Patent Application
20080261382
The present invention relates in general to dicing semiconductor wafers during semiconductor manufacturing to divide the wafers into individual chips or dice. The invention relates in particular to dicing such wafers using a scanned beam from a pulsed laser.
Wafer dicing is an operation used in the manufacture of semiconductor devices to separate a processed, silicon semiconductor wafer into individual dice, usually referred to as semiconductor “chips”. At present, most wafer dicing is done using a mechanical saw or a mechanical scribe. There is, however, a growing interest in performing dicing operations using a scanned beam from a pulsed laser. Pulsed lasers delivering radiation in the near-infrared (NIR) region of the electromagnetic spectrum and in the ultraviolet (UV) region of the electromagnetic spectrum have been investigated. Typically the IR radiation is provided by lasers using a gain-medium of neodymium-doped YAG (Nd:YAG) or neodymium-doped yttrium vanadate (Nd:YVO4) each of which is used to generate radiation having a wavelength of about 1064 nanometers (nm). UV radiation is usually provided by converting the fundamental radiation of such lasers into radiation having the third-harmonic wavelength of the fundamental radiation, for example, about 355 nm for 1064 nm fundamental radiation.
Silicon wafers have a high absorption coefficient in the UV and visible regions of the electromagnetic spectrum and are semi-transparent at wavelengths of about 1064 nm. A beam of pulsed UV radiation provides a smooth, high-quality cut but requires a relatively high peak pulse-power. This is more difficult to obtain in UV than in NIR spectral range because of a relatively low efficiency of frequency (wavelength) conversion. NIR radiation penetrates deeper into a wafer than UV radiation. This can cause thermal stresses and potential breakage of a wafer. This problem has been overcome by using relatively short pulses, for example pulses having a duration between about 1 nanosecond (ns) and 10 ns. Such short pulses can generate a plasma when focused on a wafer surface. The plasma absorbs the IR radiation, providing heat for the dicing process while avoiding thermal damage to the wafer.
It is usually desirable in semiconductor device manufacturing to have the highest possible device throughput for minimizing production cost. One way to increase throughput is to increase wafer-dicing speed. In laser dicing of wafers, cuts are made by scanning a pulsed laser beam along a line such that individual pulses overlap on the wafer. Cutting speed accordingly is dependent inter-alia on the peak power and pulse-repetition rate (PRF) in the laser beam.
Commercially available pulsed lasers than can be used for wafer dicing are typically actively Q-switched lasers. In such lasers, an electro-optical switch is used to periodically inhibit laser action in a laser resonator until energy in the gain-medium, creating by optically pumping the gain-medium, has built up to a desired level, and then is switched to allow laser action in the resonator creating a higher power, short optical output pulse. A typical PRF for such actively Q-switched pulsed lasers is about 25 kilohertz (kHz). Developmental and state-of the art actively Q-switched lasers having a PRF up to about 250 kHz have been reported. Problems encountered when operating Q-switched solid-state lasers at a PRF in excess of about 100 kHz include pulse instabilities and pulses having a longer than desirable pulse-duration, for example, about 30 ns or greater. A Q-switched, cavity-dumped, pulsed solid-state NIR laser operating at a PRF of about 100 kHz with pulses having a duration of about 1 ns is described in U.S. Pat. No. 5,870,421. This performance, however was obtained at the cost of using a complicated optical scheme and an expensive high voltage intracavity electro-optical modulator (Q-switch) for cavity-dumping to generate the short pulses.
If laser dicing of wafers is to be widely commercially practiced there is a need for a relatively simple, stable pulsed laser system having a pulse repetition rate preferably in excess of about 100 kHz. The pulses from such a laser should preferably have a duration less than about 30 ns, a peak power preferably greater than less 1 kilowatt (KW) and a wavelength in the in IR, visible, or UV ranges of the electromagnetic spectrum.
The present invention is directed to apparatus for dicing a semiconductor wafer. In one aspect apparatus in accordance with the present invention comprises a master oscillator arrangement for providing a sequence of optical pulses at a predetermined pulse-repetition frequency (PRF), an optical amplifier arrangement including at least one amplification stage arranged to amplify optical pulses in the sequence thereof to provide a dicing beam including a sequence of amplified optical pulses. An arrangement is provided for directing the dicing beam onto the semiconductor wafer and effecting relative motion between the dicing beam and the semiconductor wafer. The master oscillator arrangement includes one of a directly modulated semiconductor laser and a continuous wave (CW) laser, the output of each of which is modulated by an external modulator.
Embodiments of the invention can be operated at a PRF of about 100 kHz or greater, with pulse-durations less than about 30 ns. In a particular example, the PRF is between about 300 kHz and 500 kHz with pulse-durations between about 1 ns and 15 ns. The use of an externally modulated master oscillator provides flexibility is selecting an optimum pulse-duration for a particular arrangement.
Turning now to the drawings, wherein like features are designated by like reference numerals,
The above discussed fiber MOPA arrangement using a modulated seed-source provides higher processing speed operation with pulse repetition frequencies compared with electro-optically (E-O) Q-switched solid-state lasers. The all-fiber arrangement simplifies design of the apparatus, reduces alignment procedures and reduces cost of the system by avoiding a need for a high speed and high voltage electro-optic Q-switch device in a solid-state laser resonator (cavity).
A purpose of solid-state amplifier 70 is to increase the NIR power delivered by fiber MOPA 30 while keeping the spectral linewidth of the amplified NIR radiation narrow enough for efficient frequency-conversion. The solid-state amplification stage is preferred over another fiber-amplifier stage because in a fiber amplifier, nonlinear optical effects such as stimulated Raman scattering (SRS), stimulated Brillouin scattering (SBS), and spectral broadening due to four-wave mixing eventually limit peak pulse-power that can be obtained from a fiber MOPA. Efficiency of the frequency-conversion process in 3HG-stage 60 grows with peak pulse power in the NIR range spectral region. A solid-state amplifier with a large beam cross-section (typically ˜1 mm compared to between about 10.0 micrometers (mm) and about 50.0 in amplifier fibers) in the amplifier gain-element provides little nonlinear effects in the gain-element and increase a peak-power beyond a fiber MOPA limit. Accordingly higher peak and average powers in the UV spectral region are available in apparatus 10B than in apparatus 10A. A suitable solid state amplifier could be based on either an Nd:YAG or Vandadate gain medium.
Referring again to
One option for generating a short optical seed-pulses is to use a continuously operating (cw) optical source modulated by an external modulator. In such a system, a seed source, such as a diode-laser or a fiber laser, which is a single-frequency source operating in one longitudinal mode with a narrow linewidth of 1 kHz-50 MHz, is modulated with an external modulator providing pulses having a duration between about 0.1 ns-15 ns. A modulator, for a diode-laser or a fiber laser is preferably an electro-optical crystal in a waveguide Mach-Zehnder configuration. This type of modulator requires much less voltage (˜5 Volts) than bulk a electro-optical modulator (>100 Volts) used in solid-state laser cavities and is less expensive than a bulk modulator.
A second option for generating a short optical seed-pulses is to use a directly modulated diode-laser as a seed source. Such an approach is, in general, less expensive, and provides higher peak power (above 1 W) from the seed laser compared to modulated cw light (peak power is less than 300 mW using a waveguide LiNbO3 modulator).
A detailed description for both an externally modulated and a directly modulated diode-laser is provided in U.S. Pre-Grant Publication No. 2006/0222372, the complete disclosure of which is hereby incorporated by reference.
Directly modulating a diode laser has the advantage of independent control of pulse length and pulse repetition rate. Preferably a narrow line DBR laser (Distributed Bragg Reflector) or DFB (Distributed Feedback) laser are used as a seed source. Such lasers have a short cavity length (<5 mm) that provides a fast response time to applied electrical pulses, for example pulses having a pulse duration less than 100 picoseconds (ps).
A new type of diode-laser, referred to as a long-cavity diode-laser, has recently been developed In contrast to DFB and DBR diode-lasers, a long-cavity diode-laser has its output mirror placed not on a diode chip, but out of the chip, namely, in a fiber pigtail or waveguide optically coupled to a diode-laser in a chip. A fiber Bragg grating (FBG) written into the fiber pigtail functions as an output coupler. This provides for operation of the diode-laser in either in a single-longitudinal mode (linewidth about 50 MHz or less) or many longitudinal modes (linewidth between about 10 and 100 gigahertz). With overall cavity length of less than about 8 mm, such a laser exhibits fast response time, for example less than about 1 ns, to external modulation. That response is fast enough to generate pulses of having a pulse-duration between about 1 and about 15 ns. Long cavity diode-lasers are available from Lumics GmbH of Berlin, Germany.
It should be noted here that in apparatus 10, wherein NIR radiation is used directly for wafer dicing, the linewidth of the fiber MOPA output does not play an important role in the dicing process. Narrow linewidth, for example less than about 1 nm, linearly polarized and high peak power (>2 kW) is required in apparatus 10A and in apparatus 10B for efficient conversion of IR light into visible and UV range. Additionally, the signal from a master oscillator must have a higher spectral contrast ratio, for example greater than about 20 dB, between spectral amplitude at the peak of the spectrum and the spectral amplitude at wings of the spectrum. This helps to reduce pulse spectral broadening due to four-wave mixing.
In apparatus wherein fiber MOPA 12 has a diode-laser master oscillator, the output peak power of the diode-laser, typically between about 0.1 W and about 1 W, has to be amplified to multi-kilowatt level in a multi-stage fiber amplifier as described above. Taking into account that the pulse spectrum does suffer some broadening due to effects discussed above, a single-frequency (single longitudinal mode) master oscillator is preferable in a MOPA 30 in apparatus 10A and apparatus 10B.
The present invention is described above in terms of a preferred and other embodiments. The invention is not limited, however, to the embodiments described and depicted. Rather, the invention is limited only by the claims appended hereto.
This application claims the priority of provisional patent application Ser. No. 60/925,219 filed Apr. 19, 2007, the complete disclosure of which is hereby incorporated by reference.
| Number | Date | Country | |
|---|---|---|---|
| 60925219 | Apr 2007 | US |
