Analysis Of Key Laser Parameters For GaAs Stealth Dicing

Theoretical analysis of laser parameters is crucial to the process demand program and technical requirements, invisible cutting should be based on the wafer material properties to select the appropriate laser wavelength, so that the laser can be transmitted through the wafer surface layer, forming a focal point inside the wafer (the so-called semi-transparent wavelength). The primary condition is that the laser photon energy is less than the absorption band gap of GaAs material, which is optically transparent characteristics. Only when the photons are not absorbed by the material or a small amount of it, the optics will show transparent characteristics. Photon absorption can cause electrons in different states between the jump, so that the electrons from the low energy level jump to the high energy level. The strength of light energy absorption in semiconductors is usually described by the absorption coefficient. Assuming a light intensity of I(x) and an absorption coefficient of α (in cm-1) per unit distance, the absorbed energy in dx is:
dI(x)=-α-I(x)dx (1)
Then the internal light intensity of the semiconductor can be expressed as I(x) = I(0)-e -α-x) (2)
where the absorption coefficient is a function of the light energy, and the dependence of the absorption coefficient on the light energy (wavelength, wave number or frequency) is called the absorption spectrum. Figure 1 shows the absorption spectra of common semiconductor materials (e.g., Si, Ge, GaAs, etc.), the wavelength in the vicinity of 0.87 μm GaAs absorption coefficient undergoes a drastic change is due to the absorption of photon energy by the carriers of GaAs, so that it is generated by jumping from the low energy level to the high energy level. In this regard, a laser beam with a wavelength shorter than 0.87 μm cannot pass through a GaAs wafer, while a wavelength greater than 0.87 μm can pass through GaAs. this wavelength is the long-wavelength limit of λ0 for GaAs materials.

The wavelength of light corresponding to the long-wavelength limit λ0 determines the minimum photon energy that can cause intrinsic absorption in semiconductors, and there exists a frequency limit v 0 corresponding to the frequency. When the frequency is lower than v 0 (or the wavelength is longer than λ0), it is impossible to produce intrinsic absorption, and the absorption coefficient decreases rapidly, and this wavelength λ0 (or the frequency limit v 0) is called the intrinsic absorption limit of semiconductors.
The wavelength of the light wave at which intrinsic absorption can occur is less than or equal to the forbidden bandwidth, that is:
hν=Eg=hc/λ0 (3)
Where: Eg is the forbidden bandwidth of semiconductor materials; h is Planck's constant; c is the speed of light. Substitution can be obtained:
λ0=1.24/Eg (4)
Calculation can be obtained Si long-wave limit λ0 ≈ 1.1 μm, GaAs long-wave limit λ0 ≈ 0.867 μm for the chip three-dimensional integration of GaAs wafers, although the thickness of the wafer, impurity composition and its content of factors such as the spectral absorbance has an impact on the GaAs material mainly absorbs wavelengths of 0.87 μm or less, including near-ultraviolet wavelengths of light, and near-infrared wavelengths of the longer light The pass rate is better for the longer wavelengths of near-infrared light. Therefore, stealth cutting GaAs material wafers, usually choose the wavelength of 1064 nm infrared laser (laser full cut generally choose ultraviolet laser); stealth cutting Si material wafers, usually choose the wavelength of 1342 nm infrared laser, so that the laser light through the surface of the wafer, in the focusing lens, such as the role of the optical institutions, the wafer in the middle of the upper and lower surfaces of the wafer between the surface of the focusing of the selectable. At the same time, as far as possible to reduce the incident surface and the laser focus between the material layer of the laser absorption effect.
GaAs stealth dicing selects an ultrashort pulsed infrared laser beam with high repetition frequency, laser power greater than 5 W, and pulse width time less than 100 ns, to compress the laser absorption energy to the threshold level in order to obtain a more desirable effect of the modification layer and to control the heat-affected region. The absorption coefficient actually increases exponentially with increasing temperature. Therefore, the pulse width parameter is also very critical, not too small to ensure that enough energy is absorbed in the focusing region to form the modified layer, and not too large to allow the temperature of the region around the modified layer to be too high. Figure 2(a) shows the GaAs wafer sample after invisible cutting, and Figure 2(b) shows the cut section of the GaAs wafer sample after invisible cutting by microscope, which shows that along the thickness direction of the 100 μm thick sample, a few micrometers wide and 30 μm thick modification layer is formed in the middle layer of the wafer. From Fig. 16(b), a vertical crack line can be observed extending from the top and bottom of the SD layer to the front and back surfaces of the chip. The chip separation effect depends greatly on how far this vertical crack extends to the front and back surfaces of the chip.