JPH063620A - Method and device for applying laser beam - Google Patents

Abstract

PURPOSE:To increase the depth of focus while keeping a beam spot diameter small by applying laser beams with the beam waist positions of the laser beams at a condensing portion being slightly deviated from each other in the direction of an axis. CONSTITUTION:A laser beam L emitted from a laser diode 1 is incident on a special prism 2 equipped with a polarizing beam slitter formed integrally with a beam separation means and an optical path difference generation means and is separated into a P polarized beam and an S polarized beam, which are then emitted from the special prism 2 as a composite laser beam L' after being provided with a predetermined amount of optical path difference, the composite laser beam L' having its axis adjusted to that of the laser beam L. The laser beam L' passes through a collimator lens 3 and is thereby converted into parallel rays of light, which are then deflected at the mirror surface 4a of a polygon mirror 4 rotating at constant angular velocity and enter an ftheta lens 5; the laser beam L' transmitted through the ftheta lens 5 is image- formed on the surface of a photosensitive material 6 disposed on an image- forming surface.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laser irradiation method and apparatus for increasing the depth of focus while keeping the light spot diameter small.

[0002]

2. Description of the Related Art In the field of plate making, for example, in order to record an image on a photosensitive material (photosensitive film), a laser beam modulated by an image signal is focused and the laser beam is applied to a photosensitive material as a surface to be irradiated. A laser recording device for irradiation is used.

Further, in the field of semiconductor manufacturing, in one step of forming a photomask, a laser beam modulated according to a mask pattern is focused on a photosensitive material (photoresist) applied on the surface of a mask member. A mask drawing device that irradiates the light is used.

The laser recording device and the mask drawing device are
A two-dimensional image and a mask pattern are obtained by relatively moving the laser beam and the photosensitive material in the main scanning direction and the sub scanning direction. Regarding the movement of the photosensitive material,
A method of sticking a photosensitive material on a rotating drum and rotating at a constant speed,
A method in which the photosensitive material is conveyed at a constant speed or stepwise by a conveying mechanism, or the photosensitive material is placed on a stage to
A method of controlling the position of a two-axis or two-axis system is used.

In order to improve the resolution of the image recorded on the photosensitive material and the mask pattern to be drawn, it is desirable to reduce the beam diameter of the laser beam applied to the photosensitive material to a small light spot. Due to the characteristics of the laser beam, if the spot diameter is reduced, the depth of focus becomes shallow in proportion to its square.

This is concretely shown by the figures and mathematical formulas as follows.

FIG. 10 is a diagram showing a state of the laser beam B at the condensing portion. FIG. 10A is a diagram when the light spot diameter of the laser beam B is narrowed down to the smallest. The relationship between the beam radius (hereinafter referred to as the “beam waist radius”) ω 0 and the beam divergence angle θ at the beam waist BW at the different position is generally expressed by the following equation, where λ is the wavelength of the laser beam B.

[0008]

[Equation 2]

The depth of focus is usually defined as the distance from the position of the beam waist BW until the beam radius ω reaches a constant magnification (about 1.1 times) of the beam waist radius ω 0. , Where d is the depth of focus, it is expressed by the following equation.

[0010]

[Equation 3]

The following equation is derived from the equations (2) and (3).

[0012]

[Equation 4]

From the equation (4), it can be seen that the smaller the beam waist radius ω 0, that is, the light spot diameter, the shallower the focal depth of the laser beam B becomes in proportion to the square thereof.

This is clear even when compared with the depth of focus d1 when the beam waist radius ω1 is large as shown in FIG. 10 (b).

[0015]

As described above, when the light spot diameter of the laser beam is reduced in order to improve the resolution, the depth of focus becomes shallow in proportion to the square of the light spot diameter.

This means that in order to improve the resolving power, not only the light spot diameter of the laser beam must be reduced, but also the irradiated surface must be accurately and continuously arranged at the beam waist position of the laser beam. ,That's what it means. In other words, when the beam waist position and the irradiated surface are relatively displaced in the irradiation direction, as is apparent from FIG. 10A, the light spot diameter of the laser beam irradiated on the irradiated surface is It will fluctuate easily and the resolution will rather decrease.

However, the beam waist position of the laser beam is likely to fluctuate with respect to temperature changes, and it is difficult to maintain it at a specific position, and the means for moving the photosensitive material also fluctuates with respect to the irradiation direction. ,
It is also difficult to move the photosensitive material while maintaining the specific position in the same direction. Therefore, it is not easy to actually improve the resolution of an image or a mask pattern by using a laser beam having a shallow focal depth even if the spot diameter is small.

Similar problems occur not only in the laser recording device and the mask drawing device as described above, but also in the device for converging a laser beam and irradiating the surface to be irradiated, such as a precision processing device using a laser and a laser printer. It has become a challenge.

The present invention solves the above problems,
An object of the present invention is to provide a laser irradiation method and an apparatus therefor capable of increasing the depth of focus while keeping the light spot diameter small.

[0020]

In order to achieve the above object, a laser irradiation method according to the invention of claim 1 is a method of converging a laser beam and irradiating the surface to be irradiated with a plurality of laser beams which do not interfere with each other. It is characterized in that the laser beams are applied to the irradiation surface while making the central axes of the respective laser beams coincide with each other and slightly shifting the beam waist positions of the respective laser beams in the converging portion in the central axis direction.

The invention of claim 2 is the laser irradiation method of claim 1, wherein the plurality of laser beams irradiated onto the surface to be irradiated are two first and first laser beams having the same total energy and the same beam waist radius. It is a two-laser beam, and the distance x between the beam waists is defined by the following equation.

[0022]

[Equation 5]

According to the invention of claim 3, in the laser irradiation method of claim 2, the first laser beam is a P-polarized beam separated from the basic laser beam, and the second laser beam is separated from the basic laser beam. It is an S-polarized beam, and the optical path lengths of the first and second laser beams are different by a length corresponding to the separation distance x.

Further, the invention of claim 4 relates to a laser beam irradiating device, wherein the laser beam emitted from the laser light source is condensed by a condensing optical system to irradiate the surface to be irradiated. A beam separating means for separating a laser beam emitted from a laser light source into a P-polarized beam and an S-polarized beam, an optical path length of the P-polarized beam and an optical path length of the S-polarized beam are slightly different from each other, and An optical path difference generating unit that emits a combined laser beam having the same central axis is provided, and the combined laser beam is applied to the irradiated surface via the condensing optical system.

[0025]

In the laser irradiating method according to claim 1, since the beam waist positions of the respective laser beams are slightly deviated from each other in the central axis direction, the energy density distribution in the converging portion is obtained by combining the respective laser beams. And is expanded in the central axis direction between the beam waists at both ends.

In the laser irradiating method according to the second aspect, when the first and second laser beams are used and the separation distance x of each beam waist satisfies the formula (5), the specific energy density is maintained in the entire range of the separation distance x. An energy density of (t) or higher is obtained.

In the laser irradiating method according to the third aspect, the P-polarized beam and the S-polarized beam separated from the basic laser beam are used, so that they do not interfere with each other and the total energy and the beam waist radius are equal to each other.
You are getting a laser beam. By making the optical path lengths of the first and second laser beams different, the positions of the beam waists are displaced by the distance x.

In the laser irradiation apparatus according to the fourth aspect, the laser beam emitted from the laser light source is separated into a P-polarized beam and an S-polarized beam by the beam separating means,
The P-polarized beam and the S-polarized beam are combined laser beams whose optical path lengths are slightly different by the optical path difference generating means and the central axes of the two beams are coincident with each other. Then, the synthetic laser beam is applied to the surface to be illuminated via the condensing optical system.

[0029]

Embodiments of the present invention will now be described in detail with reference to the drawings.

First, the concept of the depth of focus in the present invention will be described.

In a laser beam that is usually used, the relationship between the distance r from the central axis and the energy density I in a plane perpendicular to the central axis that coincides with the principal ray constitutes a Gaussian distribution as shown in FIG. Therefore, it is called Gaussian beam.

In general, the energy density I at a distance r from the central axis can be expressed by a function of r, and this function I
(R) is shown by the following equation.

[0033]

[Equation 6]

However, P is the total energy of the laser beam, and ω is the beam radius at a predetermined position. The beam radius ω is the beam radius ω 0 at the beam waist, the wavelength λ of the laser beam, The distance (defocus amount) Z from the beam waist on the central axis is expressed by the following equation.

[0035]

[Equation 7]

Consider, for example, the case where a photosensitive material such as a photosensitive film is exposed by a laser beam.

Generally, when exposing a light-sensitive material, the exposure amount gradually increases, and when a certain exposure amount is reached, blackening is started. The slope of the characteristic curve of the exposure amount and the blackening amount at this time is called gamma, and the larger this gamma, the more rapidly the blackening occurs.

Generally, since a laser recording device outputs a binary image, a light-sensitive material having a large gamma is suitable, and when the energy density of a laser beam to be irradiated reaches a certain value t or more, blackening occurs at once (this energy density). t
Hereinafter, it is referred to as a “photosensitive limit value” in the sense of the minimum energy density required for blackening. ).

If the photosensitivity limit value t in the energy distribution of FIG. 6 is I (0) / 2, blackening can be achieved up to a distance of ± r1 from the central axis. R at this time
1 is called the photosensitive radius and is indicated by ρ.

In general, the photosensitive radius ρ is expressed by the following equation by expanding the equation of Equation 6 for r and setting I (r) as t.

[0041]

[Equation 8]

For example, ω 0 = 15 μm, λ = 780n
In the case of m, the expression of Expression 7 is substituted into the expression of Expression 8, and ρ is a function of Z, and the value of each t is shown in a graph as shown in FIG.

The depth of focus d for the photosensitive material is determined by the defocus amount Z until the photosensitive material is no longer exposed to the laser beam, that is, the photosensitive radius ρ becomes "0". In the graph of FIG. 7, for example,
When the photosensitive limit value t is I (0) / 2, the defocus amount Z at which the photosensitive radius ρ becomes “0” is 0.9 mm, and the depth of focus actually extends in the −Z direction. The value of the depth d is ± 0.9 mm.

FIG. 9A is a diagram showing the relationship between the defocus amount Z and the energy density distribution for the above-mentioned single beam.

As can be easily understood from the figure, in the case of a single beam, the defocus amount Z changes by a slight amount I
The (0) value decreases extremely, and immediately falls below the exposure limit value t. This indicates that the depth of focus d is small.

However, as shown in FIG. 5, by combining two laser beams and setting the distance between the beam waist positions of the respective beams to an amount satisfying a predetermined condition, the depth of focus can be increased. .

FIG. 5 is a diagram showing how the two combined laser beams B1 and B2 are overlapped at the converging portion. In the same figure, the central axis of the first laser beam B1 and the central axis of the second laser beam B2 coincide with each other, and with the central axis as the CA axis, each beam waist BW1,
The separation distance of BW2 is x, and the middle point is "0" on the CA axis.
Take the coordinates so that Further, for example, the first laser beam B1 is a P-polarized beam and the second laser beam B2 is
Is an S-polarized beam, and both beams B1 and B2 do not interfere with each other.

The first laser beam B1 and the second laser beam B2 are combined to form a combined laser beam. The energy density I (r) of a plane perpendicular to the CA axis of this combined laser beam is Based on the equation of Equation 6, it is given by the following equation.

[0049]

[Equation 9]

In order to facilitate the comparison with the case of the single beam described above, ω 0 = ω 0 ′ = 15 μm, λ = 780 nm,
P1 = P2, t = I (0) / 2, the beam waist position of the first laser beam B1 is CA = -1.25 mm,
The beam waist position of the second laser beam B2 is CA
= 1.25 mm, defocus amount Z
The relationship between the energy density distribution and the energy density distribution is as shown in FIG.

In the figure, the dotted line b1 is the energy density curve of the first laser beam B1 and the alternate long and short dash line b2 is the second
Energy curve of laser beam B2, solid line b3
Shows the energy density curve of the synthetic laser beam, and the energy density curve b3 of this synthetic laser beam
9A is compared with the change of the energy density curve in the case of the single beam in FIG. 9A, the decrease of I (0) is small with respect to the change of the defocus amount Z, and it is easy to increase the depth of focus. Can understand.

FIG. 8 is a graph showing the relationship between the photosensitive radius ρ and the defocus amount Z in the case where the photosensitive limit value t = I (0) / 2 in the combined laser beam.

The solid lines 81 and 82 indicate the distance x between the beam waists of the synthetic laser beam being 1.5, respectively.
It is a graph at the time of setting to mm and 2.5 mm. Since the depth of focus d is determined by the defocus amount Z when ρ = 0 as described above, the distance x between the beam waists is 1.5.
It is possible to obtain a focal depth d = ± 1.6 mm when mm, a focal depth d = ± 2.2 mm when x is 2.5 mm, and a focal depth d when a single beam (dotted line 83) is obtained.
= More than twice as compared with ± 0.9 mm.

From these results, it can be said that the larger the distance x between beam waists is, the larger the depth of focus d is, even if the same synthetic laser beams are the same. As shown in the concave portion 82a of 82, the synthetic energy density in the central portion between the beam waists begins to decrease. The minimum value of the combined energy density in this central portion is the defocus amount Z = 0.
Is given by the energy density I (0) at
If it is expressed as in (I), t
Distance between beam waists in the range of <Min (I) x
Is prescribed.

The range of x as above is obtained for the first and second laser beams B1 and B2 when the powers and beam waist diameters of both beams are equal to each other as follows.

That is, in the equation (9), r = 0, Z
= 0, P1 = P2 = P, and .omega.0 = .omega.0 ', the following equation is first obtained.

[0057]

[Equation 10]

On the other hand, the photosensitive limit value t is r = 0 in the equation (6).
Maximum energy density I (0) of each beam
Can be defined as S times the following.

[0059]

[Equation 11]

However, if the photosensitivity limit value t is larger than the sum of the maximum energy densities of the two laser beams, the photosensitivity does not occur. Therefore, the magnitude of the coefficient S is determined within the range of 0 <S <2. Become.

As described above, since the condition for exposing between the beam waists is t <Min (I),
Substituting the equations of equation 10 and equation 11 into this conditional equation,

[0062]

[Equation 12]

If we obtain the following equation and solve this for x,

[0064]

[Equation 13]

To obtain

However, the coefficient S is calculated from the equation (11).

[0067]

[Equation 14]

Is given by

Therefore, by setting the beam waist separation distance x within the range of the equation (13), the focal depth d is increased while always keeping the maximum value of the energy density between the beam waists at or above the exposure limit value t. be able to.

In the above embodiment, the case of two laser beams was described, but theoretically, three or more laser beams can also be combined laser beams obtained by combining these laser beams. The depth of focus can be increased more and more by defining the distance between the beam waists so that the maximum value of the energy density in the plane perpendicular to the central axis does not fall below a predetermined value between the beam waists. In this case, there is a greater possibility that the energy density will be reduced by the interference of each beam than in the case of two beams, but the wavelength of each beam, the focal length of the condensing optical system used, and the separation distance of each beam waist are By properly adjusting, the influence of interference in the image forming portion can be avoided.

Next, an example of a laser recording apparatus for recording an image on a photosensitive material using a synthetic laser beam will be described.

FIG. 1 is a diagram showing the structure of the laser recording apparatus 100.

The laser beam L emitted from the laser diode 1 is incident on a special prism 2 with a polarization beam splitter which is integrally formed with a beam separating means and an optical path difference generating means, and is separated into a P polarized beam and an S polarized beam. , A special prism 2 as a combined laser beam L ′ whose central axes are aligned after a predetermined optical path difference is provided.
Is emitted from.

FIG. 2A is a plan view showing the structure of the special prism 2, and FIG. 2B is a perspective view thereof.

The special prism 2 includes two prism members 2
It is formed by joining 1 and 22 with a transparent adhesive, and a polarized beam split surface 23 is formed on the joining surface of the prism members 21 and 22 by vapor deposition. The polarized beam splitting surface 23 transmits the P component of the laser beam L and reflects the S component thereof, thereby separating the laser beam L into two beams Lp and Ls.

The beam Lp is totally reflected at the point B on the inner side surface 21a of the prism member 21, and the beam Ls is totally reflected at the point D on the inner side surface 22a of the prism member 22. Then, they are united to be coaxial and emitted from the special prism 2 as a laser beam L '.

Now, if the refractive indices of the prism members 21 and 22 are both n, the optical path difference Δl between the beams Lp and Ls is

[0078]

[Equation 15]

It is expressed as

In FIG. 1, the laser beam L ′ passes through the collimator lens 3, is converted into parallel rays, is deflected by the mirror surface 4 a of the polygon mirror 4 rotating at a constant angular velocity, and is incident on the fθ lens 5.

The fθ lens 5 is a lens formed so as to always form beams having different incident angles on the same plane, and the laser beam L ′ passing through this lens is
An image is formed on the surface of the photosensitive material 6 arranged on the image plane, and the beam spot of the laser beam L'moves on the surface of the photosensitive material 6 at a constant speed according to the rotation of the polygon mirror 4.

Now, assuming that the ratio of the focal lengths of the collimator lens 3 and the fθ lens 5 is 1: 4, the optical longitudinal magnification between the laser diode 1 and the photosensitive material 6 is 1: 2 of its squared ratio. Become 16.

Further, the difference between the optical path length of ABC and the optical path length of ADC in the special prism 2 is 0.24 mm, and the prism members 21 and 22 having a refractive index n of 1.51 are used.
If the optical path difference Δl is obtained from the formula of Expression 15 and multiplied by 16,
The image forming position of each of the beams Lp and Ls included in the laser beam L ′, that is, the position of the beam waist is about 2.5 mm.
It will shift.

When the cleavage surface of the chip of the laser diode 1 is set to be inclined at 45 ° with respect to the paper surface of FIG. 1, the beam intensity ratio of the beams Lp and Ls separated by the special prism 2 becomes 1: 1. Since all energies are equal, if each beam waist radius ω0 is set to 15 μm, the solid line 82 in the graph of FIG. 8 gives a depth of focus d of ± 2.2 mm, which is ± This is more than double the size of 0.9 mm.

The surface of the photosensitive material 6 is preferably set at a position intermediate between the beam waists of the beam Lp and the beam Ls included in the laser beam L '.

In the above embodiment, the beam Lp is formed by using the special prism 2 with the polarization beam splitter in which the beam separating means and the optical path difference generating means are integrally formed.
Since an optical path difference is provided between the beam and the beam Ls,
If you prepare several kinds of special prisms with different optical path differences so that they can be easily replaced, the wavelength of the laser beam,
This is convenient because the beam waist separation distance x can be easily changed according to the type of photosensitive material on the image plane.

Further, the same action as that of the special prism 2 can be obtained in the modified embodiments shown in FIGS. 3 and 4.

That is, in FIG. 3, two polarization beam splitters 7a and 7b are diagonally arranged, and the P polarization beam Lp passing through the point E of the polarization beam splitter 7a.
Is reflected by the point F of the mirror 8a and is incident on the point G of the polarization beam splitter 7b, while the S-polarized beam Ls reflected by the point E of the polarization beam splitter 7a is reflected by the mirror 8b.
Is reflected at the H point of the above, is reflected at the G point of the polarization beam splitter 7b, is emitted coaxially with the P polarized beam Lp,
By making the triangle EFG and the triangle EHG asymmetric, a predetermined optical path difference can be obtained.

Further, as shown in FIG. 4, the triangle EFG and the triangle EHG are made symmetrical so that the optical path lengths are equal, and a transparent parallel plane member 9 having a width W and a refractive index n is provided in the middle of one optical path. It may be inserted.

In the case of the same figure, the optical path length of the S-polarized beam Ls can be made shorter than the P-polarized beam Lp by W / n. It is convenient that the optical path difference can be easily changed by appropriately setting the width W and the refractive index n of the parallel plane member 9 to be inserted.

As described above, according to the method in which the laser beam is separated into the P-polarized beam and the S-polarized beam, and a predetermined optical path difference is provided between them, and then the laser beam is incident on the condensing optical system,
There is no risk of interference between the two beams, and the distance between the beam waists can be easily adjusted.

The polygon mirror 4 and the f.theta. Lens 5 in the embodiment of FIG. 1 may use other focusing optical systems, if necessary. For example, the laser beam is fixed without being deflected. Alternatively, the photosensitive material 6 may be moved.

The intensity ratio of the P-polarized beam and the S-polarized beam to be separated does not necessarily have to be 1: 1, and λ / is provided between the laser diode 1 and a predetermined beam separating means.
By inserting 2 plates and rotating the λ / 2 plate,
The intensity ratio of the two may be changed as necessary.

Further, although the laser recording apparatus has been described in the present embodiment, the use of the present invention is not limited to this, and a processing apparatus using a laser, a foreign substance inspection apparatus, a mask drawing apparatus, etc. It can be applied to all devices that use a laser beam and require a certain depth of focus while maintaining the light spot diameter and energy density.

[0095]

As described above, according to the laser irradiation method of the first aspect, the energy density distribution at the condensing portion of a plurality of laser beams can be expanded in the central axis direction between the beam waists at both ends. The focal depth can be increased even if the light spot diameter of the beam is reduced. Therefore, improvement in resolution can be easily achieved.

According to the laser irradiation method of the second aspect, since the energy density of the specific energy density (t) or more can be obtained over the entire range of the separation distance x of the beam waists of the first and second laser beams, the certainty is ensured. The depth of focus can be increased.

Further, according to the laser irradiation method of the third aspect, since the P-polarized beam and the S-polarized beam are separated from the basic laser beam, they do not interfere with each other and the total energy is equal to the beam waist radius. And the second
A laser beam can be easily obtained. And the first
And the position of each beam waist can be easily set by making the optical path length of the second laser beam different.

According to the laser irradiating apparatus of the fourth aspect, the depth of focus can be increased only by providing the beam separating means and the optical path difference generating means. Therefore, it is practical to add a simple structure to the conventional apparatus. It is possible to provide a laser irradiation device having a high resolution.

[Brief description of drawings]

FIG. 1 is a block diagram showing a configuration of a laser recording apparatus according to an embodiment of the present invention.

FIG. 2 is a diagram showing a configuration of a special prism in the laser recording apparatus of FIG.

FIG. 3 is a diagram showing another embodiment of the optical path difference generating means.

FIG. 4 is a diagram showing still another embodiment of the optical path difference generating means.

FIG. 5 is a diagram showing a state of overlap in a converging portion of a combined laser beam.

FIG. 6 is a diagram showing the relationship between the distance from the central axis and the energy density in a single beam.

FIG. 7 is a diagram showing a relationship between a photosensitive radius and a defocus amount in a single beam.

FIG. 8 is a diagram showing a relationship between a photosensitive radius and a defocus amount in a combined laser beam.

FIG. 9 is a diagram showing a relationship between a defocus amount and an energy density distribution in a single beam and a combined laser beam.

FIG. 10 is a diagram showing a state of a beam waist portion of two types of single beams having different light spot diameters.

[Explanation of symbols]

 1 Laser diode 2 Special prism 3 Collimate lens 4 Polygon mirror 5 fθ lens 6 Photosensitive material 7a, 7b Polarizing beam splitter 8a, 8b Mirror 9 Parallel plane members 21, 22 Prism member 23 Polarizing beam splitting surface

Claims (4)

[Claims] 1. A method for converging a laser beam to irradiate a surface to be irradiated, wherein a plurality of laser beams which do not interfere with each other are made to coincide with each other in central axis of each laser beam, and the beam of each laser beam at a converging part A laser irradiation method comprising irradiating the surface to be irradiated with the waist positions being slightly shifted from each other in the central axis direction. 2. The plurality of laser beams applied to the surface to be illuminated are two first and second laser beams having the same total energy and beam waist radius, respectively.
The laser irradiation method according to claim 1, wherein the distance x between the beam waists is defined by the following equation. [Equation 1] 3. The first laser beam is a P-polarized beam separated from a basic laser beam, the second laser beam is an S-polarized beam separated from the basic laser beam, and the first and second laser beams are provided. 3. The laser irradiation method according to claim 2, wherein the optical path lengths of the laser beams are changed by a length corresponding to the separation distance x. 4. An apparatus for irradiating a surface to be illuminated with a laser beam emitted from a laser light source, which is condensed by a condensing optical system, wherein the laser beam emitted from the laser light source is a P-polarized beam and an S-polarized beam. Beam separation means for separating the optical path length of the P-polarized beam from the optical path length of the S-polarized beam, and an optical path difference generation means for emitting a combined laser beam having the central axes of both beams coincident with each other. And irradiating the surface to be irradiated with the synthetic laser beam through the condensing optical system.