CN102645853A - Diffractive annular illumination generator and method for manufacturing same - Google Patents

Abstract

Disclosed are a diffractive annular illumination generator used for an off-axis illuminating system of an ultraviolet lithography machine and a method for manufacturing the diffractive annular illumination generator. The diffractive annular illumination generator is characterized by comprising a first diffractive element and a second diffractive element, the surface of the first diffractive element and the surface of the second diffractive element are perpendicular to an optical axis, the first diffractive element transforms a parallel light beam into an annular divergent light beam, an included angle theta is formed between a divergent light beam generated by the annular light beam and the optical axis, and the second diffractive element transforms the annular divergent light beam into an annular parallel light beam. The light intensity distribution size of a pupil plane can be continuously adjusted, the diffractive annular illumination generator and the method have the advantages of simple structure and mature processing technology, and production cost of a lithography machine can be effectively reduced.

Description

Diffraction type ring illumination generator and preparation method thereof

Technical field

The present invention relates to lithographic equipment, particularly a kind of ultraviolet photolithographic machine off-axis illumination system that is used for is regulated diffraction type ring illumination generator of pupil plane light distribution size and preparation method thereof continuously.

Background technology

In the illuminator of advanced litho machine, need strengthen the resolution of etching system according to the structure and the suitable pupil plane illumination light intensity distributed dimension of size Selection of mask figure, increasing depth of focus, and improve the photoetching contrast, thus improve photoetching quality.

The conciliation of pupil plane light distribution size at present adopts the conical mirror group that is made up of two refractions of concave, convex conical mirror to realize.Two conical mirrors are arranged along the direction of optical axis, and place perpendicular to optical axis, through changing the continuous adjusting of light intensity distributed dimension on the distance realization pupil plane between two conical mirrors.The shortcoming of this method is that the processing of refraction conical mirror difficult completion, the particularly processing of concave surface conical mirror all also are difficult to realize both at home and abroad.Reason is that the processing of conical mirror need be accomplished by grinding with glossing, and the processing of concave surface conical mirror central area is difficult point always, and the ultraviolet photolithographic system material that can utilize is limited in addition; The fused quartz price is lower; But be difficult to accomplish accurate face type processing, the hardness of calcium fluoride material is higher, and is favourable to processing; But price is very expensive, is unfavorable for the reduction of photoetching complete machine cost.

Formerly among technology " ring-shaped light spot conversion mirror " (referring to patent CN 2163387Y), disclose a kind of ring-shaped light spot and transformed mirror, utilized the refraction conical mirror to produce the hot spot of annular.Less demanding Laser Processing can meet the demands this method for surface figure accuracy, but receives the restriction of processing technology, and difficulty is transplanted in the lithographic equipment.

Summary of the invention

The objective of the invention is to overcome the above-mentioned deficiency of technology formerly; A kind of diffraction type ring illumination generator that is used for ultraviolet photolithographic machine off-axis illumination system and preparation method thereof is provided; This diffraction type ring illumination generator substitutes traditional refraction conical mirror group, can regulate pupil plane light distribution size continuously, and simple in structure; The characteristics that processing technology is ripe can effectively reduce the production cost of litho machine.Be applicable to the diffraction type ring illumination generator of any ultraviolet light wave band.

Technical solution of the present invention is following:

A kind of diffraction type ring illumination generator; Characteristics are that its formation comprises first diffraction element and second diffraction element of Surface Vertical in optical axis; First diffraction element is transformed into parallel beam the divergent beams that become a generation with optical axis become angle theta with optical axis annular divergent beams; Second diffraction element is transformed into the annular parallel beam with the annular divergent beams; Through regulating the continuous adjusting of the distance realization output beam size between two diffraction elements, the ultimate range between first diffraction element and second diffraction element is that d is confirmed by formula;

${\mathrm{\φ}}_{\mathrm{out}}-{\mathrm{\φ}}_{\mathrm{in}}=2d\sin \mathrm{\θ}\·\left[\frac{\cos {\mathrm{\θ}}_1}{\cos (\mathrm{\θ}+{\mathrm{\θ}}_1)}\right]$

Wherein: θ ₁Be intermediate parameters, satisfy equation: sin (θ+θ ₁)/sin (θ ₁)=n, n are the material refractive index of first diffraction element and second diffraction element under the illuminating bundle wavelength X,

Be the bore of incident beam, Be the external diameter of annular outgoing beam, the scope of choosing of angle theta is 10 °～60 °;

Described first diffraction element and second diffraction element constitute by regularly arranged rectangular cells, and the transmitance of each unit is identical, only has certain phasic difference, and described first diffraction element is that the length of side is D ₁Square transparent panel, the initial point of rectangular coordinate is positioned at the center of first diffraction element, the plane of incidence is the plane, exit facet has the position and distributes mutually, its phase distribution matrix does

Wherein: x and y represent rectangular coordinate, and choosing of x and y coordinate is at-D ₁/ 2 to D ₁Be divided into 2 in/2 the scope ^NIndividual, N is an integer, chooses 10 to 12 usually, and D ₁/ 2 ^NAt least greater than 1 μ m,, mod{} representes right

Get remainder divided by 2 π;

Described second diffraction element is that the length of side is D ₂Square transparent panel, the initial point of rectangular coordinate system is positioned at the center of second diffraction element, the plane of incidence has its position and distributes mutually, exit facet is the plane, its phase distribution matrix does

${\mathrm{\φ}}_2(x,y)=\mathrm{mod}\left\{\right[-\sqrt{x^2+y^2}\·\tan \left({\mathrm{\θ}}_1\right)]\·(n-1)/\mathrm{\λ},2\mathrm{\π}\};$

Wherein: x and y represent rectangular coordinate, and choosing of x and y coordinate is at-D ₂/ 2 to D ₂Be divided into 2 in/2 the scope ^NIndividual, N is an integer, and mod{} representes right

Get remainder divided by 2 π.

Described illuminating bundle wavelength X comprises 365nm, 248nm, 193nm, even shorter wavelength.

The preparation method of described diffraction type ring illumination generator, its characteristics are that this method comprises the following steps:

1. the bore that comprises illuminating bundle wavelength X, incident beam according to the actual conditions of litho machine Maximum outgoing beam external diameter

The refractive index n of the base material of selected first diffraction element and second diffraction element; Confirm the divergent beams of first diffraction element generation and the angle theta of optical axis, the scope of choosing of angle theta is pressed sin (θ+θ after being 10 °～60 ° selected angle theta ₁)/sin (θ ₁)=n is calculated as intermediate parameters θ ₁

2. calculate the ultimate range d between first diffraction element and second diffraction element by following formula:

${\mathrm{\φ}}_{\mathrm{out}}-{\mathrm{\φ}}_{\mathrm{in}}=2d\sin \mathrm{\θ}\·\left[\frac{\cos {\mathrm{\θ}}_1}{\cos (\mathrm{\θ}+{\mathrm{\θ}}_1)}\right];$

3. calculate the position phase distribution matrix of described first diffraction element by following formula

Wherein, x and y represent rectangular coordinate, and n is the refractive index of material, θ ₁Be the intermediate parameters that 1. step tries to achieve, mod{} representes right

Get remainder divided by 2 π;

4. confirm the position phase distribution matrix

of described second diffraction element by following formula

Wherein, X and y represent rectangular coordinate; N is the refractive index of material, and mod{} representes

got remainder divided by 2 π;

5. adopt existing ripe lithography process prepared to have first diffraction element and second diffraction element that described position distributes mutually;

6. assemble described diffraction type ring illumination generator.

With compared with techniques formerly, the present invention has following technological merit:

(1) the present invention adopts diffraction optics method to realize the ring illumination generator; Because the very thin thickness of diffraction element; Help reducing litho machine complete machine size, can effectively avoid the constructive interference of traditional conical mirror group simultaneously, simplified corresponding structure with other litho machine parts.

(2) first diffraction element of the present invention and second diffraction element can utilize existing photoetching process to process; Have the advantages that processing is ripe, machining precision guarantees easily; In addition; Can on the lower fused quartz material of price, realize the processing of diffraction type ring illumination generator, effectively reduce the manufacturing cost of litho machine.

Description of drawings

Fig. 1 is the fundamental diagram of diffraction type ring illumination generator of the present invention.

Fig. 2 is the schematic diagram of diffraction type ring illumination generator of the present invention.

Fig. 3 is the simulation result of one embodiment of the invention.

Embodiment

Below in conjunction with accompanying drawing and instance the present invention is further described, but should limit protection scope of the present invention with this.

See also Fig. 1 earlier, Fig. 1 is the fundamental diagram of diffraction type ring illumination generator of the present invention, is used for producing the required ring illumination pupil of ultraviolet photolithographic machine and distributes.Visible by figure; Diffraction type ring illumination generator of the present invention; Comprise first diffraction element 102 and second diffraction element 103 of Surface Vertical in optical axis; First diffraction element 102 is transformed into parallel beam the divergent beams that become a generation with optical axis become angle theta with optical axis annular divergent beams; Second diffraction element 103 is transformed into the annular parallel beam with the annular divergent beams, and through regulating the continuous adjusting of the distance realization output beam size between two diffraction elements, the ultimate range between first diffraction element 102 and second diffraction element 103 is that d is confirmed by formula;

${\mathrm{\φ}}_{\mathrm{out}}-{\mathrm{\φ}}_{\mathrm{in}}=2d\sin \mathrm{\θ}\·\left[\frac{\cos {\mathrm{\θ}}_1}{\cos (\mathrm{\θ}+{\mathrm{\θ}}_1)}\right]$

Wherein: θ ₁Be intermediate parameters, satisfy equation: sin (θ+θ ₁)/sin (θ ₁)=n, n are the material refractive index of first diffraction element 102 and second diffraction element 103 under the illuminating bundle wavelength X,

Be the bore of incident beam, Be the external diameter of annular outgoing beam, it is 10 °～60 ° that angle theta is chosen scope;

Described first diffraction element 102 and second diffraction element 103 constitute by regularly arranged rectangular cells, and the transmitance of each unit is identical, only has certain phasic difference, and described first diffraction element 102 is that the length of side is D ₁Square transparent panel, the initial point of rectangular coordinate is positioned at the center of first diffraction element 102, the plane of incidence is the plane, exit facet has the position and distributes mutually, its phase distribution matrix does

Wherein: x and y represent rectangular coordinate, and choosing of x and y coordinate is at-D ₁/ 2 to D ₁Be divided into 2 in/2 the scope ^NIndividual, N is an integer, chooses 10 to 12 usually, and D ₁/ 2 ^NAt least greater than 1 μ m,, mod{} representes right

Get remainder divided by 2 π;

Described second diffraction element 103 is that the length of side is D ₂Square transparent panel, the initial point of rectangular coordinate system is positioned at the center of second diffraction element 103, the plane of incidence has its position and distributes mutually, exit facet is the plane, its phase distribution matrix does

${\mathrm{\φ}}_2(x,y)=\mathrm{mod}\left\{\right[-\sqrt{x^2+y^2}\·\tan \left({\mathrm{\θ}}_1\right)]\·(n-1)/\mathrm{\λ},2\mathrm{\π}\};$

Wherein: x and y represent rectangular coordinate, and choosing of x and y coordinate is at-D ₂/ 2 to D ₂Be divided into 2 in/2 the scope ^NIndividual, N is an integer, and mod{} representes right

Get remainder divided by 2 π.

First diffraction element 102 and second diffraction element 103 are placed the center of two diffraction elements and optical axis coincidence perpendicular to optical axis.Incident beam is circular illumination light field 101, behind first diffraction element 102 and second diffraction element 103, in output face 104, obtains the illumination light field 105 of annular.The one side of first diffraction element 102 is the plane, and another side contains the position and distributes mutually, and for example it distributes surface structure figure mutually shown in 106 among the figure.The one side of second diffraction element 103 is the plane, and another side contains the position and distributes mutually, and for example the position distributes surface structure figure mutually shown in 107 among the figure.During the work of diffraction type ring illumination generator, through regulating the distance between first diffraction element 102 and second diffraction element 103, maximum is no more than d, realizes the continuous adjusting of ultraviolet photolithographic machine off-axis illumination system pupil plane light distribution size.

Fig. 2 is the schematic diagram of diffraction type ring illumination generator of the present invention.Phantom line segments AE equivalence among the figure is first diffraction element 102; Its function is that parallel beam is transformed into the annular divergent beams that become a special angle with optical axis; Phantom line segments BC equivalence among the figure is second diffraction element 103, and its function is that annular divergent beams are transformed into annular parallel beam.

Bore For light beam through first diffraction element 102 after, produce with optical axis included angle be the annular divergent beams of θ, through behind second diffraction element 103, be transformed into annular parallel beam again, its external diameter does

Among the figure, line segment AE is parallel with BC, and with the angle of line segment BD be θ ₁, line segment BD is perpendicular to optical axis.According to the plane geometry principle, we can obtain following relation:

BC＝BD/cosθ ₁；

BC/AC＝sinθ/sin(90°-θ-θ ₁)；

Wherein BC is the length of line segment BC; BD is the length of line segment BD; And

AC is the length of line segment AC, and AC=d.Through these two relational expressions, we can obtain:

${\mathrm{\φ}}_{\mathrm{out}}-{\mathrm{\φ}}_{\mathrm{in}}=2d\sin \mathrm{\θ}\·\left[\frac{\cos {\mathrm{\θ}}_1}{\cos (\mathrm{\θ}+{\mathrm{\θ}}_1)}\right]---\left(1\right)$

In addition, according to the refraction law of geometrical optics, can obtain:

sin(θ+θ ₁)/sin(θ ₁)＝n； (2)

In conjunction with above-mentioned two relational expressions; We have just set up the angle theta of divergent beams and optical axis and the relation of the ultimate range d between first diffraction element (102) and second diffraction element (103), the source of two formula of Here it is this patent diffraction type ring illumination generator.

Secondly; The ultimate principle of following geometrical optics deflecting light beams direction according to the direction of diffraction optics principal maximum diffracted beam is [referring to strongly fragrant road silver peace talk permanent English; Engineering optics, China Machine Press, 1999; The 247-251 page or leaf], make up the position distribution matrix mutually of first diffraction element 102 and second diffraction element 103.Regard last every the coordinate of line segment AE and CF as equivalence position mutually, respectively they are carried out discretize and handle, and be normalized in 0～2 π, thereby obtain a mutually distribution matrix of first diffraction element 102 and second diffraction element 103 at optical axis direction.Concrete computing formula is following: the position phase distribution matrix of first diffraction element 102 is

Wherein: x and y represent rectangular coordinate, and choosing of x and y coordinate is at-D ₁/ 2 to D ₁Be divided into 2 in/2 the scope ^NIndividual, N is an integer, chooses 10 to 12 usually, and D ₁/ 2 ^NAt least greater than 1 μ m,, mod{} representes right

Get remainder divided by 2 π;

The position phase distribution matrix of described second diffraction element 103 is

${\mathrm{\φ}}_2(x,y)=\mathrm{mod}\left\{\right[-\sqrt{x^2+y^2}\·\tan \left({\mathrm{\θ}}_1\right)]\·(n-1)/\mathrm{\λ},2\mathrm{\π}\}---\left(4\right)$

Wherein: x and y represent rectangular coordinate, and choosing of x and y coordinate is at-D ₂/ 2 to D ₂Be divided into 2 in/2 the scope ^NIndividual, N is an integer, and mod{} representes right

Get remainder divided by 2 π.

Through a specific embodiment, further specify the structure of the diffraction type ring illumination generator of using in the ultraviolet photolithographic illuminator below.At first confirm following parameter: ultraviolet light beam wavelength X=248nm; Fused quartz material refractive index n=1.5 under this wavelength, the bore of incident beam 101:

maximum outgoing beam 105 external diameters

are then according to following steps design diffraction type ring illumination generator 102 and 103:

1. confirm divergent beams and the angle theta of optical axis and the ultimate range d between first diffraction element 102 and second diffraction element 103 by 102 generations of first diffraction element

Both relations can be represented by following system of equations:

${\mathrm{\φ}}_{\mathrm{out}}-{\mathrm{\φ}}_{\mathrm{in}}=2d\sin \mathrm{\θ}\·\left[\frac{\cos {\mathrm{\θ}}_1}{\cos (\mathrm{\θ}+{\mathrm{\θ}}_1)}\right];$

sin(θ+θ ₁)/sin(θ ₁)＝n；

Wherein, θ ₁Be intermediate parameters.Through above-mentioned system of equations, and choose the angle theta of suitable divergent beams and optical axis, ultimate range d and intermediate parameters θ between first diffraction element (102) and second diffraction element (103) according to practical structure restriction and processing conditions ₁Here choosing θ is 18.59 °, then θ ₁Being 30 °, is 24mm apart from d accordingly.

2. confirm the position phase distribution matrix of described first diffraction element 102

First diffraction element is selected square structure for use, its bore D ₁D ₁Should be slightly larger than the bore of incident beam

Present embodiment is elected 21mm x 21mm as.Its position distributes mutually can be expressed as a two-dimensional matrix, and can try to achieve through following formula:

Wherein, x and y represent rectangular coordinate, and n is the refractive index of material, θ ₁Be the intermediate parameters that 1. step tries to achieve, mod{} representes right

Get remainder divided by 2 π.Choosing of x and y coordinate is in the scope of 10.5mm, to be divided into 2 at-10.5mm ^NIndividual, present embodiment N gets 11, then D/2 ^N=10.25 μ m are greater than 1 μ m.X that substitution is different and y coordinate can obtain a two-dimensional matrix, and this matrix is exactly the position phase distribution matrix

of first diffraction element (102)

3. confirm the position phase distribution matrix

of described second diffraction element 103

Second diffraction element is selected square structure for use, its bore D ₂D ₂Should be slightly larger than the external diameter of outgoing beam

Present embodiment is elected 42mm x 42mm as.Its position distributes mutually can be expressed as a two-dimensional matrix, and can try to achieve through following formula:

Wherein, x and y represent rectangular coordinate, and n is the refractive index of material, θ ₁Be the intermediate parameters that 1. step tries to achieve, mod{} representes right

Get remainder divided by 2 π.M is a constant, is relative because the position of calculating distributes mutually, so the M here can be taken as zero.Choosing of x and y coordinate is in the scope of 21mm, to be divided into 2 at-21mm ^NIndividual, identical in 3. of N value and said step.X that substitution is different and y coordinate can obtain a two-dimensional matrix, and this matrix is exactly the position phase distribution matrix

of second diffraction element 103

4. design is accomplished

Through obtaining position distribution matrix

and and the ultimate range d mutually of described first diffraction element 102 and second diffraction element 103, just accomplished required design.During work, through regulating the distance between described first diffraction element 102 and second diffraction element 103, maximum is no more than d, realizes the continuous adjusting of ultraviolet photolithographic machine off-axis illumination system pupil plane light distribution size.

The simulation result that has shown present embodiment among Fig. 3, emulation is based on fresnel diffraction theory [referring to the permanent English of strongly fragrant road silver peace talk, engineering optics, China Machine Press, 1999,247-251 page or leaf].The 301st, the surface of intensity distribution of incident beam, the gray scale of figure is represented light intensity, and incident beam is circular light distribution.When the distance of first diffraction element 102 and second diffraction element 103 is 8mm; In output face, obtain annular light distribution 302, when distance is 16mm, in output face, obtain annular light distribution 303; When distance is 24mm, in output face, obtain annular light distribution 304.Therefrom can find; Change along with distance between first diffraction element 102 and second diffraction element 103; The external diameter of ring illumination light field is in continuous expansion in the output face; And the endless belt width of ring illumination field does not change with the variation of distance between first diffraction element 102 and second diffraction element 103, remains the radius value of the circular light field of incident.Above-mentioned 2 these diffraction element groups of proof have realized the function of ring illumination generator.

With compared with techniques formerly, the present invention has following technological merit:

(1) the present invention adopts the method for diffraction optics to realize the ring illumination generator; Because the very thin thickness of diffraction element; Help reducing litho machine complete machine size, can effectively avoid the constructive interference of traditional conical mirror group simultaneously, simplify corresponding structure design with other litho machine parts.

(2) the diffraction element group of the present invention's design utilizes existing photoetching process to realize; Have the advantages that processing is ripe, machining precision guarantees easily; In addition, can on the lower fused quartz material of price, realize the processing of diffraction type ring illumination generator, effectively reduce the manufacturing cost of litho machine.

Claims (3)

1. diffraction type ring illumination generator; Be characterised in that its formation comprises first diffraction element (102) and second diffraction element (103) of Surface Vertical in optical axis; First diffraction element (102) is transformed into parallel beam the divergent beams that become a generation with optical axis become angle theta with optical axis annular divergent beams; Second diffraction element (103) is transformed into the annular parallel beam with the annular divergent beams; Through regulating the continuous adjusting of the distance realization output beam size between two diffraction elements, the ultimate range between first diffraction element (102) and second diffraction element (103) is that d is confirmed by formula;

${\mathrm{\φ}}_{\mathrm{out}}-{\mathrm{\φ}}_{\mathrm{in}}=2d\sin \mathrm{\θ}\·\left[\frac{\cos {\mathrm{\θ}}_1}{\cos (\mathrm{\θ}+{\mathrm{\θ}}_1)}\right]$

Wherein: θ ₁Be intermediate parameters, satisfy equation: sin (θ+θ ₁)/sin (θ ₁)=n, n are the material refractive index of first diffraction element (102) and second diffraction element (103) under the illuminating bundle wavelength X, Be the bore of incident beam,

Be the external diameter of annular outgoing beam, the scope of choosing of angle theta is 10 °～60 °;

Described first diffraction element (102) and second diffraction element (103) constitute by regularly arranged rectangular cells, and the transmitance of each unit is identical, only has certain phasic difference, and described first diffraction element (102) is that the length of side is D ₁Square transparent panel, the initial point of rectangular coordinate is positioned at the center of first diffraction element (102), the plane of incidence is the plane, exit facet has the position and distributes mutually, its phase distribution matrix does

Wherein: x and y represent rectangular coordinate, and choosing of x and y coordinate is at-D ₁/ 2 to D ₁Be divided into 2 in/2 the scope ^NIndividual, N is an integer, chooses 10 to 12 usually, and D ₁/ 2 ^NAt least greater than 1 μ m,, mod{} representes right Get remainder divided by 2 π;

Described second diffraction element (103) is that the length of side is D ₂Square transparent panel, the initial point of rectangular coordinate system is positioned at the center of second diffraction element (103), the plane of incidence has the position and distributes mutually, exit facet is the plane, its phase distribution matrix does

${\mathrm{\φ}}_2(x,y)=\mathrm{mod}\left\{\right[-\sqrt{x^2+y^2}\·\tan \left({\mathrm{\θ}}_1\right)]\·(n-1)/\mathrm{\λ},2\mathrm{\π}\};$

Wherein: x and y represent rectangular coordinate, and choosing of x and y coordinate is at-D ₂/ 2 to D ₂Be divided into 2 in/2 the scope ^NIndividual, N is an integer, and mod{} representes right

Get remainder divided by 2 π.

2. diffraction type ring illumination generator according to claim 1 is characterized in that described illuminating bundle wavelength X comprises 365nm, 248nm, 193nm, even shorter wavelength.

3. the preparation method of the described diffraction type ring illumination of claim 1 generator is characterized in that this method comprises the following steps:

1. the bore that comprises illuminating bundle wavelength X, incident beam according to the actual conditions of litho machine Maximum outgoing beam external diameter The refractive index n of the base material of selected first diffraction element and second diffraction element; Confirm the divergent beams of first diffraction element (102) generation and the angle theta of optical axis, the scope of choosing of angle theta is pressed sin (θ+θ after being 10 °～60 ° selected angle theta ₁)/sin (θ ₁)=n is calculated as intermediate parameters θ ₁

2. calculate the ultimate range d between first diffraction element (102) and second diffraction element (103) by following formula:

${\mathrm{\φ}}_{\mathrm{out}}-{\mathrm{\φ}}_{\mathrm{in}}=2d\sin \mathrm{\θ}\·\left[\frac{\cos {\mathrm{\θ}}_1}{\cos (\mathrm{\θ}+{\mathrm{\θ}}_1)}\right];$

3. calculate the position phase distribution matrix

of described first diffraction element (102) by following formula

Wherein, x and y represent rectangular coordinate, and n is the refractive index of material, θ ₁Be the intermediate parameters that 1. step tries to achieve, mod{} representes right Get remainder divided by 2 π;

4. confirm the position phase distribution matrix

of described second diffraction element (103) by following formula

Wherein, x and y represent rectangular coordinate, and n is the refractive index of material, θ ₁Be the intermediate parameters that 1. step tries to achieve, mod{} representes right Get remainder divided by 2 π.M is a constant, is relative because the position of calculating distributes mutually, so the M here can be taken as zero;

5. adopt existing ripe lithography process prepared to have first diffraction element (102) and second diffraction element (103) that described position distributes mutually;

6. assemble described diffraction type ring illumination generator.

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