sample relative to the reference plane Acquire an interference signal generated at (Pixel). The height at any pixel point during the vertical scan is set to the place where the interference signal is the largest, and this is performed for all the pixels in the camera image to calculate the three-dimensional shape along with the height of the measurement sample.

Without considering the influence of the numerical aperture (N.A) of the objective lens, the interference signal of the white light interferometer is expressed by the following equation.

I (z) = I ₀ + v (hz) cos (2kc (hz) + φ)

In the above equation, I is a white light interference signal, which is represented by the average light intensity and visibility function v of the interference signal represented by I ₀ , and the cosine function represented by the white light center frequency kc and the phase φ. The visibility function v is a size function of the interference fringe determined by the reflectivity of the measurement object, the numerical aperture of the objective lens, the frequency distribution of the light source, and is generally expressed as a left-right symmetric function such as a Gaussian shape or a quadratic function of two or more orders of magnitude.

The maximum interference signal is generated at the point where the optical path difference between the reference plane and the measurement point becomes zero, and also shows the highest visibility when the light intensity of the reference plane reflected light and the measurement plane reflected light is the same.

The reference plane of the interference lens is composed of a high reflectivity metal film or a dielectric film coating. When measuring a sample having a low reflectance, if the intensity of the reflected light differs greatly from the reference reflectance light and the low reflectance test specimen, which are synthesized to generate an interference pattern, the two The visibility of the interference signal is lowered by the difference in the average amount of light therebetween.

In general, a photodetector represented by a camera has a certain range of dark noise with unique characteristics. Such noise acts as an error in signal analysis when the detection signal is very low. Since the error affects more, the larger the measurement error appears in the analysis of the interference signal.

Therefore, the visibility should be as large as possible in order to increase the measurement accuracy against the measurement error of the detector. For this purpose, the light intensity of the reference plane reflected light and the measurement plane reflected light should be the same.

In the case of a heterogeneous reflectance measurement sample composed of a material having a large difference in light reflectance such as a high reflectance metal pattern on a low reflectance substrate in a measurement area in a microscopic measurement using a conventional white light scanning interferometer, two samples having different reflectances because they have one reference plane It is not possible to set the optimal light quantity measurement conditions at the position. In addition, because the reflectance of the reference plane is fixed, it cannot be adjusted to obtain the best interference signal with a high signal-to-noise ratio for various measurement samples.

For example, in the surface inspection of a printed circuit board, it is difficult to obtain a detection signal for the substrate due to the low reflectance of the printed circuit board when measuring conditions such as lighting settings are set for the high reflectivity metal circuit of the printed circuit board. If the illumination is set on a low reflectivity substrate to increase the visibility of a printed circuit board having a high degree of visibility, a saturation phenomenon of the photodetector occurs for the metal pattern with high reflectance, and thus a measurement signal cannot be obtained.

As described above, in the case of a microscopic measurement using a conventional white light scanning interferometer, it is difficult to obtain a clear signal of high visibility when measuring various measurement specimens such as low reflectance samples or heterogeneous reflectance samples, so that measurement accuracy is lowered.

The present invention relates to various measurement samples such as a low reflectance measurement sample or a heterogeneous reflectance measurement sample in which a low reflectance sample and a high reflectance sample are mixed in a three-dimensional surface shape measurement device using optical interference such as a white light scanning interferometer. In order to solve the problem, the visibility of the interference signal can be adjusted, and in the case of hetero reflectance samples, a surface capable of more accurate measurement by selecting an interference signal having better visibility among the individual interference signals of two separate optical interference paths. An interferometer for shape measurement is proposed.

The optical interference measuring method and apparatus of the present invention can obtain an ideal interference signal visibility by adjusting a reference plane, which is a reference, to have a similar light reflectance with respect to a measurement plane in generating an optical path difference to obtain interference light intensity. A method and apparatus for more accurately measuring a heterogeneous reflectance measurement sample having a difference in reflectance by applying a polarization split interferometer having heterogeneous reference planes having different light reflectances.

The configuration and measuring principle of the present invention is shown in FIG.

The white light emitted from the light source 200 becomes parallel light through the condenser lens 201 and enters the polarizer 202.

The parallel light passing through the polarizing device 202 through the condenser lens 201 becomes light in an arbitrary polarization state. That is, the polarizer 202 serves to make the incident parallel light equal in magnitude to the horizontally polarized light and the vertically polarized light.

Incident light having light horizontally and vertically polarized by the polarizer 202 is incident to the secondary luminous flux splitter 204 via the primary luminous flux splitter 203.

The secondary beam splitter 204 separates incident light into the reference planes 210 and 212 and the measurement plane 206. The secondary beam splitter 204 may be selectively configured such that the light intensities separated by the reference planes 210 and 212 and the measurement plane 206 have a ratio of 5: 5, 3: 7, and the like.

The measurement light separated by the secondary light splitter 204 is incident on the measurement surface 206 via the objective lens 205, and the reference light is incident on the polarization splitter 208.

The reference light incident on the polarization splitter 208 is divided into vertically polarized light and horizontally polarized light by the polarization splitter 208 and is incident on the vertical polarization reference plane 210 and the horizontal polarization reference plane 212, respectively. The vertically polarized light is reflected by the vertical polarization reference plane 210 and the horizontally polarized light is reflected by the horizontal polarization reference plane 212.

The vertical polarization reference plane 210 and the horizontal polarization reference plane 212 are composed of reference planes having different reflectances, or the light transmittance control filters 209 and 211 are formed before the vertical polarization reference plane 210 and the horizontal polarization reference plane 212. By adjusting the light transmittance, the reflectance of each reference light is adjusted.

The light transmittance control filters 209 and 211 may be a gray filter (Neutral density filter), or a linear variable gray filter or a variable gray filter wheel having different light transmittance in stages. As a light reflectance control device using a variable light attenuator using a liquid crystal cell that adjusts the light transmittance by adjusting the arrangement of dichroic molecules as an electrical signal. Can be configured.

After passing through the objective lens 205, the measurement light reflected from the measurement surface 206 and the reference light reflected from the respective reference planes 210 and 212 are synthesized on the same optical path to form interference light. The interference information of the measurement plane 206 with respect to the reference plane 210 and the interference information of the measurement plane 206 by the horizontal polarization reference plane 212.

That is, the vertically polarized light reflected from the measurement plane 206 forms an interference signal on the same optical path as the vertically polarized reference light reflected from the vertical polarization reference plane 210, and the horizontally polarized light reflected from the measurement plane 206 An interference signal is formed on the same optical path as the number of polarized reference light reflected from the horizontal polarization reference plane 212.

Interfering light formed from each of the reference planes 210 and 212 is incident on the polarization splitter 214 positioned before the photodetectors 216 and 217 via the primary beam splitter 203 and the imaging lens 213. The imaging lens 213 serves to form an image of the workpiece on the photodetectors 216 and 217.

The polarization splitter 214 located before the photodetectors 216 and 217 serves to separate the vertically polarized and the horizontally polarized interference light by the photodetector 1 216 and the photodetector 2 217, respectively. The interfering signal is detected by the photodetector 1 216 and the photodetector 2 217.

The separated interference signal detected by each of the photodetectors 216 and 217 obtains an interference signal having high visibility by adjusting the reflectances of the vertical polarization reference plane 210 and the horizontal polarization reference plane 212 with respect to the reflectance of the measurement sample.

Photodetector 1 (216) and photodetector 2 (217) while moving at a small interval of several tens of nm in the optical axis direction to obtain an interference signal having the height information of the measurement sample 206 for each reference plane (210, 212). The interfering signal is generated in all pixels of the pixel, and the data obtained from the photodetectors 216 and 217 are combined and analyzed to calculate a three-dimensional shape of the measurement sample.

Fig. 3 shows the effect in the interference signal detection by the optimized optical interference visibility when the reflectance between the reference plane and the measurement plane is set similarly.

Referring to the monochromatic light as an example, the equation of the light waves reflected after the incident light beams are incident on the reference planes 210 and 212 and the measurement plane 206 is as follows.

Er = Aexp (i2kl)

Em = Bexp [i (2k (l + h-z) -α)]

here

Er: reference plane reflected light, Em: workpiece reflected light.

A, B: amplitude of light waves.

k = 2 π / λ: Propagation constant of monochromatic light

α: phase change at the measuring point

l: distance from the beam splitter to the reference plane

h: shape of workpiece

z: Optical axis position of the workpiece.

An interference fringe generated by combining two reflected lights on the same optical path is represented by the following equation.

I = <(Er + Em) ² > = A ² + B ² + 2ABcos (2k (hz) + α) = I _k [1 + vcos (ΔΦ)] Equation 1

here

<>: Time average

I _k = A ² + B ² : average intensity of light

v = (2AB) / (A ² + B ² ): Interference pattern visibility at single wavelength.

ΔΦ = 2k (h-z) + α: Synthetic phase difference due to optical path difference and phase change at measuring point

[Vcos (ΔΦ)] in the above equation is an interference term that exhibits an interference effect, and the brightness of the interference fringe is determined by a visibility function and a cos value.

3 (a) shows light waves (reference light = Er and measurement light = Em) for the reference light and the measurement light when the difference in reflectance between the reference plane and the measurement object is large.

The vertical axis represents the detection size of the photodetector and shows a difference in the detection size due to the difference in reflectance between the reference light and the measurement light at the same light intensity.

These reference light and measurement light are synthesized on the same path to show an interference signal having a small detection size S represented by a dotted line of (c) in accordance with the micro movement in the optical axis direction of the interferometer.

FIG. 3B illustrates light waves (reference light = Er and measurement light = Em) when the reflectance difference between the reference plane and the measurement object is large and the reflectance of the reference light is adjusted to have the same reflectance as the measurement light. That is, when the light intensity of the reference light is adjusted to a light intensity similar to the measurement light, the average light intensity of each light becomes the same.

These reference light and measurement light are synthesized on the same path to show an interference signal having a large detection size L represented by the solid line of (c) in accordance with the small shift in the optical axis direction of the interferometer.

From Equation 1 (I = A ² + B ² + 2ABcos (2k (hz) + α) = I _k [1 + vcos (ΔΦ)])

The maximum illuminance of the interference pattern

I _max = A ² + B ² + 2AB, where constructive interference occurs with ΔΦ = 0 and ± 2nπ (n = 1.2.3 ..).

The minimum illuminance of the interference pattern

I _min = A ² + B ² -2AB, where the extinction interference occurs as ΔΦ = 0 and ± (2n + 1) π (n = 1.2.3 ..).

Interference pattern visibility at single wavelength, v = (2AB) / (A ² + B ² ), has the highest value when the light waves with the same light intensity are synthesized, and the amplitude between the constructive and destructive interference of the interference pattern is maximized. do. In the case of such an interference signal having a large detection size, there is an effect of stable signal detection that excludes the influence of an external noise signal such as camera noise in data acquisition and processing.

In general, the greater the bandwidth of the light source, the lower the visibility of the optical interference fringe is greater than the optical interference visibility at a single wavelength. In other words, in the surface shape measurement apparatus using a white light scanning interferometer using a wide bandwidth light source, the difference in light reflectance between the reference plane and the measurement plane further decreases the visibility of the optical interference, which indicates a smaller detection signal size. In addition, it is sensitive to the influence of external noise signals such as the inherent noise of the camera, resulting in greater error in the measurement result through the interference signal analysis.

In measuring the shape of the workpiece, the light reflectances of the reference light and the measurement light are set equal to each other, and the average light intensity by them is set to half of the camera detection size, so that the maximum amplitude of constructive interference and destructive interference is achieved by the optimized interference signal visibility. When the detection size of the camera is set over the entire area, it is easy to detect a stable signal, excluding the influence of an external noise signal affecting a measurement such as camera noise in the interference signal detection, which measures the surface shape of the workpiece through the interference signal analysis. Minimize the error of the value.

4A is a conceptual diagram illustrating a method for measuring the surface shape of a hetero reflectance measurement sample in which a low reflectance sample and a high reflectance sample coexist in a measurement region.

For the hetero-reflectance measurement sample, in step 1, the interference signal for the low reflection sample and the high reflection sample is separated through the heterogeneous polarization separation reference plane, and then the interference signal for the hetero-reflection sample is obtained through the heterogeneous photodetector (camera). Detect. That is, as an example, the photodetector 1 216 obtains a vertically polarized interference signal for the high reflectance pattern A, and the photodetector 2 217 obtains a horizontal polarized interference signal for the low reflectivity substrate B. Dotted lines in the photodetectors indicate the selected Region of interest for each photodetector.

In step 2, the interference signal obtained through each photodetector is analyzed and the surface shape of the entire measurement sample is calculated by merging the image information between the photodetectors.

Figure 4b shows a first embodiment of the interference signal detection method for measuring the shape of the hetero reflectance measurement sample.

When there is a high reflectance pattern (A) on the low reflectivity substrate (B) (or vice versa, a low reflectance pattern on the high reflectivity substrate), the light source (white light) is adjusted based on the high reflectance pattern (A) and then vertical The reflectance of the vertical polarization reference plane 210 for polarization interference is adjusted to be similar to that of the high reflectance pattern A (adjust the light transmittance adjusting filter 209). In this case, the horizontal polarization reference plane 212 for the horizontal polarization interference is configured by adjusting the reflectance similar to the reflectance of the low reflectivity substrate B (adjusting the light transmittance adjusting filter 211).

The vertically polarized or horizontally polarized interference signal of the vertical polarization reference plane 210 and the horizontal polarization reference plane 212 having the optimized visibility for each sample is determined by the photodetector 1 216 and the photodetector 2 ( 217).

Photodetector 1 216 obtains the vertically polarized interference signal for the high reflectance pattern A, and photodetector 2 217 obtains the horizontally polarized interference signal for the low reflectance substrate B.

When the white light is adjusted based on the high reflectance pattern A and the photodetector 2 217 that obtains the interference data of the low reflectance substrate B has a low sensitivity, it is difficult to detect the signal. By increasing the gain of the gain (Gain) for the interference information on the low reflectivity substrate (B).

That is, as shown in FIG. 4A, the interference signals for the high reflectance pattern A and the low reflectance substrate B are separated in step 1 to obtain interference information in each of the photodetector 1 216 and the photodetector 2 217. In step 2, the step shape of the sample is calculated by merging the images of the individual photodetectors and combining the measurement results.

Figure 4c shows a second embodiment of the interference signal detection method for measuring the shape of the hetero reflectance measurement sample.

The method different from the first embodiment adjusts the white light illumination based on the low reflectance substrate B, and then adjusts the reflectance of the horizontal polarization reference plane 212 similarly to the low reflectance sample B for the horizontal polarization interference. Transmittance adjustment filter 211). At this time, the reflectance of the vertical polarization reference plane 210 for the vertical polarization interference is configured by adjusting (adjusting the light transmittance control filter 209) similar to the reflectance of the high reflectance pattern (A).

The horizontal polarization or vertical polarization interference signals of the horizontal polarization reference plane 212 and the vertical polarization reference plane 210 having optimized visibility for each sample are separated and detected by the photodetector 2 217 and the photodetector 1 216. do.

Photodetector 1 216 obtains the vertically polarized interference signal for the high reflectance pattern A, and photodetector 2 217 obtains the horizontally polarized interference signal for the low reflectance substrate B.

When the white light illumination is adjusted based on the low reflectance substrate B, and the photodetector 1 216 that obtains the interference data of the high reflectance pattern A is saturated due to high light intensity and cannot detect the signal, the photodetector After setting the exposure (Shutter adjustment) of 1 (216) to a detectable area, interference information on the high reflectance pattern (A) is obtained.

That is, as shown in FIG. 4A, the interference signals for the high reflectance pattern A and the low reflectance substrate B are separated in step 1 to obtain interference information in each of the photodetector 1 216 and the photodetector 2 217. In step 2, the step shape of the sample is calculated by merging the images of the individual photodetectors and combining the measurement results.

As described above, for the measurement of dissimilar reflectance

Adjusting the reflectance of the vertical or horizontal polarization reference plane to match the heterogeneous reflectance sample;

Forming an ideal interference signal visibility polarized vertically or horizontally;

Adjusting each photodetector setting to obtain interference information for the heterogeneous reflectance sample;

Obtaining an interference signal from photodetectors 1 and 2 through micromovement of the optical interferometer;

Combining and analyzing the interference signals of each photodetector including shape information on the measurement sample;

The three-dimensional shape of the measurement sample is calculated through the above.

In addition, in the embodiment of the present invention, in order to adjust the reflectance of the vertical or horizontal polarization reference plane to apply a light transmittance control filter, in order to obtain a high-visibility interference signal from each reference plane for heteroreflective samples

Horizontal polarization reference plane reflectivity: Vertical polarization reference plane reflectance = N%: (100-N)%, consisting of a fixed reflectivity reference plane of N = 1.2.3 .. From these reference planes, better visibility interference to the hetero reflectance measurement sample It is also possible to obtain surface shape information by selecting a signal.

The present invention obtains an optimized interference signal visibility by measuring light and reference light by adjusting the reflectance of the reference plane with respect to the measurement plane, and also a heterogeneous reflectance in which a low reflectance sample and a high reflectance sample are mixed in a low reflectance sample or a measurement region. Disadvantages of measuring methods and devices using conventional optical interference in measuring samples are that the interference signal visibility is lowered due to the difference in reflectance between the low reflectance sample and the reference plane and the sensitivity of the photodetector to the heterogeneous reflectance sample is different. Overcome the lack of accurate surface measurements.

That is, the present invention forms an optimized interference signal visibility by adjusting the reference plane reflectance for the low reflectance sample, and selectively separates the interference signal having high visibility for the heterogeneous reflectance measurement sample through the heterogeneous light reflectivity adjustable reference plane. Accordingly, the present invention provides a measuring method and apparatus using optical interference having an active measuring response capable of obtaining interference signals having high visibility with respect to various measurement samples.

Claims (5)

In measuring the surface shape of a heterogeneous reflectance measurement sample in which a low reflectance sample and a high reflectance sample coexist in the measurement region. A light source unit irradiating a light source emitting illumination light to a measurement sample and a heterogeneous reference plane by using a beam splitter; An optical path separator configured to separate the light irradiated onto the heterogeneous reference plane into a vertically polarized light and a horizontally polarized light by a polarization splitter and to enter the vertically polarized reference plane and the horizontally polarized reference plane; An interference signal generation unit configured to form two interference signal visibilitys vertically or horizontally polarized based on the measurement light reflected from the measurement sample and the reference light reflected from the heterogeneous reference plane; A photodetector sensitivity adjusting unit for adjusting a setting of a heterogeneous photodetector (photodetector 1, photodetector 2) so as to obtain vertically or horizontally polarized interference information with respect to the measurement sample; A micro-transfer unit which micro-moves the optical interferometer which forms the interference signal with respect to the measurement sample by using a transfer module; A signal detector having a photo detector for detecting the interference light signal; A method and apparatus for measuring the surface shape using optical interference for analyzing the information on the transport distance obtained from the micro-transporter and the interference signal of the heterogeneous photodetector and merging the interference signal images to calculate the surface shape of the hetero reflectance measurement sample. The heterogeneous reference plane composed of the horizontal polarization reference plane and the vertical polarization reference plane has a surface shape using optical interference including a reflectance control unit for individually adjusting the reflectance of the heterogeneous reference plane to obtain an optimized optical interference visibility for the heterogeneous reflectance measurement sample. Measuring device. 2. The heterogeneous reference plane of claim 1 comprising a horizontal polarization reference plane and a vertical polarization reference plane has a horizontal polarization reference plane reflectance: vertical polarization reference plane reflectance = N%: (100-N)%, where N = 1.2.3 .. Surface shape measuring apparatus using the optical interference is configured. The surface shape measuring apparatus using optical interference according to claim 1, wherein the heterogeneous photodetector is configured with a polarization splitter on an optical path incident to the individual photodetector to obtain vertically or horizontally polarized separated interference information. The surface shape measurement apparatus using optical interference according to claim 1, wherein the light source comprises at least one light source of a multi-wavelength light source and a short wavelength light source.

Images