JP3257807B2 - Method for forming periodic microstructure on solid surface - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a periodic ripple structure having a fine period suitable for manufacturing a DFB laser, a grating type optical coupler, a quantum wire structure, or a quantum dot structure. The present invention relates to a method for forming a periodic fine structure on a solid surface such as a periodic dot structure having a fine period.

[0002]

2. Description of the Related Art Conventionally, as a method of forming a periodic ripple structure or a periodic dot structure having a fine period of, for example, 1 μm or less, electron beam exposure using a resist, mask transfer, holography An exposure method, a holographic direct etching method without using a resist, and the like are known.

[0003] Among these methods, the electron light exposure method using a resist has a problem that a plurality of steps are required and a cycle is limited by the resolution of the resist.

Further, in the holographic direct etching method without using a resist, the etching solution used is opaque at the laser wavelength, and there is a dark reaction in which the etching proceeds even in a portion not irradiated with light, so that the depth of the groove is reduced. There is a problem that the width ratio (aspect ratio) is reduced, and furthermore, since the pitch width changes depending on the incident angle, there is also a problem that it is necessary to give the incident angle a large degree of freedom in order to change the pitch width.

Further, formation of periodic ripples due to surface electromagnetic waves (mainly considered to be surface plasma waves) has been reported. The present inventors have used GaAs as a sample and Q as a short-wavelength light source to verify this. Perform holographic exposure using the switch Nd: YAG laser 4th harmonic, CH
As a result of laser etching with ₃ Br (methane bromide) gas, it was observed that fine ripples almost parallel to the holographic grating were formed together with the holographic grating, and the relationship between this formation and surface electromagnetic waves was considered [Laser Research, Vol. 18, No. 7 (199
0)].

However, the periodic ripple structure obtained by this method has a problem that it has a wavy irregular structure and is not practical.

[0007] In any of the above methods,
Nothing can produce a periodic dot structure in one step.

[0008]

As described above, the electron beam exposure method using a resist has problems that a plurality of steps are required and that the period is limited by the resolution of the resist. In the holographic direct etching method that does not use, the etching liquid used is opaque at the laser wavelength, or there is a dark reaction in which the etching proceeds even in a portion not irradiated with light, so that the ratio of the depth to the width of the groove (aspect ratio) ) Is reduced, and since the pitch width changes with the incident angle, there is a problem that it is necessary to have a large degree of freedom in the incident angle in order to change the pitch width. The obtained periodic ripple structure has a wavy irregular structure,
There was a problem of lack of practicality.

SUMMARY OF THE INVENTION The present invention has been made to solve such a conventional difficulty, and a highly accurate periodic ripple structure or periodic dot structure having an arbitrary pitch can be obtained in one step.
In addition, it is an object of the present invention to provide a method for forming a periodic fine structure on a solid surface that can obtain a high aspect ratio because there is no dark reaction.

[0010]

That is, according to the first aspect of the present invention, there is provided a method for forming a periodic fine structure on a solid surface, the method comprising: Irradiating the laser light at a predetermined incident angle, a holographic grating generated by the interference of the laser light, and an interference grating generated by the interference of surface electromagnetic waves excited on the solid surface by the laser light and the laser light irradiation In the method of exciting the reaction gas according to the interference intensity and forming a periodic fine structure on the surface of the solid sample by the etching action of the reaction gas generated by decomposition, the interval between the holographic gratings is set to A periodic ripple structure is formed as an integral multiple of the interval.

According to a third aspect of the present invention, in the method for forming a periodic fine structure on a solid surface, a plurality of laser beams are applied to a surface of a solid sample set in an irradiation container filled with a reaction gas. Irradiation at an incident angle, the holographic grating generated by the interference of the laser light, and the laser light and the interference intensity of the interference grating generated by the interference of the surface electromagnetic waves excited on the solid surface by the irradiation of the laser light, In a method of exciting the reaction gas and forming a periodic fine structure on the surface of the solid sample by an etching action of the reaction gas generated by decomposition, by changing a polarization angle of the laser light, the holographic grating and It is characterized in that a periodic dot structure is formed by changing the crossing angle with the interference grating.

The laser beam includes a fundamental wave (1064 nm) and harmonics (266 nm, 355 nm, 53 nm) of an Nd: YAG laser.
2nm), XeF laser with 351nm wavelength, XeCl with 308nm wavelength
Laser, KrF laser with 248 nm wavelength, Ar with 193 nm wavelength
An F laser, an F ₂ laser having a wavelength of 157 nm, or the like can be used.

On the other hand, if the reactivity of the reaction gas is low, it takes a long time to form a periodic fine structure, and if the reactivity is too high, it is difficult to form a periodic structure having a fine pitch. Becomes From such a viewpoint, methane bromide is preferable as the reactive gas used in the present invention.

[0014]

In the method for forming a periodic fine structure on a solid surface according to the present invention, the reaction gas used, for example, methane bromide is
Methyl radicals and bromine atoms are released by photodecomposition by laser light irradiation, and these are combined with constituent atoms of a solid sample, for example, a GaAs substrate, and are released into the air to perform etching. Since this reaction occurs more strongly at high light waves due to interference between a plurality of incident laser beams and interference between the incident laser beam and the surface electromagnetic wave generated by the irradiation of the laser beam, these holographic gratings and interference gratings are formed on the solid surface. A periodic fine structure corresponding to the above is generated.

Next, a case where a periodic ripple structure is formed by the method for forming a periodic fine structure on a solid surface according to the present invention will be described.

The interference between the incident laser beams, that is, the laser etching speed by holography is performed at a constant speed irrespective of the elapse of time, but the reaction speed of the laser etching by the interference between the incident laser beam and the surface electromagnetic wave is very short immediately after irradiation. The reaction rate increases exponentially with the lapse of time, and the reaction becomes dominant over the entire etching rate after a certain time. This is because, when the surface electromagnetic wave is excited by the incident laser light, the surface electromagnetic wave and the incident laser light interfere with each other, causing optical etching due to the intensity of the interference wave, and the resulting groove causes a large amount of scattering of the incident light. This is because electromagnetic wave excitation occurs more strongly, and this positive feedback loop rapidly increases the etching rate.

The periodic ripple structure obtained by the present invention has a very high regularity, and a parallel pattern having the same pitch is formed. This is caused by a plurality of interference gratings of laser light which are initially generated through etching. It is considered that the surface electromagnetic wave is strongly excited in the portion of the etching pattern caused by the laser beam, and this becomes the basis of the laser etching pattern due to the interference between the laser light and the surface electromagnetic wave. In other words, a surface electromagnetic wave is generated with good controllability by utilizing grooves of direct etching by holographic exposure, instead of surface irregularities that are accidentally present on the surface.

That is, first, a groove (FIG. 2A) having excellent regularity is formed on a solid surface by holographic laser etching using a plurality of laser beams, and then the interference grating between the incident laser beam and the surface electromagnetic wave is formed based on the groove. (FIG. 2B), and a periodic ripple structure (FIG. 2B) that partitions the groove intervals (FIG. 2A) at equal intervals.
Becomes

The pitch width of the periodic ripple structure is determined as follows.

Holographic grating period,
That is, the period (D) of the holographic groove is given by the following equation.

D = (laser light wavelength) / 2 sin θ (1) Here, θ is the angle of incidence of two beams that are symmetrically incident from both directions in the incident plane.

On the other hand, the period of the interference grating generated by the interference between the laser light and the surface electromagnetic wave excited on the solid surface by the irradiation of the laser light is determined as follows.

That is, the surface electromagnetic wave propagating at the interface between the solid sample and the reaction gas (the dielectric constant ε _D of the reaction gas can be assumed to be ε _D = 1) is in the TH mode, and its dispersion relation is given by the following equation.

−ε _m / δ = ε _D / β (2) where ε _m is the complex permittivity of the solid sample depending on the frequency. Furthermore, [delta], beta, respectively [delta] = [k _‖ ^{2 - (ω 2 / c 2} ) ε m] 1/2 (3) β = [k _‖ ^{2 - (ω 2 / c 2} ) ε D] 1 / ² (4). Here, k _‖ is a component parallel to the interface of the complex propagation vector, ω is the frequency of the surface electromagnetic wave, and c is the speed of light. By solving the dispersion relation of the equation (2) using the complex permittivity for the solid sample, _k‖ (= k ₁ + jk ₂ ) is obtained. When the electric field component of the laser light is polarized by the angle φ, the spatial frequency of the interference grating matches k ₁ + k ₀ sin θ abs (cos φ). Therefore, the theoretical period d of the periodic structure due to the surface electromagnetic wave is represented by d = (2π) / [k ₁ + k ₀ sin θ abs (cos φ)] (5) Here, k ₁ is the wave number of the surface electromagnetic wave, and k ₀ is the wave number of the laser light, that is, (2π) / (laser light wavelength)
It is.

Here, when the polarization angle φ of the electric field component of the laser beam is zero, the expression (5) is as follows: d = (2π) / (k ₁ + k ₀ sin θ) (6) This is the case where the direction of the holographic grating is parallel to the direction of the interference grating due to the interference between the surface electromagnetic wave and the incident laser light. Here, D = m · d
(M is an integer), then sin θ = (k ₁ / k ₀ ) / (2m −1) (7) The value of (k ₁ / k ₀ ) is 266 n of GaAs.
Using the value for the wavelength of m, sin θ = 1.169 · (2m−1) (8), 22.9 degrees for m = 2 and 13.5 for m = 3
Degrees, 9.6 degrees for m = 4.

A periodic ripple structure having a period (d) is formed according to the specific incident angle. For example, m
= 2, a fine ripple of d = 171 nm is formed.

When the wavelength of the laser beam is changed,
Since ₀ and k ₁ is changed, (6) the formula, periodic ripple pitch (d) is changed. Therefore, a fine ripple structure having a free pitch width can be obtained by changing the wavelength of the laser beam. Further, since the ratio between k ₁ and k ₀ is almost constant, it is not necessary to give the irradiation optical system a large degree of freedom.

In the method for forming a periodic fine structure on a solid surface according to the third aspect of the present invention, by changing the polarization angle φ of the electric field component of the laser light of the above equation (5),
As shown in the figure, the holographic grating G
_1, changing the intersection angle φ between the interference grating G _2, to form a periodic dot structure of the desired shape.

Therefore, a periodic dot structure having a desired shape can be formed in one step. Further, as shown in the equation (1), the period D of the holographic grating G ₁ can be changed by changing the wavelength and the incident angle of the laser light to be irradiated, and the period d of the interference grating G ₂ is As shown in the equation (5), it can be changed by changing the wavelength, incident angle, and polarization angle of the laser light to be irradiated.

[0030]

Next, an embodiment of the present invention will be described.

FIG. 1 is a view schematically showing an apparatus used in the first embodiment of the present invention.

This irradiation device is a laser generator (Q-switched Nd: YAG laser generator (Quantel YG681)).
1, spatial filter 2, mirror optical system 3a, 3
b, etc., and the irradiation cell is composed of an irradiation container 4 made of a stainless steel container having a quartz window, a gas introduction system 5, an exhaust system (oil rotary pump) 6, and the like.

In this embodiment, first, Ga
The As substrate was set, methane bromide (1%, helium diluted) was introduced from one end of the container, and the oil was evacuated from the other end by an oil rotary pump 6 so that the pressure in the container 4 became 1 atm.

Next, after removing the second harmonic component from the fourth harmonic light from the laser generator 1 through a quartz prism, the light is shaped by a pinhole 2a and divided into two beams by a semi-transmissive mirror 3a. Both beams are irradiated by the reflecting mirror 3b into the Ga inside the irradiation container 4.
The light was incident on the As (100) substrate so that an interference grating was formed parallel to the (011) direction (P-polarized light). The wavelength of the laser light used is 266 nm, the pulse width is about 5 ns, the repetition rate is 10 Hz,
The output energy is 150-350mJ.

After the laser irradiation, the surface of the GaAs substrate was observed with a scanning electron microscope (SEM). FIG. 2 shows an angle θ = 22.9 obtained when m = 2 in equation (4) using the apparatus of FIG.
N-type GaAs (10
0) is a sectional view of a periodic ripple on the surface. The period “a” of the groove A having a deep periodic ripple was 355 nm. The theoretical period given by equation (1) is 341 nm, which indicates a good agreement. On the other hand, the shallow groove B in FIG. 2 is an etching groove generated by a surface electromagnetic wave. The period b of the etching groove generated by this surface electromagnetic wave was 169 nm, which was in good agreement with the theoretical value of 171 nm.

FIG. 3 is a diagram showing the depth of a groove obtained by direct etching by holographic exposure as a function of laser irradiation time. This etching is linear with respect to the laser irradiation time. Etching rate is 1.13nm / m
was in.

On the other hand, FIG. 4 shows the dependence of the depth of the etching groove generated by the surface electromagnetic wave on the laser irradiation time.
It is. The etching groove grows exponentially with respect to the laser irradiation time, indicating that the etching using the surface electromagnetic wave is based on the stimulated scattering effect. When holographic exposure was performed for 50 minutes, the depth of the groove obtained by direct etching was 57 nm, whereas the depth of the etching groove using surface electromagnetic waves was almost the same as 48 nm.

Next, a second embodiment of the present invention will be described. FIG. 5 is a diagram schematically showing an apparatus used in the second embodiment of the present invention, and the same reference numerals are given to the same portions as those in the above-described embodiment of FIG. As shown in the figure, in the present embodiment, the irradiation device is provided with λ / 2 as a polarizing means.
A wave plate 10 is provided.

Then, a third harmonic (wavelength: 355 nm) is generated from the laser generator 1 and shaped by the pinhole 2a, and then polarized by the λ / 2 wavelength plate 10 (parallel to the sample entrance surface). Is converted into s-polarized light, and is further divided into two beams by a semi-transmissive mirror 3a, and both beams are reflected by an all-reflective mirror 3b on an n-GaAs (100) substrate in the irradiation container 4, [0111] The light was incident so that an interference grating was formed parallel to the direction. The laser output power after shaping by the pinhole 2a was about 100 mW at 10 Hz, the pulse width was 5 ns, and the incident angle of the laser beam was 19.1 °.
The etching gas was set under the same conditions as in the above-described embodiment, and laser light irradiation was performed for 10 minutes.

The GaAs thus etched is
The substrate surface was observed with a scanning electron microscope (SEM). As a result, as shown in FIG. 6, a group of etching grooves E formed periodically and parallel in the vertical direction, and these etching grooves E
And a group of etching grooves F formed periodically and parallel in the horizontal direction so as to be perpendicular to the above, a periodic fine structure separated into dots was observed. The period e of these etching grooves E is 544.8 nm, and the etching grooves F
Was 369.7 nm.

Here, the above equation (1) shows that the wavelength λ = 35
The theoretical period D of the holographic grating obtained by substituting 5 nm and the incident angle θ = 19.1 ° is 541.2 nm. Therefore, it is understood that the etching groove E is based on the holographic grating due to the interference between the laser beams.

The etching groove F is considered to be based on an interference grating caused by interference between the surface electromagnetic wave and the laser beam. Here, in the above equations (2), (3) and (4), the complex permittivity ε _m = ε ₁ + of GaAs at a wavelength of 355 nm.
Solving the dispersion relation as jε ₂ = 8.08 + j13.57 gives k ₁
^{= 1.74 × 10 5 cm -1,} k 2 = 1.48 × 10 4 cm -1 becomes k _‖ (=
k ₁ + jk ₂ ). When a surface electromagnetic wave propagates with a propagation constant of k ₁ and the incident laser light has an electric field component in the same direction as the surface electromagnetic wave, the light intensity distribution on the surface is in the x direction as an interference effect, and I (x) = sin [ (K ₁ + k ₀ sin θ) x]. In this embodiment, since the incident surface of the laser beam is substantially perpendicular to the sample surface, k ₀ for s-polarized light.
The term of sin θ is 0, and the spatial frequency of k ₁ and the fringe of the interference grating match. Accordingly, in the above-mentioned (5), the theoretical period d of the periodic structure by the surface electromagnetic wave at the s-polarized light, d = (2 π) / k 1 , and the value becomes 361.1Nm. Therefore, it can be seen that the above-described period f = 369.7 nm of the etching groove F is in good agreement with this theoretical value.

[0043]

As is clear from the above embodiments, according to the present invention, a high-precision periodic ripple structure or periodic dot structure with an arbitrary pitch can be obtained in one step, and there is no dark reaction. Therefore, a high aspect ratio can be obtained.

[Brief description of the drawings]

FIG. 1 is a diagram schematically showing an apparatus used in a first embodiment of the present invention.

FIG. 2 is a sectional view showing a periodic ripple structure on a solid surface obtained according to the present invention.

FIG. 3 is a diagram showing the depth of a groove obtained by direct etching by holographic exposure as a function of laser irradiation time.

FIG. 4 is a view showing the laser irradiation time dependency of the depth of an etching groove generated by a surface electromagnetic wave.

FIG. 5 is a diagram schematically showing an apparatus used in a second embodiment of the present invention.

FIG. 6 is a diagram showing a periodic dot structure on a solid surface obtained according to the present invention.

FIG. 7 is a diagram for explaining a method of forming a periodic dot structure.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Laser generator 2 ... Spatial filter 3 ... Mirror optical system 4 ... Irradiation container 5 ... Gas introduction system 6 ... Exhaust system (oil rotary pump)

Continuation of front page (56) References JP-A-2-196425 (JP, A) JP-A-3-76289 (JP, A) JP-A-3-67101 (JP, A) JP-A-62-18035 (JP) , A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H01L 21/3065 C23F 4/02 H01L 21/00 H01L 29/00

Claims (5)

(57) [Claims] 1. A holographic grating generated by irradiating a plurality of laser beams at a predetermined incident angle onto a surface of a solid sample set in an irradiation container filled with a reaction gas, the holographic grating being generated by interference of the laser beams, and the laser beam The solid sample is excited by exciting the reaction gas in accordance with the interference intensity of the interference grating generated by interference of light and the surface electromagnetic wave excited on the solid surface by the irradiation of the laser light, and etching the reaction gas caused by decomposition. In a method of forming a periodic fine structure on a surface, the interval between the holographic gratings is set to an integral multiple of the interval between the interference gratings, and the periodic fine structure on the solid surface is characterized by forming a periodic ripple structure. Forming method. 2. The method for forming a fine structure on a solid surface according to claim 1, wherein the period of the periodic ripple structure is changed by changing the wavelength of the laser light to be irradiated. 3. A holographic grating generated by the irradiation of a plurality of laser beams at a predetermined incident angle onto a surface of a solid sample set in an irradiation container filled with a reaction gas, the laser beam being interfered with the laser beam. The solid sample is excited by exciting the reaction gas in accordance with the interference intensity of the interference grating generated by interference of light and the surface electromagnetic wave excited on the solid surface by the irradiation of the laser light, and etching the reaction gas caused by decomposition. In a method of forming a periodic fine structure on a surface, by changing a polarization angle of the laser beam, changing a crossing angle between the holographic grating and the interference grating to form a periodic dot structure. For forming a periodic microstructure on a solid surface. 4. The period of the holographic grating is changed by changing the wavelength and the incident angle of the laser light to be irradiated, and the period of the interference grating is changed by changing the wavelength, the incident angle and the polarization angle of the laser light to be irradiated. 3. The method for forming a fine structure on a solid surface according to claim 2, wherein the method is changed by changing the following. 5. The method for forming a fine structure on a solid surface according to claim 1, wherein the reaction gas is methane bromide.