JP2003209032A - Gallium nitride wafer and marking method for gallium nitride wafer - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To mark an indicator on a GaN (gallium nitride) wafer, absorbing no visible rays and being transparent for the visible rays. <P>SOLUTION: Marking on the GaN (gallium nitride) wafer, transparent for the visible rays, can not be formed by the fundamental ray (λ=1.06 μm) or a second harmonic (λ=532 nm) of an Nd-YAG laser beam employed for Si and GaAs. Accordingly, a light beam of a shorter wavelength of less than 400 nm or a light beam of a longer wavelength of more than 5000 nm is employed to irradiate the GaN wafer and imprint a mark, composed of letters and symbols, on the front or rear surface of the GaN wafer. When a mirror-writing letter is written on the rear surface of the wafer, the letter can be seen as a normal-writing letter from the front side whereby the right and wrong side of the wafer can be distinguished at a glance. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for imparting marks such as letters and symbols to the front, back and inside of a single crystal free-standing gallium nitride (GaN) wafer. It is difficult to manufacture a GaN single crystal, and even now, a large free-standing (freestanding film) wafer having a size of more than 35 mm square is hardly commercially available. Wafers having a size of about 1 inch square are manufactured for research. Circular wafers have not yet been manufactured on a commercial basis. GaN crystals are transparent to visible light. It is significantly different from Si and GaAs, which are opaque and have metallic luster.

Semiconductor wafers such as Si and GaAs are circular,
There are shapes such as squares. In some cases, wafers having only flat and smooth surfaces are shipped as they are. If a mark is placed on a part of the wafer, that part may not be usable, and in terms of cost, it is often the case that the mark cannot be placed.

However, in order to identify a wafer, a mark such as a character or a symbol (a wafer number, a lot number, a manufacturer, a characteristic, a history, etc.) may be attached to a part of the peripheral portion of the wafer. For Si or GaAs wafers, the laser marking method is usually performed using a YAG laser (λ = 1.06 μm).

However, there is still no laser-marked wafer for a GaN wafer. This may be because there are no large wafers in the first place, and there is no room for experiments to damage expensive wafers. It was still unclear whether or not opaque Si or GaAs and transparent GaN could be used for marking on GaN wafers.

The present invention relates to a method for marking a GaN wafer and a labeled GaN wafer.

[0006]

2. Description of the Related Art Marking is a process of marking characters or symbols on a front surface or a back surface of a wafer with a laser in order to indicate a lot number, a wafer number and the like. It is common practice to write numbers, letters, and bar codes to indicate lot, quality, manufacturer, etc. on Si wafers or GaAs wafers by Nd-YAG laser. YAG laser fundamental wave (λ = 1.06 μm) or second harmonic (λ = 53)
A beam of 2 nm) is focused by a lens to irradiate the wafer, and the beam or the wafer is relatively moved to write characters and symbols. In many cases, a mark is formed by melting a part of the wafer in a dot shape with a pulsed laser. For example,

Japanese Patent Laid-Open No. 2001-68390 (2001)
"Published on March 16, 2013)" Semiconductor wafer "is a GaAs wafer having a diameter of 4 inches and a depth of 0.1 to 1 µm and a diameter of 1 using a second harmonic of a 532 nm Nd-YAG laser.
It proposes a method of writing a symbol or a character consisting of a set of dots having an outer ring height of 5 μm to 50 μm and an outer ring height of 0.1 μm to 0.5 μm.

GaAs wafers are often used as substrates for LEDs and LDs, which are small-sized light emitting devices. Since the light emitting element (LED) is formed into a small chip of about 300 μm square, the chip can be collected over the entire area of the circular wafer. Moreover, it is necessary to manufacture at a low cost with a minimum cost. In that case, the circular wafer of GaAs is not marked.

A GaAs wafer may also be used as a substrate for electronic devices such as amplifiers (AMPs) and logic elements (ICs) that take advantage of the high speed of electrons. In that case, in order to make a large chip of about 5 mm square and make it into an element,
A region that cannot be taken as a chip remains in the peripheral portion of the circular wafer. Further, since it is an expensive product for use, it is necessary to mark the outer peripheral portion of the circular wafer so that the quality can be identified. A large number of Si wafers and GaAs wafers are manufactured and sold, and wafers with markings on the back and front surfaces of the wafers are also shipped. Laser marking technology has already been established.

JP-A-11-58043 (Heisei 11
Published on March 2, 2012) "Laser marking method and device"
Is GaA with the light of an Nd-YAG laser (1.06 μm).
Since particles are emitted when marking s and contaminate the wafer, turn the wafer upside down and irradiate the wafer with a YAG laser beam to blow gas such as nitrogen and oxygen onto the wafer to remove the particles and suck the gas. To prevent the particles from contaminating the wafer. Marking is not preferable from the viewpoint of the wafer itself because the marking damages the wafer, and a powdery substance is formed by the marking, and a device for aspirating it to prevent the wafer from being soiled has been proposed.

Japanese Unexamined Patent Publication No. 9-278494 (published on October 28, 1997) “Marking method for glass substrate”
Proposes a method of marking a part of a thick and wide glass substrate for a liquid crystal panel. Since glass is transparent and does not absorb visible light, the usual laser marking method using the second harmonic of YAG and the fundamental wave cannot be used. It is stated that carbon dioxide laser of 10.6 μm cannot be used because it causes cracks in the glass. This is a Q switch, N
Laser marking is performed on a glass substrate using 262 nm ultraviolet pulsed light of a d-YLF harmonic laser. YLF is an abbreviation for YLiF4 (yttrium / lithium / fluorine). Laser light is emitted by doping this with Nd. It is a type of solid-state laser. It is a new laser that utilizes its harmonics and is not easily available.

As described above, a marked wafer already exists for a wafer such as Si or GaAs. The production amount of unmarked wafers is larger, but the production of marked wafers is also considerable.

[0013]

In recent years, GaN-based materials and ZnSe-based materials have emerged as materials for blue light emitting devices. Both are semiconductors with a wide band gap, and are important materials for blue LEDs and blue LDs. In semiconductors with a wide band gap, there are some problems to be overcome, such as difficulty in doping p-type and n-type impurities and many crystal defects.

In the case of a GaN type, the active layer is an InGaN layer, but the buffer layer, the cladding layer, the contact layer, etc. are G layers.
Since it is aN, it is called a GaN system. Good G with few defects
Sapphire (α-A
GaN-based LEDs and LDs are manufactured using l ₂ O ₃ ) as a substrate. Due to the heteroepitaxial growth on the sapphire substrate, the GaN-LED has crystal defects of 10 ⁹
It is extremely large, such as cm ⁻³ to 10 ¹⁰ cm ⁻³ .
However, despite the extremely large number of crystal defects, deterioration does not occur and it has a practical life (10,000 hours or more). So G
The aN-based LED is ahead of another leading wide-gap semiconductor ZnSe-based LED.

GaN-LED which has been put to practical use and used
However, since the current density of GaN-LD is much larger than that of LED, it may be deteriorated by current injection. Moreover, since a sapphire substrate is used, there is no natural cleavage. There is no reason to make a laser cavity by natural cleavage. Since the sapphire substrate is an insulator,
Since the mold electrode cannot be formed on the bottom, it is necessary to cut both sides of the chip to the n-type GaN buffer layer and attach the n-type electrode there. In other words, it is a convex element shape of p on n. For this reason, some GaN-LDs on a sapphire substrate have been reported to be continuous oscillation at room temperature, but they are still not practical in terms of process yield such as cleavage, high cost due to chip area yield, and longevity.

As substrates for GaN-based LEDs and LDs,
A GaN substrate should be more suitable than a sapphire substrate. However, since GaN does not melt even if it is exposed to high heat, single crystal growth cannot be performed by the pulling method (Choralski method) or the Bridgman method.

The GaN thin film is formed by vapor phase epitaxy (HVPE method, M
Heteroepitaxial growth can be carried out on a sapphire substrate by OCVD method, MOC method) or the like. Therefore, an attempt has been made to grow a thick GaN crystal into a substrate crystal by a vapor phase growth method. However, since it is thin, it grows epitaxially on the sapphire substrate, but if it becomes thicker, the strain increases and it cannot be grown epitaxially.
Not only a single crystal but also many defects occur.
Therefore, a mask having many windows at equal intervals is attached on a GaAs substrate, and GaN crystal grains are grown independently from the windows to laterally grow beyond the mask to reduce dislocations. Lateral overgrowth (Epitaxial Late Growth)
A clever GaN growth method called the ral overgrowth method has been proposed.

Japanese Patent Application No. 9-298300   Japanese Patent Application No. 10-9008   Japanese Patent Application No. 10-102546

This has made it possible to considerably reduce the number of dislocations in GaN. However, it was still insufficient, and a thick crystal could not be formed, and the dislocation density in the GaN crystal was about 10 ⁹ cm ^-2 . Furthermore, a new innovation method capable of reducing dislocations in addition to such a direction has been devised by the applicant.

Japanese Patent Application No. 11-273882
001-102307) Now, it is finally possible to manufacture a large GaN single crystal. Even so, the GaN single crystal that can still be manufactured by the method has a small diameter of about 35 mm. However, larger GaN single crystal wafers will soon be available.

GaN single crystal wafers are becoming available. Since no GaN wafer existed until then, the peripheral technology regarding the GaN wafer is naturally immature. The technique of marking with laser has not been solved yet. It is not known whether the technique of marking a GaAs wafer with an Nd-YAG laser as described above can also be applied to a GaN wafer.

GaAs and Si wafers are opaque to visible light. That is, it absorbs visible light. Therefore, if you squeeze the visible light, it will be heated locally and that part will be melted and a shallow hole will open. However, GaN is transparent to visible light. That is, it hardly absorbs visible light. GaA is a transparent wafer that looks like glass and has a gray metallic luster.
The appearance is quite different from the s-wafer.

In both the Si wafer and the GaAs wafer, the YAG fundamental wave (1.06 μm) or the second harmonic wave (5
A laser beam of 32 nm) is commonly used for marking. The above-mentioned prior art is based on Nd-YA on GaAs.
A second hole (532 nm) of G was applied to punch a dot hole.

Since GaN is transparent to 532 nm, there is no absorption and it may be difficult to heat the GaN wafer with its beam. Even if it is transparent, there is a slight absorption, so if you hit the second harmonic with a very large output G
It may be possible to heat aN. However, the term harmonic means that the YAG light of 1.06 μm is converted into a nonlinear optical crystal (KNb
O ₃ , KTP, LiTaO ₃ , LiNbO ₃ ), and is produced by the action of the nonlinear coefficient. Even under the best conditions, the conversion efficiency is only about 3%. Nd-YA
The excitation light of G (1060 nm, 946 nm) is, for example, 10
Even at 00 mW, the second harmonic is only about 30 mW. So it may not be possible to use the raw technique of YAG second harmonics.

[0025]

[Means for Solving the Problems] The fundamental wave (1.06 μm) and the second harmonic wave (5) of YAG used for Si and GaAs.
It has been found that a laser of 32 nm) cannot mark a GaN wafer that is transparent to visible light. If the power is increased to increase the absorption, the GaN wafer will crack or break.

In the present invention, a GaN wafer which is transparent to visible light is irradiated with a light beam having a wavelength of 400 nm or less or a light beam having a wavelength of 5 μm or more to engrave markers such as characters and symbols.

GaN is transparent to visible light. The light transmission coefficient is highest around 500 nm. Transmission coefficient is 500n
It decreases with increasing distance from m. Absorption coefficient is 400
It becomes high for ultraviolet rays of nm or less and infrared rays of 5000 nm or more. It was found that GaN cannot be marked unless the light has a wavelength with a large absorption coefficient. Therefore, a light source with a wavelength of 400 nm or less, or 50
A light source with a wavelength of 00 nm or more is used.

If letters and symbols are to be written as a set of dots, a pulsed light source should be used, but some pulsed lasers operating in this range exist and may be newly manufactured. Ah

The present invention can print on the front surface (device manufacturing surface) or the back surface of a GaN wafer. Not only that, but it can also be printed inside (the middle of the front and back).

Since the GaN wafer is transparent, characters in the opposite direction can be printed and read as erect characters from the surface in the opposite direction. If reverse characters are printed on the back surface that is not used for device manufacturing, it can be read as a normal character from the front surface (device manufacturing surface).

The GaN wafer is transparent and the distinction between the front side and the back side is difficult to visually recognize, but the front side and the back side can be seen immediately by observing the direction of the characters.

[0032]

BEST MODE FOR CARRYING OUT THE INVENTION In order to carry out the present invention, 400n
A high-power laser capable of generating light having a wavelength of m or less
A high-power laser capable of generating light with a wavelength of μm (5000 nm) or more is required. A carbon dioxide laser is a high-power laser that emits light having a long wavelength (infrared) of 5 μm or more. This has an oscillation wavelength of about 10.6 μm or about 9 to 10 μm. A pulsed laser is suitable because it writes discrete symbols and signs. There are some carbon dioxide gas lasers that oscillate in pulses, and they are easily available. The carbon dioxide laser beam cannot be condensed by an ordinary lens, but ZnS
It can be stopped down using the lens e. The upper limit of the laser oscillation wavelength that can be marked is 20 μm (20,000 n
m).

There are not many practical high-power lasers that emit ultraviolet rays of 400 nm or less. The excimer laser can emit laser light having such a short wavelength. The KrF laser has an oscillation wavelength of 248 nm, and the ArF laser has a lasing wavelength of 19 nm.
It has an oscillation wavelength of 3 nm. Excimer lasers currently existing satisfy the above conditions in terms of wavelength. A high power excimer laser can be used to mark a GaN wafer. The lower limit of the oscillation wavelength of the laser that can be marked is about 100 nm.

The third harmonic of YAG (λ = 355 nm) is promising from the point of wavelength (355 <400 nm), and 2
There is also an output of about 50 mW, so use GaN
It can be used for marking wafers.

I-line (365 nm) of a mercury lamp is 400
Since it is less than nm, it can be used for GaN marking if the power is increased. Although this is not a laser, it can be pulsed and used for marking.

The beam can be focused by the lens and the height of the focal point can be finely adjusted to mark not only the front surface and the back surface of the wafer but also the inside thereof. Even if you say the front side and the back side,
Either of the reverse letters can be used, and there is a degree of freedom in which part of the wafer is marked. Since the device is manufactured on the front surface, it is better to mark on the back surface. GaN is transparent, so if you mark it on the back with reverse characters, it looks like a normal character when viewed from the front. You can easily tell from the direction of the letters whether it is the front side or the back side.

FIG. 1 is a plan view of a 2-inch diameter GaN wafer. In fact, a 2-inch circular wafer is in the laboratory stage and not commercially available. What is currently available is a small self-supporting film of about 10 mm square (about 400 mm) shown in the examples.
(μm thickness). However, in the near future, 2-inch wafers will be commercially available. In FIG. 1, the line segment on the lower side is the first orientation flat. The line segment on the left side is the second orientation flat. The orientation flat is a line segment obtained by cutting a part of the peripheral portion of the wafer to match the orientation in the wafer process.
GaN has no inversion symmetry and has (0001) plane and (000-
1) The faces are not the same. Two orientation flats are necessary to show the distinction between the front and back.

Since the cleavage plane of GaN is {1-100}, the angle formed by the cleavage plane is 60 degrees and is not a right angle. Therefore, neither the first nor the second orientation flat is a cleavage plane. Either one is the cleavage plane.
Here, the "front surface" means the device manufacturing surface (surface).
That is.

FIG. 2 shows an example in which regular characters are engraved along the first orientation flat on the front surface (front surface).
When viewed from the back side, the letters are reversed so that the front side and the back side can be distinguished.

FIG. 3 shows an example in which regular characters are engraved along the second orientation flat on the front surface.
When viewed from the back side, the letters are reversed so that the front side and the back side can be distinguished.

FIG. 4 shows an example in which regular characters are engraved along the first orientation flat on the back surface (back surface). When viewed from the surface, it is the reverse character.

FIG. 5 shows an example in which regular characters are engraved along the second orientation flat on the back side (back side). When viewed from the surface, it is the reverse character.

FIG. 6 shows an example in which reverse characters are engraved along the first orientation flat on the back surface (back surface). When viewed from the front side, the characters are legitimate, so the front and back sides can be distinguished.

FIG. 7 shows an example in which reverse characters are marked along the second orientation flat on the back side (back side). When viewed from the front side, the characters are legitimate, so the front and back sides can be distinguished.

6 and 7 are the most characteristic of the present invention in that the reverse characters are drawn on the transparent wafer and viewed from the opposite side as the normal characters. In each of the examples, the characters are engraved, and the direction of the surface can be known by that, so the second orientation flat is no longer necessary and can be omitted.

[0046]

EXAMPLES [Example 1 (GaN substrate; carbon dioxide laser;
λ = 10.6 μm)] Characters were marked on the front and back surfaces of a square GaN substrate of 10 mm square and a thickness of 420 μm under the following conditions using a carbon dioxide laser with an oscillation wavelength of 10.6 μm.

[0047] (Marking conditions)   Substrate transport speed U: 3m / min   Laser irradiation time T: 100 μsec (character height 1 mm)   Laser output W: 200W-300W

[0048] (Laser beam condition)   Beam dot diameter d: 110 μmφ   Dot spacing Vertical h: 100 μm               Width k: 140 μm

(Character size) Vertical H: 1 mm (9 dots) Horizontal K: 0.7 mm (5 dots)

A regular character was engraved on the surface of the target square GaN wafer by laser irradiation, and a reverse character (a character whose left and right were reversed) was engraved on the back surface by laser irradiation.
The peak power of the laser was 170W. Since the length of one pulse is 10 ^-4 seconds, the power of one pulse is 17 m.
It is J. As a result, it was possible to print on the front and back surfaces of the transparent GaN wafer. The printing depth was about several μm.

[Example 2 (GaN substrate; YAG second harmonic; λ = 532 nm)] Since the GaN substrate is transparent, it is not necessary to engrave on the front surface and back surface, and characters are engraved inside. Good. Therefore, an attempt was made to mark characters inside the GaN substrate by the IGM (Inner Glass marking) method. 1
Characters were marked by the IGM method under the following conditions at a depth of 70 μm inside a square GaN substrate of 0 mm square and a thickness of 420 μm, using a YAG second harmonic having an oscillation wavelength of 532 μm.

[0052] (Marking conditions)   Dot count: 1 count / dot   Laser output W: 4W   Frequency f: 1000 Hz (4 mJ / count)

The GaN wafer has irregular cracks,
It was not possible to secure sufficient dot visibility. It is 4 mJ per pulse. This is not too much power. GaN in the second harmonic of YAG (532 nm)
It turns out that the marking of is not possible.

Example 3 (GaN Substrate; YAG Third Harmonic; λ = 355 nm) Since the YAG second harmonic was not able to engrave characters inside the GaN substrate, the YAG third harmonic. Wave (λ
= 355 nm), an attempt was made to imprint characters inside the same GaN substrate. The GaN substrate is the same as the previous example.
It is a square having a side of 420 mm and a thickness of 420 mm. Inside 7 of GaN substrate by IGM method (Inner Glass Marking)
Characters were marked at a depth of 0 μm.

[0055] (Marking conditions)   Laser output W: 4W   Dot count: 1 count / dot   Frequency f: 2000 Hz (120 to 280 μJ / count)

[0056]

Although the power per pulse was 2 mJ, this enabled visible printing. It was found that marking can be performed with the YAG third harmonic of 355 nm.

[0058]

EFFECT OF THE INVENTION Si wafer, G, which is opaque to visible light
For the aAs wafer, the second harmonic (5
32 nm) or a fundamental wave (1064 nm) laser beam can be used to imprint signs such as symbols on the front and back surfaces. The glass substrate also has an Nd-doped YLF laser 26
A technique for marking with a strong light of 2 nm has been proposed. Although glass is transparent, it is strong, tough, and resistant to thermal shock, and it is thick and inexpensive. It is easy to experiment with strong light. There is still no marking technology for GaN wafers. Since GaN is transparent to visible light, it has little absorption and is difficult to label. Since it is more brittle and thinner than glass, it will crack or break if treated in the same way as glass. Moreover, since it is much more expensive than Si, glass, etc., it is not possible to produce defective products due to failure in marking.

Although GaN is transparent to visible light,
There is considerable absorption for light having a wavelength of nm or less and light having a wavelength of 5000 nm or more. In the present invention, the GaN is labeled with light having a wavelength of 400 nm or less or light having a wavelength of 5000 nm or more.

Surface of GaN wafer (surface for making device)
Also, it is possible to mark on the back surface (the surface on which the device is not made) or the middle portion (inside). It is possible to write symbols and numbers as a set of dots using pulsed light.

Unlike Si and GaAs, GaN is transparent, so if you write a reverse letter (mirror-symmetrical letter) on the back side, you can see it from the front side, and you can see well which side it is. Such is not possible with Si or GaAs.

[Brief description of drawings]

FIG. 1 is a plan view of a 2-inch GaN wafer. There are first and second orientation flats.

FIG. 2 is an example in which regular characters are engraved along a first orientation flat of a front surface (front surface). When viewed from the back side, the letters are reversed so that the front side and the back side can be distinguished.

FIG. 3 is an example in which regular characters are engraved along a second orientation flat on the front surface (front surface). When viewed from the back side, the letters are reversed so that the front side and the back side can be distinguished.

FIG. 4 is an example in which regular characters are engraved along a first orientation flat on the back surface (back surface). When viewed from the surface, it is the reverse character.

FIG. 5 is an example in which regular characters are engraved along a second orientation flat on the back side (back side). When viewed from the surface, it is the reverse character.

FIG. 6 is an example in which reverse characters are engraved along the first orientation flat on the back surface (back surface). When viewed from the front side, the characters are legitimate, so the front and back sides can be distinguished.

FIG. 7 is an example in which reverse characters are engraved along the second orientation flat on the back surface (back surface). When viewed from the front side, the characters are legitimate, so the front and back sides can be distinguished.

Claims (3)

[Claims] 1. A gallium nitride substrate transparent to visible light and a light beam having a wavelength of 400 nm or less or 5000 n
A gallium nitride wafer characterized by being illuminated with a light beam having a wavelength of m or more to engrave a mark made up of letters and symbols on the front surface, the back surface, or the inside. 2. A gallium nitride wafer characterized in that a reverse character is engraved on the back surface on which the device is not manufactured so that it looks like a positive character when viewed from the front surface on which the device is manufactured. 3. A light beam having a wavelength of 400 nm or less or 5000 nm on a gallium nitride substrate transparent to visible light.
A method for marking a gallium nitride wafer, which comprises irradiating a light beam having the above wavelength to imprint a mark consisting of letters and symbols on the front surface, back surface or inside of the GaN wafer.