Particles attached to the surface of the object are mainly oxides and dust. YELu et al. divided laser cleaning into two types: one for dry laser cleaning (drylasercleaning) and the other for vapor laser cleaning. Laser cleaning is a new type of laser surface treatment technology that has developed rapidly in the past 10 years and has been blown by To avoid damage to the surface of the object being cleaned, a lower energy density can be used, and the number of pulses is increased to obtain high cleanliness. Selecting a higher energy density without damaging the surface condition can reduce the number of pulses and improve the cleaning efficiency.
Cleaning: air bearing (23 TiC) surface ZrO2 particles, laser: KrF excimer, f = 5 Hz. 2.4 laser beam incident direction 2.2 energy density unit area cleaning power peak increases linearly with energy density. Therefore, the cleanliness also increases substantially linearly with the energy density.
When the energy density is too low, the cleaning power per unit area is too small to produce a cleaning effect. For a laser beam of a certain wavelength, the cleaning effect can only be produced when the energy density reaches a certain value, that is, the cleaning force reaches a certain level. The energy density at this time is the threshold at which the laser beam at the wavelength begins to produce a cleaning effect (see).
At a pulsed laser beam irradiation above the energy density threshold, the excimer laser beam is irradiated from the front and back sides of the quartz substrate to clean the particles adhering to the surface as shown. The surface of the quartz was observed with an optical microscope before and after the irradiation, and the cleanliness of the cleaning was measured. At an energy density of 100 m/cm2, a repetition rate of 10 Hz, and after 100 pulses of the same position, the cleanliness of the beam from the frontal incidence is only 24%, and the cleanliness of the backside incidence is 100%. Visible, for the permissive excimer In the case of laser quartz, it is more effective to remove particles adhering to the surface from the back side than from the front side. It can be clearly seen that the threshold of energy density for cleaning aluminum particles from the quartz surface, whether the laser beam is incident from the back or from the front, is 50 m/cm2. The difference is that the incident from the back is exceeded after exceeding the energy density threshold. Cleanliness increases with increasing energy density at a faster rate.
Backside incidence provides higher cleanliness than frontal incidence. This is because the former has a higher temperature rise at the interface between the particles and the substrate, resulting in a larger cleaning force, and the cleaning power per unit area of ​​the back side is not affected by the size of the particles. For frontal incidence, the cleaning power per unit area decreases as the particle size increases (at a constant energy density), as shown. For example, when the energy density is 50 m/cm 2 , the cleaning power per unit area incident on the back surface is approximately equal to the adhesion force per unit area, and the cleaning power of the front surface incident particles of various sizes is smaller than the adhesion force. For example, when the energy density is 100 m/cm2, only the cleaning force of the aluminum particles having a diameter of less than 0.5 wn is greater than that of the quartz, and the cleaning is performed, while the aluminum particles larger than 0.5 ym remain on the quartz surface. Obviously, the backside incident will be thoroughly cleaned at this time.
2.5 The metal film deposited by the polarization state of the beam and the piezoelectric or electromagnetic material have better cleaning effect by polarized light. The substrate can be glass, quartz, silicon, and metal alloys. Experiments have shown that polarized laser cleaning is better than non-polarized laser cleaning. Silica glass, chrome film, nickel film, aluminum film, tin alloy, indium tin oxide, lithium niobate, lithium niobate, acrylic and photoresist materials. .
2.6 Blowing inert gas The use of an inert gas to blow the illuminating area can greatly reduce the possibility that the washed waste will adhere to the surface of the substrate again, thereby effectively improving the cleanliness. The inert gas used is subjected to strict filtration and drying. For example, ECHarrey uses a 3nm particulate filter and an oxygen/moisture scrubber to achieve high cleanliness.
2.7 The difference in cleaning process parameters between the base material and the pollutants and the substrate with different pollutants. Table 1 shows the optimum energy density, average energy flux, and peak power for certain materials cleaned with a KrF excimer laser (maximum output energy of 600 m per pulse, maximum repetition rate of 30 Hz).
The energy density is the energy that is instantaneously applied to the target divided by the spot area. The average energy flux is the sum of the energy of multiple pulsed illumination per unit area. The peak power is the injection energy divided by the pulse density of the laser.
The laser pulse shape calculated here is similar to the top of the cap, and the pulse width is 34 ns. Most of the data in Table 1 has been experimentally verified. It can be seen from Table 1 that the suitable cleaning process parameters for different substrates and contaminants vary widely.
3 device surface particle laser cleaning optics surface when the attached particle size is equivalent to or less than the incident light wavelength, resulting in a small size effect, resulting in a sharp increase in absorption, light scattering and loss, resulting in a decrease in the spectral reflectance factor and a decrease in the laser damage threshold. If there are alkali metal ions such as Na+ and K+ on the surface, the glass substrate will be slightly corroded, which will directly affect the coating quality.
Table 1 recommends laser cleaning process parameters substrate contaminant energy density / (/cm2) average energy flux / (/cm2) peak power / MW quartz particles, paint, refers to the semiconductor industry, due to the substrate surface is not clean enough to cause the chip The loss of failure exceeds more than half of the total loss in the manufacturing process. Therefore, the development of surface cleaning technology determines the future of ultra-precision machining technology development.
In view of this, the research on ultra-smooth surface cleaning technology, especially the ultra-smooth surface nano-scale particle cleaning, has attracted the attention of workers in the fields of electronics, microelectronics, semiconductor and optics.
3.1 Large-scale astronomical telescope mirror cleaning Astronomical observation equipment in which the mirror surface is contaminated by particles, the spectral reflection factor decreases, and the image background is aggravated. This is one of the major problems encountered in astronomical observations. The Southern European Observatory has a telescope with a diameter of the main mirror! 8.2m. Such a large mirror is difficult to clean using conventional methods.
The test was cleaned with KrF excimer laser and achieved good results.
A BK7 glass coated sample was prepared on an aluminum substrate and placed in the open position of the telescope for 342 months. The main pollutants in this area are quartz sand with a diameter of several hundred microns, and some samples have water marks on the surface.
The effect of specular laser cleaning of particulate contamination is measured as a percentage increase in spectral reflectance factor. Laser cleaning is performed while ensuring that the mirror surface is not damaged. To this end, the energy density threshold at which the laser beam of the wavelength used does not cause specular damage should be accurately determined. Table 2 shows the energy density damage threshold for the coated BK7 glass aluminum mirror. As can be seen from Table 2, the damage threshold increases as the wavelength increases. Starting from avoiding damage to the mirror, it is safer to choose a longer wavelength. For example, in Table 2, the damage threshold is 115 m/cm 2 at a wavelength of 193 nm, and the energy density is very close to the threshold of 90 m/cm 2 . After 10 pulses, only the spectral reflectance factor is increased by 13%, and the cleaning effect is poor. The wavelengths are 248nm and 351nm, and the obtained spectral reflectance increase percentage is the same, but the aluminum mirror damage threshold wavelength/nm damage threshold/(m/cm2) cleaning energy density/(m/cm2) spectral reflection of BK7 glass coated in Table 2 above. Factor increase percentage/%. Note: 10 pulses per spot.
The energy used is lower. See the relationship between the energy density used in laser cleaning and the increase in spectral reflectance. When the energy density is less than 50 m/cm2, the spectral reflection factor hardly increases. With 5 pulse irradiation, the spectral reflection factor reached the highest peak at 230m/cm2, and 15 pulses were irradiated, and the spectral reflection reached the highest value at 160m/cm2.
Continued increase in energy density will cause the spectral reflectance factor to decrease as a result of the damage to the specular coating. At a wavelength of 248 nm, using 5 pulses per position, energy density cleaning at (160 ± 0) m/cm2 does not cause specular damage, and the spectral reflectance factor can be increased by more than 25%.
It is the relationship between the number of pulses per illumination point and the percentage increase in spectral reflectance. It can be seen that when the energy density is 136 m/cm 2 and 162 m/cm 2 , the cleaning effect is substantially saturated after 5 pulses of irradiation. The first pulse has increased the spectral reflectance factor to a maximum of about 43%. A satisfactory cleaning effect can be obtained with 34 pulses. At the same time of laser cleaning, auxiliary gas or air pumping should be blown in order to blow away (or suck away) the particles falling off in the irradiation area in time to prevent secondary pollution.
3.2 Cleaning of the magnetic head slider air bearing In the disk drive industry, in order to increase the recording density, the flying head flying height value is continuously reduced. Currently the height value is around 0.1 ym. Submicron fine particles can damage the slider and the disk surface, causing the drive system to malfunction. In practice, it has been found that the accumulation of submicron particles on the surface of the sliding surface is the main cause of interface damage. The traditional ultrasonic cleaning effect is very poor, therefore, cleaning the magnetic head slider air bearing is an urgent technical problem to be solved. Experiments have shown that laser cleaning is an effective new method.
The head slider air bearing is made of aluminum oxide (AI2O3) and titanium carbide (TiC), and the surface is coated with zirconium dioxide (Zr2) particles. The loose particles are blown off with a strong air flow and then laser cleaned.
The number of particles adhering to the surface was examined by an optical microscope before and after washing.
The laser cleaning equipment uses a KrF excimer laser with a wavelength of 248 nm, a pulse width of 23 ns, a maximum pulse frequency of 30 Hz, and a maximum pulse energy of 300 m. The quartz lens is focused and irradiated onto the air bearing.
The relationship between the cleanliness of the air bearing surface of the head slider and the energy density of the injection is seen using a pulse repetition frequency of 5 Hz and 100 pulses. Almost no particles were washed away below 100 m/cm2, and cleaning was started at 100 m/cm2. It can be seen that the energy density threshold of the cleaning head slider air bearing Zr2 particles is 100 m/cm2. When the temperature is 400 m/cm2, the cleanliness is close to 100%. See the number of suitable cleaning pulses.
The pulse repetition frequency has almost no effect on the cleaning effect and cleanliness. See 0. Cleaning material: Air bearing (AI2O3 TiC) surface Zr2 particle Experimental conditions: KrF excimer laser, f=5Hz, 100 pulses, energy density 200m/cm2 4 Conclusion As a new type of cleaning technology, laser cleaning technology has a wide range of applications, causing great concern in related industries. For example, in the microelectronics industry, micro-machinery industry, cultural relics protection industry, electronic transportation industry, ship military industry, aerospace industry have broad application prospects.
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