CN101354476A - Low thermal effect projection objective - Google Patents

Abstract

The invention discloses a low heat effect projection objective which selects suitable material and reasonable structural design to realize small heat effect under high exposal dosage and reduce the difficulty of the heat effect compensation; simultaneously, good and uniform image quality can be realized in the square visible field with the specification of 44mm*44mm; the working distance between the object part and the image part is 30mm and the total length is not more than 800mm; furthermore, the non-spherical lens is not included and difficulty is not added for the processing, testing, installing and debugging.

Description

Low thermal effect projection objective

Technical field

The present invention relates to a kind of optical system, relate in particular to a kind of all-refraction type projection optical system.

Background technology

At present in the semiconductor packages field, micron order resolution, the projection optical system demand of high yield increases day by day.But all run into the puzzlement of eyeglass thermal effect problem (lens heating) from ultraviolet to the deep ultraviolet wave band, correspondingly produced much discussion about the apparatus and method that how to compensate thermal effect, for example U.S. Pat 7294814, US7084952, US5898479 etc., the device of introduction all detects or compensates at the eyeglass thermal effect.

But how from the thermal effect that designs and the selection angle is prevented eyeglass, relevant report is less.U.S. Pat 7239450 has been discussed the use of quartz crystal (SILICA) and two kinds of materials of fluorite (CAF2) from the thermal effect angle.The main contribution of patent is: sum up the thick lens of little clear aperature, be subjected to the life-span influence of (lifetime) or thermal effect (lens heating) bigger than the thin lens of big clear aperture, and provided single eyeglass life-span/thermal effect susceptibility coefficient:

$K_{\mathrm{LT}/\mathrm{LH}}=\frac{D}{<D>}{\left(\frac{<\mathrm{CA}>}{\mathrm{CA}}\right)}^2$

Wherein, D represents single lens thickness; CA represents effective clear aperture of single eyeglass;＜D〉expression N eyeglass average center thickness;＜CA〉expression N eyeglass average clear aperture.

Crystal of fluoride (as CAF2) has higher transmittance than traditional quartz crystal (SILICA) in short-wave band, so because the thermal effect that low absorption produces is littler.Therefore for the susceptibility COEFFICIENT K _LT/LHHigh eyeglass considers to use crystal of fluoride to replace traditional quartz crystal.

Patent US7239450 has only inquired into the use of SILICA and two kinds of materials of CAF2 of deep ultraviolet wave band.At 400nm～440nm wave band, alternative material is more relatively, avoids thermal effect can bring into play bigger effect from the angle of selection.And the thermal effect of eyeglass is not only relevant with transmitance, with the heat conduction of material, thermal refractive index coefficient, material heat expansion characteristic etc. confidential relation is arranged all, merits attention very much.

Summary of the invention

The object of the present invention is to provide a kind of projection objective, to reduce the harmful effect that thermal effect produces projection optical system with low thermal effect.

In order to reach above purpose, the invention provides a kind of low thermal effect projection objective, comprise the first to the 12 lens that are arranged in order along optical axis from the object side to the image side, be provided with diaphragm between the described the 6th and the 7th lens, described first to the 6th lens and described the 12 to the 7th lens are about described diaphragm symmetry; Wherein, described first, second, the 6th lens have positive light coke, described the 3rd, the 4th, the 5th lens have negative power; In described 12 lens, all adopted the 10mm internal optical transmission to reach 0.999 material and made, and described 12 lens all do not comprise non-spherical surface.

Further, described first, second lens are formed the burnt group of negative light; Wherein, described first lens are biconcave lens, use the SILICA material to make; Described second lens are the meniscus lens of concave surface over against object plane, use the BSL7Y material to make.

Described the 3rd, the 4th lens are formed the burnt group of positive light; Wherein, described the 3rd lens are approximate plano-convex lens, and the less one side of its curvature uses the BSL7Y material to make towards object space; Described the 4th lens are biconvex lens, use the BSL7Y material to make.

Described the 5th, the 6th lens are positive negative power combination; Wherein, described the 5th lens are biconvex lens, use fritter CAF2 material to make, and are used for correcting chromatic aberration; Described the 6th lens are biconcave lens, use the PBL1Y material to make.

Low thermal effect projection objective of the present invention is by selecting suitable material and passing through reasonable structural design, can be implemented in thermal effect less under the high exposure dose, reduced the difficulty of thermal effect compensation, can in the square field range of 44mm * 44mm, realize simultaneously good picture element uniformly, object space reaches 30mm as side working distance, length overall is no more than 800mm, and does not comprise aspherical lens, does not give processing, tests and debug the increase difficulty.

Description of drawings

Fig. 1 is the structural representation of the low thermal effect projection objective of the present invention's one specific embodiment.

Fig. 2 is the aberration figure of the projection objective of the present invention's one specific embodiment.

Embodiment

Below in conjunction with accompanying drawing the specific embodiment of the present invention is further described.

Referring to Fig. 1, the projection objective of present embodiment is taked symmetrical structure, and preceding latter half of system is about diaphragm AS symmetry, and one has 12 eyeglass L1～L12.Comprise 6 eyeglasses in half system of, first, second, the 6th eyeglass L1, L2, L6 have negative power, the the 3rd, the 4th, the 5th eyeglass L3, L4, L5 have positive light coke.And in 400nm～420nm wave band, all select high permeability low thermal effect material for use, considered the functional need of lithographic objective under the high exposure dose.

Ultraviolet submicron lithography object lens are extremely harsh to the requirement of ultraviolet optical material, under the especially high exposure dose, if lens materials selects improper meeting that eyeglass is heated up the several years under the irradiation of ultraviolet light intensity.Below table 1 listed the characterisitic parameter of some uv materials, refractive index n and Abbe number V are the approximate values in 400nm～440nm wave band in the table, back four row are temperature characterisitics (thermal property) of material.

Table 1

Material Material	Refractive index Refractive Indices n	Abbe number V	Internal optical transmission Internal Transmittance τ 10mm	Pyroconductivity Thermal Conductivity k (W/mK)	Relative index of refraction temperature coefficient Temperature Coefficients of Refractive Index dn/dt relative (10-6/ ℃)	Thermal expansivity Expansion Coefficients (30～+ 70 ℃) α (e-7/ ℃)
							SFPL51Y	1.50585	144.64	0.999	0.78	-6.6	136
BSM51Y	1.61752	107.6	0.998	0.961	3.9	63
							PBM18Y	1.61882	62.789	0.998	0.865	5.9	88
PBM2Y	1.64615	57.985	0.998	0.814	6	86
							PBL25Y	1.603	66.451	0.998	0.899	4.7	87
SILICA	1.468	124.29	0.999	1.31	10.425	5
							CAF2	1.44	169.15	0.999	9.71	-11.3	167
BAL15Y	1.5705	103.11	0.998	1	3.7	76
							PVM8Y	1.61853	63.573	0.998	0.878	5.2	85
PBM18Y	1.61882	62.789	0.998	0.856	5.7	88
							SFSL5Y	1.49739	128.1	0.999	1.002	-0.4	89
PBL6Y	1.54785	82.485	0.999	1.016	3.9	83
							PBL1Y	1.56602	76.322	0.999	0.951	3.3	93
BSL7Y	1.52788	115.65	0.999	1.182	3.7	68

Reduce the objective lens design of thermal effect and should consider that the present invention meets following several principle of design from selection and structure two aspects:

1, material is selected

Internal optical transmission (Internal Transmittance): the 10mm internal optical transmission of listed material all is higher than 0.998 in the table 1, and thermal effect comes from the energy of absorbed, and the high more then absorption of transmittance is few more, and thermal effect is also more little.The 10mm internal optical transmission of four kinds of materials that present embodiment uses all is 0.999.

Pyroconductivity (Thermal Conductivity): because the lens barrel outer wall has thermostatic control, the material that pyroconductivity is high can pass the heat that absorbs morely.

Relative index of refraction temperature coefficient (Temperature Coefficients of Refractive Index) and thermal expansivity (Expansion Coefficients): the eyeglass temperature rises, the refractive index of material is changed, and, finally picture element is exerted an influence along with thermal expansion also can change by the generating plane type.These two kinds of variations can obtain embodying by relative index of refraction temperature coefficient and thermal expansivity, and the big more influence of numerical value is big more.But as CaF2, the two coefficient symbols is opposite, and the thermal effect of generation can partly be cancelled out each other.

2, consider the distribution of focal power from reducing the thermal effect angle, the material big to thermal effect do not distribute too big focal power.

3, on the thick lens of little clear aperture, use this material, meet eyeglass that U.S. Pat 7239450 proposes life-span/eyeglass that thermal effect susceptibility coefficient is big in principle and use the criterion of the little material of thermal effect characteristic, that is:

$K_{\mathrm{LT}/\mathrm{LH}}=\frac{D}{<D>}{\left(\frac{<\mathrm{CA}>}{\mathrm{CA}}\right)}^2$

Wherein, D represents single lens thickness; CA represents effective clear aperture of single eyeglass;＜D〉expression N eyeglass average center thickness;＜CA〉expression N eyeglass average clear aperture.

4, because aspheric surface precision requires height,, should avoid putting on the big material of thermal effect and (not use aspheric surface in the example of the present invention) if adopt aspheric surface.

Below in conjunction with the drawings and specific embodiments, for a more detailed description to projection objective of the present invention.

As shown in Figure 1, light projection photoetching objective lens of the present invention is 12 chip architectures, and lens surface all is a sphere, and preceding latter half of system symmetry, forms-1 times enlargement ratio, and vertical axial aberration compensates automatically.First L1 near object plane is biconcave lens, and effect is that more off-axis ray is entered, and realizes the big visual field of 44mm * 44mm, the negative curvature of field that it comes balance back lens with a large amount of positive curvature of field, and pincushion distortion has simultaneously guaranteed the object space heart far away.Second L2 is the negative meniscus of concave surface towards object plane, and focal power is very little, and the off-axis ray of edge bore is not produced big aberration.The 3rd L3 is nearly plano-convex lens, and convex surface is towards picture side, and the 4th L4 is biconvex lens, third and fourth two L3, L4 assemble off-axis rays simultaneously balances spherical aberration and distortion.Proofreaied and correct most of aberration with the 6th L5, L6 for the 5th.

Three sets of curves that run through whole object lens among Fig. 1, the from left to right corresponding successively light path trend that projects to image planes from 0 visual field, 0.7 visual field and the visual field, edge of object plane.

Each parameter request of example of the present invention is as shown in table 2:

Table 2

Operation wavelength	400nm～440nm
		Thing/as number formulary value aperture NA	0.1
Enlargement ratio	-1
		Visual field (diameter)	31.467mm
Object image distance from	800mm
		Thing/as side's work distance	30mm

Table 3 has provided the specific design value of the projection objective of present embodiment, definition is the coordinate axis positive dirction from the object plane to image planes along optical axis direction, then positive radius value is represented the coordinate of the coordinate of the center of curvature greater than surface vertices, and negative radius value is represented the coordinate of the coordinate of the center of curvature less than surface vertices.Between optical element thickness or two optical elements is to go up distance to the axle on next surface at interval.All dimensional units all are millimeters.

In the table 3, " S# " presentation surface numbering, " STOP " expression aperture diaphragm AS, in the surface type item, " FLT " represents the plane, " SPH " represents sphere.In the radius item, " INF " represents infinitely great." CC " represents concave surface, and " CX " represents convex surface.Listed material is all represented the trade mark of material in the table, and is general in GB and industry, specifically can be with reference to GB GB/T 903-1987 colouless optical glass.

Table 3

Each eyeglass in the table 3, material are taken from transmittance in the table 1, pyroconductivity is higher, thermal refractive index coefficient, moderate four kinds of material-BSL7Y, PBL1Y, CAF2, the SILICA of thermal expansivity; Wherein fluorite CAF2 is the especial dispersion material, and correcting chromatic aberration and second order spectrum are worked, and considers that higher being difficult to of big high-quality fluorite price obtain, and the bore that the fluorite material uses in this example is less than 60mm.

The eyeglass thermal effect sensitivity coefficient K of present embodiment _LT/LHAs shown in table 4:

Table 4

Lens	K LT/LH
		L1	0.7
L2	0.8
		L3	0.8
L4	0.9
		L5	2.01
L6	1.27

Wherein, the 5th eyeglass L5 is thermal effect sensitivity coefficient K _LT/LHZui Da eyeglass considers to use the material of CAF2 as the 5th eyeglass from reducing thermal effect and correcting chromatic aberration angle relatively.Because the pyroconductivity of CAF2 is the highest in the table, and the opposite in sign of thermal refractive index coefficient and thermal expansivity, the two thermal effect partial offset.

Fig. 2 has shown astigmatism, the curvature of field and the distortion result of the projection objective of present embodiment, and as can be seen from Figure 2 the curvature of field of the projection objective of present embodiment is in 6 μ m, and astigmatism is less than 3 μ m.In addition, wave aberration is less than 12nm, and heart degree far away is less than 3.5mrad, and the side-play amount of image point position relative ideal picture point reaches the image quality of diffraction limit less than 10nm.

In order to verify that projection objective design of the present invention has the characteristics of low thermal effect really, has below analyzed in the square field range of 44mm * 44mm 3300mj/cm ²Under the exposure dose, the temperature variation of object lens and image quality.

The inner initial temperature of emulation hypothesis object lens of thermal effect is 23 ℃, constant 23 ℃ of lens edge air themperature.After thermal effect reached stable state, each eyeglass had produced the thermograde that increases progressively from the edge to the center.The maximum temperature of each center of lens point is as follows:

Table 5

Lens	Temperature (℃)
		L1	23.535
L2	23.792
		L3	23.835
L4	23.716
		L5	23.147
L6	24.14
		L7	24.113
L8	23.14
		L9	23.657
L10	23.739
		L11	23.6556
L12	23.439

As seen, heat up the highest be in the middle of two sheet material be eyeglass L6, the L7 of PBL1Y, but the highest intensification also has only 1.14 ℃.

According to this simulation result, the several microns of only mobile focal plane can make wave aberration return to the 20nm level, but this moment, multiplying power was still undesirable with distortion.If use last eyeglass L12 as movable lens, then the multiplying power recoverable is to 5ppm, and distortion can be corrected to 100nm.

The thermal effect of actual projection objective also has the influence of a lot of uncertain factors, but emulation can be judged the quality of object lens thermal effect characteristic to a certain extent.Projection objective design bond material of the present invention is selected and structural design, has finally realized lower thermal effect.

Claims (4)

1, a kind of low thermal effect projection objective, comprise the first to the 12 lens that are arranged in order along optical axis from the object side to the image side, be provided with diaphragm between the described the 6th and the 7th lens, described first to the 6th lens and described the 12 to the 7th lens are about described diaphragm symmetry; It is characterized in that: described first, second, the 6th lens have positive light coke, described the 3rd, the 4th, the 5th lens have negative power; In described 12 lens, all adopted the 10mm internal optical transmission to reach 0.999 material and made, and described 12 lens all do not comprise non-spherical surface.

2, projection objective according to claim 1 is characterized in that: described first, second lens are formed the burnt group of negative light; Wherein, described first lens are biconcave lens, use the SILICA material to make; Described second lens are the meniscus lens of concave surface over against object plane, use the BSL7Y material to make.

3, projection objective according to claim 1 is characterized in that: described the 3rd, the 4th lens are formed the burnt group of positive light; Wherein, described the 3rd lens are approximate plano-convex lens, and the less one side of its curvature uses the BSL7Y material to make towards object space; Described the 4th lens are biconvex lens, use the BSL7Y material to make.

4, projection objective according to claim 1 is characterized in that: described the 5th, the 6th lens are positive negative power combination; Wherein, described the 5th lens are biconvex lens, use the CAF2 material to make; Described the 6th lens are biconcave lens, use the PBL1Y material to make.

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