JP2810435B2 - Laser processing method - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laser processing method, for example, a laser processing method suitable for processing a fine recess such as a V-shaped groove formed in a solar cell substrate. is there.

<Prior Art> In recent years, with the diversification of laser sources and the progress of laser devices, laser processing has been increasingly applied in a wide range of fields. In the field of semiconductors, trimming of resistors, drilling of substrates, etching, and the like have been performed. It is used for various purposes.

By the way, a variety of fine pits having different depths depending on locations, such as antireflection V-shaped grooves of a solar cell substrate and element isolation grooves of an integrated circuit, have been widely used, and most of them are currently formed by chemical etching. ing. For example, in a silicon solar cell, a process called texture etching as shown in FIG. 6 is performed on a silicon substrate 61 to reduce the reflectance on the light receiving surface and increase the energy conversion efficiency, and pyramid-shaped irregularities 62 are formed on the surface. Is formed.

<Problems to be Solved by the Invention> In the case of texture etching used in the silicon solar cell, pyramid-shaped irregularities are formed well on a single-crystal silicon substrate and greatly contribute to improvement of energy conversion efficiency. In this case, the pyramid-shaped irregularities are not formed well, and the energy conversion efficiency is not so large. This is because texture etching uses anisotropic etching. In the case of a polycrystalline silicon substrate, the ratio of the {100} surface effective for forming pyramid-shaped irregularities to the substrate surface is small (for example, low cost). 10% or less for cast substrates developed as substrates). Attempts have been made to form a V-shaped groove on such a polycrystalline silicon substrate by photolithography using isotropic etching, but it is necessary to repeat the application, exposure and phenomena of the resist a number of times, which is troublesome. It did not reach practical use. In texture etching, it is difficult to control the size of the pyramid-shaped irregularities even when a single crystal substrate is used. The formation of the fine dents having different depths depending on the locations by the chemical etching has problems such as difficulty in controlling the shape and limitation of the substrate.

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and provides a method of easily forming the above-described depression on various substrates with good controllability by using a laser beam.

<Means for Solving the Problems> In order to solve the above problems, the present invention provides a method for irradiating a processing surface such as a solar cell substrate with a laser beam, the method being predetermined in accordance with the processing shape of the processing surface. A laser processing method characterized by performing a fine linear depression of a substantially V-shaped groove while changing an irradiation shape of a laser beam at a varied speed and moving an irradiation position.

<Operation> The depth of the pit formed on the surface of the workpiece by laser light is determined by the irradiation time. The irradiation shape of the laser beam to a suitable size,
The beam must be moved for each location having different depths, and processing must be performed for each location, which takes a long time.

In contrast, in the processing method according to the present invention, for example, as shown in FIG. 1, the surface of the substrate 11 is irradiated with laser light, and the laser light irradiation shape 13 is changed to FIG. 1 (a) → (b) → (c) →
By decreasing over time as in (d),
The area of the melted / evaporated layer 12 due to the irradiation of the laser beam also becomes smaller accordingly, whereby pits having different depths depending on locations are formed in the same time as processing the deepest portion at one place by the conventional method. That is, in the processing method according to the present invention, the irradiation shape of the laser light applied to the surface of the workpiece is changed with time, so that a distribution occurs in the irradiation time of the laser light for each place. The processing can be completed in a short time.

In addition, since the irradiation shape of the laser beam can be precisely and finely controlled by the aperture of the optical system, it is possible to form a fine depression having a different depth depending on the location such as a V-shaped groove with good control.

<Example> Example 1. An example of the present invention in surface processing of a polycrystalline silicon solar cell substrate will be described.

XeCl light (wavelength λ = 308nm) as laser light source,
The processing of the polycrystalline silicon substrate was performed using a laser processing apparatus having a rectangular slit which was variable by computer control in the X and Y directions in a range of 1 to 50 μm.
The XeCl light was used as the light source because, when processing the semiconductor substrate, using an ultraviolet laser, especially one with a wavelength of 360 nm or less, the penetration depth into the substrate is very shallow and the effect on the substrate by heat is small, This is because the absorption coefficient is large and processing can be performed efficiently, and in particular, XeCl light is excellent in stability. This device reduces the laser light energy intensity by 2
When a laser beam was irradiated onto a 400 μm thick polycrystalline silicon substrate at an oscillation frequency of 100 Hz at 3.6 J / cm ² , the substrate penetrated in about 30 seconds. From this data, it was found that the substrate could be processed by 1 μm in the depth direction by about eight irradiations. Therefore, as shown in FIG.
The size was reduced stepwise from a square of μm to a square of 1 × 1 μm, and an inverted pyramid-shaped depression 23 having a depth of about 35 μm was formed in 2.8 seconds. The depression 23 has a V-shaped cross section as shown in FIG. 2 (b). Further, by sequentially moving the substrate fixed to the XY stage, a depression 23 was formed on the entire surface of the substrate.

Processing into such a shape is very difficult with the conventional laser processing method in which the irradiation shape is fixed, and also requires a very long processing time. However, according to the method of the present invention, it is simple and easy. It turned out that it can be formed in a short time.

FIG. 3 shows an example of a V-shaped groove formed according to the present invention.
In this V-shaped groove 34, the irradiation shape 31 of the laser light is first made into a square of 50 × 50 μm, and the stage is moved in the X direction 32 in steps of 6 μm for each irradiation, and the irradiation position is moved to the end of the substrate 33. By repeating the process of reducing the irradiation shape by the Y-axis slit and moving the opposite stage in the X direction, a V-shape having a width of 50 μm and a depth of approximately 35 μm having a step of about 1 μm in the depth direction is repeated. By moving the stage sequentially in the Y direction, a similar V-shaped groove 34 was formed on the entire surface.

Using the substrate thus manufactured, the V-shaped groove shown in FIG.
A solar cell having a structure in which 41 and the light-receiving surface electrode 42 are orthogonal to each other was manufactured. The reason why the V-shaped groove 41 is orthogonal to the light receiving surface electrode 42 is to make the current path generated by the light irradiation to the electrode flat and short. Although the light reflectance of this solar cell is larger than that of the solar cell formed on the substrate having the inverted pyramid-shaped depression of the previous embodiment, the series resistance component is reduced because the current path is shorter than this, and as a whole, The conversion efficiency showed a higher value. In addition, it was found that the same small reflectance was obtained as compared with a conventional single-crystal silicon substrate subjected to texture etching. In addition, there was no deterioration of the substrate due to the laser processing.

By using the laser processing method according to the present invention, processing for reducing the reflectance of a polycrystalline silicon substrate, which has been difficult in the past, can be easily performed, and since a fine shape can be accurately processed, it is ideal. It was found that processing of a general shape became possible, and a light utilization rate higher than that of a conventional single crystal could be achieved. Further, since it is easy to leave the electrode forming portion and the peripheral portion of the substrate unprocessed, especially in a thin substrate aiming at low cost, the present invention can be more effectively achieved by leaving the rib as a rib for maintaining the substrate strength. I also found it available.

Embodiment 2 An application example of the present invention in the case of wiring two layers of electrodes will be described. As shown in FIG. 5 (a), a Ni lower electrode 52 is placed on a glass substrate 51.
After forming 2000mm, polyimide layer as an insulating layer on top
53 was formed at 1 μm. Thereafter, a contact hole 55 having a diameter of 50 μm is formed in the polyimide layer using the same laser device as in the first embodiment.
53 was formed. The irradiation shape of the laser beam was increased from a rectangle of 25 × 50 μm to a square of 50 × 50 μm at a constant speed in one second. For comparison, a conventional contact hole 56 shown in FIG. 5B was also formed. Thereafter, the Ni upper electrode 4 was formed to a thickness of 2000 ° by the sputtering method, and the occurrence rate of defects in the two types of contact holes was examined. The time required for processing was the same.

From the above, it has been found that the present invention is also very effective for forming contact holes.

<Effects of the Invention> Since the present invention has the above-described configuration, processing for reducing the reflectance of a polycrystalline silicon substrate, which has been conventionally difficult, can be easily performed, and a fine shape can be accurately processed. ,
Ideal shape processing becomes possible, light utilization efficiency higher than that of conventional single crystals can be achieved, and it is easy to leave the electrode formation part and substrate peripheral part unprocessed, thus reducing costs in particular. In a thin substrate aiming at the above, it is easily possible to leave it as a rib for maintaining the substrate strength.

[Brief description of the drawings]

FIG. 1 is an explanatory view of a processing method according to the present invention, FIG. 2 is a view showing an inverted pyramid-shaped depression formed on a polycrystalline silicon substrate, and FIG. 3 is a V-shape formed on a polycrystalline silicon substrate. FIG. 4 is a view showing a groove, FIG. 4 is a structural view of a solar cell fabricated on a substrate having a V-shaped groove, FIG. 5 is a view showing a contact hole of Example 2, and FIG. 6 is formed by texture etching. FIG. 3 is a diagram showing pyramid-shaped irregularities. 13: Laser light irradiation shape 21: Laser light irradiation shape 23: Inverted pyramid-shaped depression 31: Laser light irradiation shape 34: V-shaped groove 41: V-shaped groove 42: Light receiving surface electrode 43: Substrate 44: Back surface electrode 55, 56: Contact hole

Claims (1)

(57) [Claims] In a processing method by irradiating a processing surface such as a solar cell substrate with a laser beam, the irradiation position is changed while changing the irradiation shape of the laser beam at a predetermined speed according to the processing shape of the processing surface. A fine linear recess of a substantially V-shaped groove while moving the laser beam.