1. Field of the Invention
The present invention relates to a technique of laser marking of a silicon carbide semiconductor wafer.
2. Description of the Background Art
A semiconductor element using silicon carbide (SiC) is regarded as a promising element to function as a next-generation switching element capable of realizing high breakdown voltage, low loss, and high resistance to heat, and is expected to be applied in a power semiconductor device such as an inverter.
In order to easily identify and manage semiconductor wafers to be produced in large quantities in the manufacture of a semiconductor device, marking is generally employed in which identifies are engraved on surfaces of the semiconductor wafers in an initial stage of the wafer processing. Marking techniques of a conventional silicon (Si) semiconductor wafer (hereinafter called “Si wafer”) for example include marking (laser marking) to form a recessed irradiation mark by irradiating the Si wafer with a laser, and marking to cut a surface of the Si wafer with a diamond cutter, and others.
A pulsed laser repeatedly turned on and off at certain intervals is used in the laser marking of the conventional Si wafer, and which forms an irradiation mark (pulse-irradiated mark) with application of one pulse that is a relatively large mark of a size range of from several tens to several hundreds of micrometers. In order to provide visibility, several pulse-irradiated marks are partially overlaid to form a continuous irradiation mark, and the irradiation mark is formed into a great depth by applying a laser of high output power.
A basic YAG laser (λ=1,064 nm) and a green laser (λ=532 nm) are mainly employed as lasers for the laser marking of the Si wafer. Marking with the basic YAG laser (λ=1,064 nm) is called “hard marking,” and which allows formation of an irradiation mark of high visibility while causing a high probability of generation of particles. Marking with the green laser (λ=532 nm) capable of making output power low for its high absorptance (for its low transmittance) is called “soft marking,” and which suppresses generation of particles while a resultant irradiation mark has lower visibility.
As described above, in the conventional laser marking, several pulse-irradiated marks are partially overlaid to form a continuous irradiation mark in order to enhance the visibility of the mark. However, overlapping the pulse-irradiated marks results in the formation of projections in the generation of splashes in the overlapping portion. More particles are generated if the projections are dispersed. So, the laser marking involves a trade-off between suppression of particles and provision of visibility.
Japanese Patent Application Laid-Open No. 2005-101305 discloses an example of use of a harmonic (λ=266 nm) of a wavelength four times that of a YAG laser during marking of an inorganic nitride material such as a gallium nitride substrate.
Management of particles in any environments such as those in a clean room, in a semiconductor manufacturing device and on a wafer is an important issue in semiconductor wafer processing. Many adverse effects such as secondary contamination inside the clean room and the manufacturing device, failure in the manufacturing process, and resultant characteristic degradation of a semiconductor device may be generated due to particles if the particles are not managed strictly. So, reducing the amount of particle generation and taking countermeasures against generated particles are important issues to be achieved in each manufacturing device.
Marking of a semiconductor wafer particularly generates particles in large quantities as it directly processes the semiconductor wafer with a laser and the like. The particles generated by the marking are collected in a marking unit, or removed in a step of processing the semiconductor wafer. However, particles left unremoved may generate the aforementioned problems.
An SiC semiconductor wafer (hereinafter called “SiC wafer”) has higher transmittance to laser than the conventional Si wafer. So, in order to provide the visibility of an irradiation mark, the SiC wafer requires laser irradiation at higher output power even if the SiC wafer is to be subjected to marking with a laser such as a green laser having a relatively short wavelength. This results for example in the breakage of the crystalline structure of SiC if the SiC wafer is subjected to the same marking technique as that applied for the conventional Si wafer, generating particles excessively.
It is an object of the present invention to provide a method of marking capable of maintaining high visibility of an engraved pattern and capable of suppressing generation of particles during laser marking of an SiC wafer.
The method of marking of an SiC semiconductor wafer of the present invention includes steps (a) and (b). In the step (a), an SiC semiconductor wafer is prepared. In the step (b), a laser is applied from a laser head to the SiC semiconductor wafer while the laser head is caused to move relative to the SiC semiconductor wafer, thereby engraving a predetermined pattern on a surface of the SiC semiconductor wafer. The predetermined pattern has irradiation marks as a result of irradiation with the laser. The laser is a pulsed laser of a wavelength four times that of a YAG laser. In the step (b), the laser head moves at a speed that prevents overlap between irradiation marks by continuous pulses of the pulsed laser, and in an orbit that prevents one of the irradiation marks previously formed from being irradiated with the pulsed-laser again.
The pulsed laser using a harmonic of a wavelength four times that of a YAG laser, and which has a high absorptance (low transmittance) is applied to the SiC semiconductor wafer, allowing the output power of the pulsed laser to be made low. Further, irradiation marks formed as a result of irradiation with corresponding pulses do not overlap. So, the irradiation marks are given stable shapes (projections in the form of splashes are not generated), thereby suppressing generation of particles. The irradiation marks formed at low output power do not provide high visibility if they are viewed alone. However, the irradiation marks are placed densely as they are continuously formed by causing the laser head to move, so that the pattern as an aggregate of the irradiation marks is provided with visibility.
These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.
In the preferred embodiment, the pulse-irradiated marks 1 have a relatively small diameter of about 10 μm. The small pulse-irradiated marks 1 do not provide high visibility if they are viewed alone. However, the visibility of the dots 10 (namely, the visibility of the identifier 101) is provided as the pulse-irradiated marks 1 are placed densely to form the dots 10.
A method of marking the SiC wafer of the preferred embodiment is described below. The present invention employs a pulsed laser (UV laser) using a harmonic (λ=266 nm) of a wavelength four times that of a YAG laser, and which has a relatively high absorptance (low transmittance).
First, the SiC wafer 100 targeted for the marking is prepared, and the SiC wafer 100 is fixed to a marking unit capable of outputting a pulsed laser using an UV laser. Then, the pulsed laser of an UV laser is applied from a laser head of the marking unit to the SiC wafer 100 while the laser head is caused to move relative to the SiC wafer 100 while, thereby achieving marking to engrave the pattern of the identifier 101 with the pulse-irradiated marks 1 on a surface of the SiC wafer 100.
This marking step includes first and second marking steps. In the first marking step, a plurality of pulse-irradiated marks 1 not overlapping each other are formed to render one dot 10. In the second marking step, the pattern of the identifier 101 (such as the pattern of the character “A”) with a plurality of dots 10 is rendered by repeating the first marking step.
In order to form a dot 10 as an aggregate of separated pulse-irradiated marks 1 in the first marking step, a pulsed laser should be applied to a predetermined position of the SiC wafer 100 while the laser head is caused to move at a speed that prevents overlap between continuous pulse-irradiated marks 1, and in a manner that prevents a pulse-irradiated mark 1 previously formed from being irradiated with a laser again.
As described above, a pulsed laser is an intermittent laser repeatedly turned on and off. The preferred embodiment makes a cessation period (pulse interval) be sufficiently longer than a period of laser irradiation (pulse width). As a result, the laser head moves a distance longer than the diameter of a pulse-irradiated mark in the cessation period to prevent overlap between continuous pulse-irradiated marks if the laser head moves at a speed (laser head speed) higher than a certain speed. To be specific, separated pulse-irradiated marks 1 are aligned in a direction in which the laser head moves as shown in
Making the laser head move in an orbit that does not pass through the same place more than once is the easiest way in the first marking step in order to prevent a pulse-irradiated mark 1 previously formed from being irradiated with a laser again.
Various parameters (irradiation parameters) relating to irradiation with a pulsed laser are established in preparation for the first marking step. The irradiation parameters include for example output power [W], laser head speed [mm/s], and Q-switch (Q-SW) frequency [Hz]. These irradiation parameters are described below.
The output power is a parameter corresponding to the irradiation intensity of a pulsed laser, and which contributes to the depth of the pulse-irradiated marks 1 to be formed.
The speed at which the laser head moves (laser head speed) is a parameter contributing to the distance between pulse-irradiated marks 1 formed continuously.
The Q-switch frequency is a parameter contributing to the pulse period [s] of a pulsed-laser and the energy of one pulse (pulse energy) [J].
The following relationship is established between the output power of a pulsed laser [W/s], the Q-switch frequency [Hz], and the pulse energy [J]:
(pulse energy)=(output power)/(Q-switch frequency) (1)
As described above, in the preferred embodiment, the identifier 101 engraved on the SiC wafer 100 is an aggregate of separated pulse-irradiated marks 1 (more specifically, the dots 10 forming the identifier 101 are each an aggregate of the pulse-irradiated marks 1). The pulse-irradiated marks 1 each have a stable shape as the pulse-irradiated marks 1 do not overlap each other (projections in the form of splashes are not generated), thereby suppressing generation of particles.
The high absorptance (low transmittance) of an UV laser (λ=266 nm) used as a pulsed laser for marking controls an output power at a low level. This also provides the stable shape of pulse-irradiated marks to suppress generation of particles.
The pulse-irradiated marks 1 of the preferred embodiment have a relatively small size of about 10 μm. A laser requires high output power for formation of a conventional large pulse-irradiated mark, resulting in unstable shape of the pulse-irradiated mark. In contrast, the small pulse-irradiated marks 1 can be formed with a laser having low output power, so that generation of particles is suppressed more effectively. The small pulse-irradiated marks 1 provide poor visibility if they are viewed alone. However, the dots 10 each including the densely placed pulse-irradiated marks 1, and the identifier 101 as an aggregate of the dots 10 are formed into patterns with sufficient visibility.
Thus, the preferred embodiment reduces the probability of generation, dispersion, stay, dripping and the like of particles while providing the visibility of the identifier 101 formed on the SiC wafer 100, so that subsequent processes are protected from the effect of contamination due to particles.
The irradiation parameters established in the first marking step may not be constant parameters but may be changed where necessary. As an example, increasing a distance between pulse-irradiated marks 1 lowers the visibility of the dots 10. However, increase of the distance between pulse-irradiated marks 1 also advantageously reduces the amount of particle generation to increase a throughput. There is a trade-off between visibility required for the identifier 101, and the amount of particle generation and a throughput. So, suitably controlling each of the irradiation parameters in consideration of this trade-off relationship makes it possible to effectively apply a laser in response to an object of marking.
Establishing an irradiation parameter in consideration of nonuniformity of the positions or sizes of the pulse-irradiated marks 1 is an effective way in terms of the performance of the marking unit. By referring to
The present inventors have confirmed by experiment that the identifier 101 to be engraved on the SiC wafer 100 is provided with sufficient visibility if the energy of one pulse (pulse energy) is 5 μJ or higher. The present inventors have also confirmed that the pulse energy of higher than 10 μJ generates crystal damage of the SiC wafer 100, or increases particles due to excessively great depth of the resultant pulse-irradiated marks 1. So, in order to achieve both the provision of visibility and suppression of particles, the output power and the Q-switch frequency are preferably determined such that the pulse energy falls within a range of from 5 to 10 μJ.
Referring to the depth of the pulse-irradiated marks 1, it has been confirmed that the identifier 101 to be engraved on the SiC wafer 100 is provided with sufficient visibility if the depth is 0.1 μm or more. It has also been confirmed that increase of particles becomes noticeable if the depth of the pulse-irradiated marks 1 is 0.7 μm or more. So, in order to achieve both the provision of visibility and suppression of particles, the output power and the Q-switch frequency are preferably determined such that the depth of the pulse-irradiated marks 1 falls within a range of from 0.1 to 0.7 μm.
While the invention has been shown and described in detail, the foregoing description is in all aspects illustrative and not restrictive. It is therefore understood that numerous modifications and variations can be devised without departing from the scope of the invention.
Number | Date | Country | Kind |
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2011-047205 | Mar 2011 | JP | national |