CN117871044A - Device and method for measuring Littrow diffraction angle - Google Patents
Device and method for measuring Littrow diffraction angle Download PDFInfo
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Abstract
The invention discloses a device for measuring Littrow diffraction angle, belonging to the field of information optics and precision optics measuring instruments; the method comprises the following steps: a laser for emitting incident light; a grating for diffracting incident light to generate grating diffracted light; a receiving screen for receiving the grating diffracted light and exhibiting diffracted spots; a metal wire between the laser and the grating for diffracting incident light; the metal wire is also positioned between the grating and the receiving screen and used for shielding part of grating diffraction light and diffracting the grating diffraction light. The invention also discloses a method for measuring the Littrow diffraction angle. The method aims to solve the problem that the diffraction spot position cannot be accurately judged due to irregular incident light spot shape or larger spot size than the area of the grating to be measured in Littrow angle measurement, and meanwhile, zero position is not required to be set, so that the system error caused by the preset zero position is eliminated.
Description
Technical Field
The invention relates to the field of information optics and precise optical measuring instruments, in particular to a device and a method for measuring a Littrow diffraction angle.
Background
The pitch of the micro-nano grating periodic structure is commonly used for calibrating the magnification of an optical microscope or an electron microscope. The spacing of the micro-nano gratings can be measured by an optical diffraction method, namely, the average spacing of the measured gratings is calculated according to a diffraction equation by measuring the Littrow angle of the micro-nano gratings. Taking a reflective grating as an example, when the diffracted light of the grating is coincident with the incident light, the diffraction angle formed by the diffracted light and the normal line of the grating is Littrow angle, as shown in FIG. 1, wherein θ L I.e. the Littrow angle.
The accuracy of the Littrow angle measurement determines the accuracy of the grating pitch measurement. Measurement of Littrow requires that the diffraction spot be located at the same position of the photodetector each time. The traditional method is to preset a zero position on the detector, namely, when the diffracted light coincides with the incident light, the position of the diffracted light spot on the detector is located. The commonly used null detector is a four-quadrant photodetector. Also, a linear array photoelectric detector is adopted to determine the diffraction light spot position through a cross correlation method; or, a CCD is adopted as a photoelectric detector, and whether the diffraction light spot coincides with a preset zero position is judged through an image processing technology.
Whether a four-quadrant photoelectric detector or a linear array photoelectric detector is adopted or a CCD is used as a zero detector, zero position needs to be preset. While the preset zero position inevitably introduces a systematic error.
In addition, the scheme is only suitable for the condition that the incident light spot is smaller than the area of the grating to be measured. When the incident light spot is larger than the grating area, the diffraction light spot and the incident light spot generate a cross-over difference, so that a zero position cannot be accurately preset.
When the spot of the incident light is not a relatively regular circle, it is difficult to ensure that the diffraction spot irradiates the same position each time. For example, when a frequency doubling light source is used, walk-off effects in the nonlinear crystal may cause frequency doubling spot shape irregularities.
The four-quadrant photoelectric detector, the linear array photoelectric detector or the CCD is adopted as a zero detector, and real-time detection calculation is needed. The requirements on the ambient lighting conditions are met, and the complexity of the measuring device is high.
The Littrow structure requires that the diffracted light is coincident with the incident light, so a light splitting device is required to be placed in the light path to separate the diffracted light returned by the original path from the incident light, the light path is more complex, the adjustment is inconvenient, and more alignment errors are introduced.
Disclosure of Invention
The invention aims to provide a device and a method for efficiently measuring a Littrow diffraction angle.
In order to solve the above technical problems, the present invention provides a device for determining Littrow diffraction angle, comprising:
a laser for emitting incident light;
a grating for diffracting incident light to generate grating diffracted light;
a receiving screen for receiving the grating diffracted light and exhibiting diffracted spots;
a metal wire between the laser and the grating for diffracting incident light; the metal wire is also positioned between the grating and the receiving screen and used for shielding part of grating diffraction light and diffracting the grating diffraction light.
Preferably, the rotary table is further included;
the turntable is fixedly connected with the grating and used for driving the grating to rotate.
Preferably, the fringes of the grating are parallel to the axis of the turntable.
Preferably, the fringes of the grating are parallel to the length direction of the wire.
A method of measuring a Littrow diffraction angle using a device comprising the steps of:
the incident light passes through the metal wire to be diffracted, so that filament diffraction light is generated;
the light of the filament diffraction light irradiates the grating to diffract, and grating diffraction light is generated;
the grating diffracts light and irradiates the receiving screen through the metal wire to form diffraction light spots;
from the diffraction spots, the Littrow diffraction angle was determined.
Preferably, the grating diffracts light and irradiates the receiving screen through the metal wire to form diffraction light spots, and the method specifically comprises the following steps of:
one part of grating diffraction light is shielded by the metal wire, and the other part of grating diffraction light is directly irradiated to the receiving screen to form two grating diffraction spots at intervals; and meanwhile, diffraction light of the grating is diffracted by the metal wire to generate secondary filament diffraction light which irradiates the receiving screen to form a filament diffraction light spot.
Preferably, the Littrow diffraction angle is determined from the diffraction spots, specifically comprising the steps of:
when the 0-order main maximum of the filament diffraction light spot is positioned in the middle of the two grating diffraction light spots, a Littrow diffraction angle is obtained according to the incident angle of incident light.
Preferably, when the 0-order principal maximum of the filament diffraction spot is located right in the middle of the two grating diffraction spots, the Littrow diffraction angle is obtained according to the incident angle of incident light, and specifically comprises the following steps:
the grating is rotated left and right by the turntable, and incident light is respectively incident from two sides of the normal line of the grating;
when the 0-order main maximum of the filament diffraction light spot is positioned in the middle of the two grating diffraction light spots, recording the incident angle of incident light to obtain two Littrow incident angles;
obtaining a Littrow diffraction angle according to an included angle between the two Littrow incidence angles;
preferably, the Littrow diffraction angle = angle between two Littrow angles of incidence/2.
Preferably, the method further comprises the following steps:
the average distance of the gratings is calculated through a diffraction formula, wherein the diffraction formula is as follows:
wherein: θ L Is Littrow diffraction angle; beta is the angle between the incident light and the diffracted light.
Compared with the prior art, the invention has the beneficial effects that:
the invention aims to weaken the problem that the diffraction spot position cannot be accurately judged due to irregular incident light spot shape or larger spot size than the area of the grating to be measured in the measurement of Littrow angle, and meanwhile, zero position is not required to be set, so that the system error introduced by the preset zero position is eliminated, and the method specifically comprises the following steps:
1) The zero point is not required to be preset for Littrow, so that measurement errors caused by inaccurate preset zero point positions are eliminated, and the measurement of Littrow angles is more accurate.
2) The requirement on the laser beam is reduced, and the position of the Littrow angle can be accurately measured even if an irregular light spot is not needed. Thus, although the spot shape is not ideal, the laser with shorter wavelength is used for measuring the grating spacing, so that the capacity of measuring the grating spacing is improved, and smaller spacing can be measured.
Drawings
The following describes the embodiments of the present invention in further detail with reference to the accompanying drawings.
FIG. 1 is a schematic diagram of a Littrow structure diffracted by a reflection grating;
FIG. 2 is a schematic diagram of the structure of the Littrow diffraction angle measuring device of the present invention; a is a top view and b is a side view.
FIG. 3 is a schematic view of a Littrow diffracted beam of a grating passing through a wire and impinging on a receiving screen;
fig. 4 is a schematic diagram of a secondary diffraction spot on a receiving screen.
In the figure: 1-a laser; 2-grating; 3-a receiving screen; 4-wire.
Detailed Description
In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. The present invention may be embodied in many other forms than those herein described, and those skilled in the art will readily appreciate that the present invention may be similarly embodied without departing from the spirit or essential characteristics thereof, and therefore the present invention is not limited to the specific embodiments disclosed below.
The terminology used in the one or more embodiments of the specification is for the purpose of describing particular embodiments only and is not intended to be limiting of the one or more embodiments of the specification. As used in this specification, one or more embodiments and the appended claims, the singular forms "a," "an," and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It should also be understood that the term "and/or" as used in one or more embodiments of the present specification refers to and encompasses any or all possible combinations of one or more of the associated listed items.
It should be understood that, although the terms first, second, etc. may be used in one or more embodiments of this specification to describe various information, these information should not be limited by these terms. These terms are only used to distinguish one type of information from another. For example, a first may also be referred to as a second, and similarly, a second may also be referred to as a first, without departing from the scope of one or more embodiments of the present description. The word "if" as used herein may be interpreted as "at … …" or "at … …" or "responsive to a determination", depending on the context.
The invention is described in further detail below with reference to the attached drawing figures:
an apparatus for determining Littrow diffraction angle comprising:
a laser 1 for emitting incident light;
a grating 2 for diffracting incident light to generate grating diffracted light;
a receiving screen 3 for receiving the grating diffracted light and exhibiting a diffracted spot;
a metal wire 4 between the laser 1 and the grating 2 for diffracting incident light; the metal wire 4 is also positioned between the grating 2 and the receiving screen 3 and is used for shielding part of grating diffraction light and diffracting the grating diffraction light.
Preferably, the rotary table is further included;
the turntable is fixedly connected with the grating 2 and is used for driving the grating 2 to rotate.
Preferably, the fringes of the grating 2 are parallel to the axis of the turntable.
Preferably, the fringes of the grating 2 are parallel to the length direction of the wire 4.
A method of measuring a Littrow diffraction angle using a device comprising the steps of:
the incident light is diffracted through the metal wire 4 to generate filament diffracted light;
the light of the filament diffraction light irradiates the grating 2 to diffract, and grating diffraction light is generated;
the grating diffraction light irradiates the receiving screen 3 through the metal wire 4 to form diffraction light spots;
from the diffraction spots, the Littrow diffraction angle was determined.
Preferably, the grating diffracts light and irradiates the receiving screen 3 through the metal wire 4 to form a diffraction light spot, and specifically comprises the following steps:
part of grating diffraction light is shielded by the metal wire 4, and the other part of grating diffraction light is directly irradiated to the receiving screen 3 to form two grating diffraction spots at intervals; and meanwhile, the grating diffraction light is diffracted through the metal wire 4 to generate secondary filament diffraction light which irradiates the receiving screen 3 to form filament diffraction light spots.
Preferably, the Littrow diffraction angle is determined from the diffraction spots, specifically comprising the steps of:
when the 0-order main maximum of the filament diffraction light spot is positioned in the middle of the two grating diffraction light spots, a Littrow diffraction angle is obtained according to the incident angle of incident light.
Preferably, when the 0-order principal maximum of the filament diffraction spot is located right in the middle of the two grating diffraction spots, the Littrow diffraction angle is obtained according to the incident angle of incident light, and specifically comprises the following steps:
the grating 2 is rotated left and right by the turntable, and incident light is respectively incident from two sides of the normal line of the grating 2;
when the 0-order main maximum of the filament diffraction light spot is positioned in the middle of the two grating diffraction light spots, recording the incident angle of incident light to obtain two Littrow incident angles;
obtaining a Littrow diffraction angle according to an included angle between the two Littrow incidence angles;
preferably, the Littrow diffraction angle = angle between two Littrow angles of incidence/2.
Preferably, the method further comprises the following steps:
the average spacing of the gratings 2 is calculated by a diffraction formula:
wherein: θ L Is Littrow diffraction angle; beta is the angle between the incident light and the diffracted light.
The invention adopts the secondary metal wire 4 diffraction alignment technology to realize accurate measurement of Littrow angle. The first-order diffraction light in the incident light spot is selected through the first-order diffraction of the metal wire 4, so that the effect of shaping the incident light spot is achieved. When the primary diffracted light of the wire 4 passes through the wire 4 again after being diffracted by the grating 2, the primary diffracted light is diffracted by the wire 4 for the second time while being blocked by the wire 4, and a diffraction main maximum is formed in a shadow area behind the wire 4. When the diffraction main pole is located at the center of the shadow area, the diffracted light of the grating 2 forms Littrow diffraction with the incident light.
In order to better illustrate the technical effects of the present invention, the present invention provides the following specific embodiments to illustrate the above technical flow:
example 1, a method for determining the diffraction angle of Littrow, a modified Littrow structure for measuring the pitch of grating 2, as shown in fig. 2, diffracts light slightly lower than the incident light. The grating 2 is arranged on the angle measuring turntable, the surface of the grating 2 coincides with the axis of the turntable, and taking one-dimensional grating 2 as an example, the stripes of the grating 2 are parallel to the axis of the turntable. Incident light passes through the turntable axis and makes an angle beta/2 with the turntable surface, impinging on the grating 2 surface. When the turntable rotates, the diffraction direction of the diffracted light is changed along with the rotation of the turntable due to the change of the incident angle. When the incident angle is Littrow, the diffracted light is formed slightly lower than the incident light and forms an angle beta with the incident light. The plane formed by the incident light and the diffracted light coincides with the turntable axis as shown in fig. 2. A metal wire 4 with the length direction parallel to the stripes of the grating 2 is placed in the incident light path. Under Littrow conditions, the diffracted light also passes through the wire 4. The receiving screen 3 is placed a suitable distance behind the wire 4.
To form the ideal diffraction of the wire 4, the wire 4 should be straight and the surface of the wire 4 should be smooth. The metal wire 4 adopted in the embodiment is a copper wire, and other metal wires 4 which are convenient for plasticity and have a certain hardness can be used, so that the straightness of the metal wire 4 can be maintained.
According to the diffraction theory, the smaller the diameter of the metal wire 4 is, the wider the diffraction 0-order main maximum light spot is formed. The diameter of the wire 4 should be chosen so that the divergence angle of the 0 th order light diffracted by the wire 4 for the first time is not too large. Also, too thin a wire 4 will result in too wide a 0 th order spot in the diffraction of the secondary wire 4, which is wider than the shadow that the primary diffracted light leaves on the receiving screen 5 through the wire 4.
The further the wire 4 is from the grating, the higher the accuracy of the Littrow angle measurement. However, if the wire 4 is too far from the grating, the 0 th order diffracted light once diffracted by the wire 4 may reach the grating, resulting in a decrease in light intensity due to too much dispersion.
The proper distance between the metal wire 4 and the receiving screen 5 should be kept, so that not only the main maximum light spot of the second diffraction order 0 can be observed on the receiving screen 5, but also two separated light spots of Littrow diffraction light, which are formed by shielding by the metal wire 4, can be observed. If the wire 4 is too close to the receiving screen 5, the second diffraction phenomenon cannot be observed on the receiving screen 5, and only Littrow diffraction spots blocked by the wire 4 can be observed. If the wire 4 is located far from the receiving screen 5, only the main maximum light spot of the second diffraction order 0 can be observed on the receiving screen 5, and the Littrow diffraction light spot blocked by the wire 4 cannot be observed.
Regardless of the shape of the incident spot, diffraction occurs as incident light passes through the wire 4. The 0 th order main maximum of the filament diffracted light becomes a relatively regular stripe spot, and the 0 th order diffracted light diffracted by the wire 4 is irradiated on the grating 2 along the direction of the incident light, and is diffracted by the grating 2, under the influence of the shape of the wire 4. As shown in fig. 3, when the Littrow diffraction beam of the grating 2 passes through the metal wire 4 again and irradiates on the receiving screen behind the metal wire 4, the light spot center on the receiving screen 3 is blocked by the metal wire 4, a shadow of the metal wire 4 is formed, and the Littrow diffraction light spots which are not blocked by the metal wire 4 are arranged on two sides of the shadow. The main maximum of 0 order formed by diffraction of the Littrow diffraction beam by the metal wire 4 occurs in the center of a shadow area in the center of the Littrow diffraction spot. At this time, the incident angle of the incident light is Littrow angle.
And rotating the angle measuring turntable to enable incident light to be incident from two sides of the normal line of the grating 2 respectively and form a Littrow structure, and recording the two angular positions. The Littrow angle 2 times, namely 2 theta, can be obtained by subtracting the two angle positions L . By diffraction formulaThe average pitch of the diffraction grating 2 can be calculated.
Wherein: lambda is the wavelength of the incident light.
The invention is applied to a national measurement standard device, namely a nano grating 2 line distance sample plate calibrating device. The device uses a 633nm He-Ne laser doubled 316.4nm UV light source to extend the minimum measurement capability of the device from 3000 lines/mm (333 nm) to 5900 lines/mm (169 nm). The reflection grating 2 developed by the laser convergence deposition technology is measured by the device, and the interval is 212.8nm.
In the several embodiments provided by the present invention, it should be understood that the disclosed apparatus and method may be implemented in other manners. For example, the above-described apparatus embodiments are merely illustrative, and the division of modules, or units is merely a logical function division, and there may be additional divisions when actually implemented, e.g., multiple units, modules, or components may be combined or integrated into another apparatus, or some features may be omitted, or not performed.
Claims (10)
1. An apparatus for determining Littrow diffraction angle comprising:
a laser (1) for emitting incident light;
a grating (2) for diffracting incident light to generate grating diffracted light;
a receiving screen (3) for receiving the grating diffracted light and exhibiting diffracted spots;
a metal wire (4) located between the laser (1) and the grating (2) for diffracting incident light; the metal wire (4) is also positioned between the grating (2) and the receiving screen (3) and used for shielding part of grating diffraction light and diffracting the grating diffraction light.
2. The apparatus for determining the Littrow diffraction angle of claim 1, further comprising a turntable;
the turntable is fixedly connected with the grating (2) and is used for driving the grating (2) to rotate.
3. The device for determining the Littrow diffraction angle according to claim 2, wherein:
the stripes of the grating (2) are parallel to the axis of the turntable.
4. The apparatus for measuring Littrow diffraction angle as claimed in claim 3, wherein:
the stripes of the grating (2) are parallel to the length direction of the metal wires (4).
5. A method of measuring a Littrow diffraction angle using the apparatus for measuring Littrow diffraction angle according to any one of claims 1 to 4, comprising the steps of:
the incident light passes through the metal wire (4) to be diffracted, so as to generate filament diffraction light;
the filament diffraction light irradiates the grating (2) to diffract, and grating diffraction light is generated;
the grating diffraction light passes through the metal wire (4) and irradiates on the receiving screen (3) to form diffraction light spots;
from the diffraction spots, the Littrow diffraction angle was determined.
6. The method of measuring a Littrow diffraction angle apparatus as claimed in claim 5, wherein the grating diffracted light is irradiated onto the receiving screen (3) through the wire (4) to form a diffracted light spot, comprising the steps of:
part of grating diffraction light is shielded by the metal wire (4), and the other part of grating diffraction light is directly irradiated to the receiving screen (3) to form two grating diffraction light spots at a certain distance; and meanwhile, diffraction light of the grating is diffracted through the metal wire (4) to generate secondary filament diffraction light which irradiates the receiving screen (3) to form filament diffraction light spots.
7. The method of measuring a Littrow diffraction angle by the apparatus for measuring Littrow diffraction angle as claimed in claim 6, wherein the Littrow diffraction angle is measured from the diffraction spot, specifically comprising the steps of:
when the 0-order main maximum of the filament diffraction light spot is positioned in the middle of the two grating diffraction light spots, a Littrow diffraction angle is obtained according to the incident angle of incident light.
8. The method of measuring Littrow diffraction angle of the apparatus of claim 7, wherein the Littrow diffraction angle is obtained according to an incident angle of incident light when the 0-order principal maxima of the filament diffraction spots are located right in between the two grating diffraction spots, comprising the steps of:
the grating (2) is rotated left and right by the turntable, and incident light is respectively incident from two sides of the normal line of the grating (2);
when the 0-order main maximum of the filament diffraction light spot is positioned in the middle of the two grating diffraction light spots, recording the incident angle of incident light to obtain two Littrow incident angles;
and obtaining the Littrow diffraction angle according to the included angle between the two Littrow incidence angles.
9. The method for measuring a Littrow diffraction angle according to claim 8, wherein:
the Littrow diffraction angle = the angle between two Littrow angles of incidence/2.
10. The method for measuring the Littrow diffraction angle of the device according to claim 9, further comprising the steps of:
the average distance of the grating (2) is calculated by a diffraction formula, wherein the diffraction formula is as follows:
wherein: θ L Is Littrow diffraction angle; beta is the included angle between the incident light and the diffracted light; lambda is the wavelength of the incident light.
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| CN115325941A (en) * | 2022-08-09 | 2022-11-11 | 游学威 | Error compensation method and system for grating ruler |
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| NL1033095A1 (en) * | 2005-12-23 | 2007-06-26 | Agilent Technologies Inc | Littrow interferometer. |
| US20080251738A1 (en) * | 2007-04-13 | 2008-10-16 | Cymer, Inc. | Method and apparatus for vibration reduction in laser system line narrowing unit wavelength selection optical element |
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