JPH0644646B2 - Method for cleaning surface of solid-state laser device body - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for cleaning the surface of a solid-state laser element body in a step of forming an antireflection coating on the solid-state laser element body.

[Prior Art] With the recent increase in output of solid-state laser devices, the antireflection film formed on the surface of the solid-state laser device body is destroyed by laser light irradiation in order to bring various effects to the solid-state laser device body. What has been done has been regarded as a problem.

When the antireflection film formed on the surface of the solid-state laser element body is destroyed by the irradiation of laser light, optical energy loss occurs inside the laser element body that constitutes the resonator. It is easily predictable that the oscillation efficiency of the will greatly decrease.

Therefore, if the laser is to be used stably over a long period of time, the solid-state laser element body must have an antireflection coating for the solid-state laser element body, which has a high laser damage threshold value, on the surface thereof. It is coming.

Regarding optical glass and quartz glass, although various surface treatment methods such as ultraviolet irradiation, laser irradiation, etching, and plasma cleaning have been studied as a method for forming an antireflection film having a high laser damage threshold value, a solid-state laser element body about,
An antireflection film with a sufficient laser damage threshold and a sufficient performance has not yet been realized.

In addition, after mechanically precision polishing the surface of the solid-state laser element body, after performing a cleaning treatment by an ultrasonic cleaning method in the order of detergent, pure water, and alcohol, a steam drying treatment using Freon is performed, and then, It is common because a single layer or a multilayer antireflection film is applied by the vacuum vapor method.

However, in such a cleaning method, contaminants such as carbon adhering to the surface of the solid-state laser element body, colloidal silica used for precision polishing of the solid-state laser element body,
It was not possible to completely remove the polishing agent such as cerium oxide from the surface of the solid-state laser element body.

If the antireflection film is formed while these abrasive particles adhering to the surface of the solid-state laser element body remain, these abrasive particles adhering to the surface of the solid-state laser element body will absorb the laser. As a result, heat is generated and the coating film that comes into contact with the abrasive particles is locally burned.

The greater the residual amount of these abrasive particles attached to the surface of the solid-state laser element body, the greater the damage to the antireflection film and the lower the laser damage threshold value.

Therefore, the advent of a technique for obtaining a clean surface before forming an antireflection film has long been awaited.

[Problems to be Solved by the Invention] As described above, in order to improve the laser damage threshold value on the surface of the solid-state laser element body, antireflection for the solid-state laser element body is applied to the surface of the solid-state laser element body. The state of treatment of the surface of the solid-state laser device body before forming the film becomes important.

In this case, it is desirable that the surface roughness of the solid-state laser element body immediately before forming the antireflection film on the surface of the solid-state laser element body is not significantly changed by performing the surface treatment. The contaminants such as fingerprints, dust, fat and carbon that are the pollution sources on the surface of the laser element body to the solid state laser element surface, as well as the colloidal silica used for precision polishing and the polishing agent such as cerium oxide are completely removed. Must be

The present invention, just before forming an antireflection film on the surface of the solid-state laser element body, fingerprints, dust, which is a source of contamination on the surface of the solid-state laser element body, without greatly impairing the surface roughness of the solid-state laser element body. In order to completely remove contaminants such as fat, carbon and the like, and further polishing agents such as colloidal silica and cerium oxide used for precision polishing of the solid-state laser element from the surface of the solid-state laser element, the solid-state laser The purpose of the present invention is to provide a method for cleaning the surface of an element body to clean it.

[Means for Solving the Problems] The inventors of the present invention have conducted extensive studies to achieve the above-mentioned object, and as a result, immediately before the formation of the antireflection film formed on the surface of the solid-state laser element body, the solid-state laser It has been found that the objective can be achieved by immersing the surface of the device body in a cleaning solution obtained by mixing phosphoric acid and sulfuric acid, and then achieving an antireflection film on the end face of the solid-state laser device by the vacuum deposition method. The present invention has been completed.

The details of the present invention will be described below.

That is, the present invention is a method for cleaning the surface of a solid-state laser element body, which comprises immersing or contacting the surface of the solid-state laser element body on which an antireflection coating is to be formed in a mixed solution of phosphoric acid and sulfuric acid. It is preferable that the liquid mixture has a weight mixing ratio of 0.6 to 0.3 that resists sulfuric acid of phosphoric acid, and a temperature of 150 ° C to 220 ° C, and particularly, The solid-state laser element body is Nd; YAG crystal.

[Operation] Before the cleaning method according to the present invention is carried out, a small amount of abrasive particles, which adhere to the surface of the solid-state laser element body in the precision polishing step and are not removed even by the subsequent preliminary cleaning, remain, The phosphoric acid-sulfuric acid mixed solution, which is the cleaning solution of the present invention, removes adhered particles from the surface of the solid-state laser element body and cleans the surface of the solid-state laser element body.

In this case, the colloidal silica is changed into a state where it is easily aggregated by the dehydration reaction of the mixed liquid, and is removed from the surface of the solid-state laser element body.

Further, during handling of the solid-state laser element body, oil and fat, carbon particles, organic dust and the like adhering to the surface are removed from the surface of the solid-state laser element body by the oxidizing power of the mixed liquid.

However, these reactions do not show a sufficient effect when phosphoric acid or sulfuric acid is used alone for washing, and the effect is exhibited only when both are mixed and used.

Phosphoric acid for particles attached to the surface of the solid-state laser device-
Generally, the reaction rate of the cleaning solution containing sulfuric acid increases as the temperature of the cleaning solution increases, and the cleaning time can be shortened as the temperature of the cleaning solution increases.

However, if the liquid temperature is too high, the cleaning solution of phosphoric acid-sulfuric acid mixture locally and selectively dissolves the surface of the solid-state laser element body, resulting in the risk of degrading the surface smoothness. come.

From the above, the composition and treatment temperature of the phosphoric acid-sulfuric acid mixture cleaning solution must be determined by factors such as the surface cleanability of the solid-state laser element body, the type of the solid-state laser element body, the degree of smoothing accuracy, and the cleaning time. I won't.

In the present invention, a solid-state laser device body, particularly Nd: Y
In the case of targeting an AG-based single crystal, the weight mixing ratio of phosphoric acid to sulfuric acid is in the composition range of 0.6 to 3.0,
It is preferable to use a cleaning liquid having a liquid temperature in the range of 150 to 220 ° C.

In the present invention, the temperature of the cleaning liquid for cleaning the surface of the solid-state laser element body is limited to 150 to 220 ° C. immediately before forming the coating film on the surface of the solid-state laser element body. If it is less than C, the silica film often remains on the surface of the solid-state laser element body, and as a result, the laser damage threshold value is lowered, and the cleaning liquid temperature is 220 ° C.
If it exceeds, even if the immersion time in the cleaning liquid is about 100 seconds,
This is because the surface itself of the solid-state laser element body is melted to deteriorate the surface accuracy of the solid-state laser element body, and the surface roughness on the surface of the solid-state laser element body increases.

In addition, if the weight mixing ratio of phosphoric acid to sulfuric acid is in the range of 0.6 to 3.0, the cleaning liquid in the above temperature range shows a complete cleaning effect even by the cleaning treatment for the immersion time of 40 to 100 seconds. No foreign matter is observed on the surface of the solid-state laser element body after cleaning, and a clean surface state is secured.

As a method of cleaning the surface of the solid-state laser element body, a method of directly immersing the solid-state laser element body in a cleaning liquid is adopted, but a method of dropping the cleaning liquid on the surface of the solid-state laser element body or spraying the cleaning liquid on the surface of the solid-state laser element body The method of making it possible is also possible.

To complete the cleaning operation, the solid-state laser element body is taken out of the cleaning liquid and immediately immersed in cold pure water.

[Examples] Examples of the present invention will be described below.

Example 1 First, as a solid-state laser element body, Nd-doped Y ₃ Al ₅ O ₁₂ (Nd: YAG) having a diameter of 30 mm and a thickness of 5 mm was cut out from an ingot, and the surface of the solid-state laser element body was polished with colloidal silica. The surface roughness is the root mean square surface roughness (RMS).
Prepare a 0.5 nm sample at
Ultrasonic cleaning was performed with pure water.

Next, a cleaning liquid prepared by mixing phosphoric acid having a purity of 85% or more and sulfuric acid having a purity of 95% or more in a weight ratio of 1: 1 (corresponding to a weight mixing ratio of about 1.1) was prepared. C
The sample is immersed in the heated state for 60 seconds, then pulled up, washed with running water of ultrapure water for 10 minutes, and further washed with pure water and alcohol in this order with an ultrasonic cleaner. The test samples were obtained by stacking and further drying with Freon vapor.

The above test sample was attached to an electron beam vacuum deposition apparatus,
After exhausting to 1.0 × 10 ⁻⁶ torr, the substrate heating temperature is set to 300 ° C., and the optical film thickness is 266 nm.
The refractive index is 1.36 and the oscillation wavelength of the laser medium is 10
A 64 nm MgF ₂ anti-reflective coating was deposited on the above test sample.

For the test sample coated with the antireflection film as described above, N having a wavelength of 1064 nm and a pulse width of 1 ns
When a laser damage threshold value was measured by a one-on-one method in which a laser irradiation position was changed for each shot using a d ': YAG laser, a value of 8 J / cm ² was shown.

Example 2 As a solid-state laser element body, Nd having a diameter of 30 mm and a thickness of 5 mm
Dope Y ₃ Al ₅ O ₁₂ (Nd: YAG) is cut out from an ingot to make a substrate sample, and a fluid liquid is dripped on the substrate rotating at high speed, and the centrifugal force of the rotating substrate is used. After applying the colloidal silica, which is a polishing agent, to the surface of this Nd: YAG substrate sample with a thickness of 1 micron by using a "spin coater" that applies the liquid thinly and uniformly on the substrate by 1
The sample was heated in a dryer at 00 ° C. for 10 hours to evaporate the water and used as a sample for a cleaning test.

Separately prepared phosphoric acid with a purity of 85% or more and sulfuric acid with a purity of 95% or more were mixed in a weight ratio of 1: 1 to obtain a cleaning liquid, which was heated to a liquid temperature of 180 ° C. After immersing the above-mentioned sample for cleaning test therein for 60 seconds, the sample was immersed in ultrapure water to stop the cleaning action.
A silica film remaining on the sample surface of the sample obtained by performing a cleaning treatment on the sample for 0 minutes, further cleaning with pure water and alcohol in that order using an ultrasonic cleaner, and drying with fluorocarbon vapor. When the total amount of the above was calculated by measuring the weight change before and after the washing treatment, silica could not be detected at all.

[Example 3] The total amount of the silica film remaining on the sample surface was measured for all the samples treated in the same manner as in Example 2 except that the time for immersing the cleaning test sample in the cleaning liquid was 80 seconds. However, it could not be detected at all.

[Example 4] A sample treated in the same manner as in Example 2 except that the time for immersing the sample for the cleaning test in the cleaning liquid was 40 seconds and the heating temperature of the cleaning liquid was 200 ° C was applied to the sample surface. When the total amount of the remaining silica film was measured, it could not be detected at all.

Example 5 A sample treated in the same manner as in Example 2 except that the time for immersing the sample for the cleaning test in the cleaning liquid was 60 seconds and the heating temperature of the cleaning liquid was 200 ° C. When the amount of remaining carbon was measured by Auger electron spectroscopy, no peak indicating the presence of carbon was detected on the Auger spectrum line obtained from the sample surface.

[Comparative Example 1] All were treated in the same manner as in Example except that the dipping treatment was not performed by using a cleaning solution in which phosphoric acid having a purity of 85% or more and sulfuric acid having a purity of 95% or more were mixed at a weight ratio of 1: 1. For the test sample, the laser damage threshold measured in the same manner as in the example was 3 J / cm ² .

[Comparative Example 2] The surface roughness of the test sample was 1.8 nm when treated in the same manner as in Example 1 except that the temperature of the immersion treatment with the cleaning liquid was 250 ° C. It showed a significant increase in roughness compared to the roughness which was 0.5 nm.

[Comparative Example 3] A sample treated in the same manner as in Example 2 except that the time for immersing the sample for the cleaning test in the cleaning liquid was 20 seconds and the heating temperature of the cleaning liquid was 180 ° C was measured. When the total amount of the remaining silica film was measured, it was found that the cleaning treatment was insufficient and 27% of the silica film remained on the sample surface.

[Comparative Example 4] A sample treated in the same manner as in Example 2 except that the time for immersing the sample for the cleaning test in the cleaning liquid was 80 seconds and the heating temperature of the cleaning liquid was 100 ° C was measured. When the total amount of the remaining silica film was measured, it was found that the cleaning treatment was insufficient and 37% of the silica film remained on the sample surface.

[Comparative Example 5] A sample treated in the same manner as in Example 2 except that the operation of immersing the cleaning test sample in the cleaning liquid was not performed.
When the amount of carbon remaining on the sample surface was measured by Auger electron spectroscopy, a peak showing the presence of carbon was clearly observed on the Auger spectrum line obtained from the sample surface.

In addition to the above, even if the mixing ratio of phosphoric acid and sulfuric acid as the cleaning liquid is changed in the range of 0.5 for phosphoric acid to 0.5 to 1.2 for phosphoric acid and 0.8, the same as the present invention. The results have been obtained.

The laser damage threshold value is measured by changing the laser irradiation position each time the laser is irradiated, and at the laser irradiation position, a sample is magnified 200 times using a Nomarski differential interference microscope. The back side of the film was observed from the back side of, and the lowest laser power density at which laser damage was confirmed was taken as the laser threshold.

From the above results, by carrying out the present invention on Nd: YAG precisely polished, the laser damage threshold value of the MgF ₂ antireflection film applied to the end face of Nd: YAG is more than doubled. It became clear that the colloidal silica used in the precision polishing of the solid-state laser element body can be sufficiently cleaned and removed.

It should be noted that this phenomenon is due to the fact that, with respect to the Nd: YAG element that has been precisely polished, SiO ₂ / Ta which is an antireflection film other than MgF _2
2 O ₅ / SiO ₂ / Ta ₂ O ₅ / SiO ₂ or SiO ₂ /
The same effect can be obtained when a multilayer antireflection film such as Al ₂ O ₃ / SiO ₂ is processed.

[Effects of the Invention] By carrying out the present invention, contaminants such as dust adhering to the surface of the solid-state laser element body during the polishing process can be completely removed without deteriorating the surface accuracy of the solid-state laser element body obtained by precision polishing. Not only is it possible to obtain a clean surface of the solid-state laser element body by removing it, but also the processing time required for cleaning work can be treated in an extremely short time. In addition, the laser damage threshold of the antireflection film can be significantly improved, which is a great contribution to the electronic equipment industry.

Claims (3)

[Claims] 1. Cleaning of the surface of a solid-state laser element body, characterized in that the surface of the solid-state laser element body on which an antireflection film is to be formed is immersed in or brought into contact with a cleaning liquid prepared by mixing phosphoric acid and sulfuric acid. Method. 2. The method for cleaning the surface of a solid-state laser element according to claim 1, wherein the solid-state laser element is Nd: YAG. 3. The cleaning liquid according to claim 1 or 2, wherein the cleaning liquid has a weight mixing ratio of phosphoric acid to sulfuric acid of 0.6 to 3.0 and is at a temperature of 150 to 220 ° C. Method for cleaning the surface of a solid-state laser device body of.