CN101219430B - Watt-level all-solid-state ultraviolet laser cleaning machine and laser cleaning method - Google Patents

Abstract

The invention discloses a watt-level all-solid-state ultraviolet laser cleaning machine and a laser cleaning method belonging to the technical scope of light energy cleaning, which are used for cleaning microelectronic substrates such as integrated circuits. The power supply of the device is connected to the laser resonator, and two nonlinear optical crystals, a gate mirror and a reflector are set on the optical path of the pulsed laser emitted by the laser resonator; a rotating mirror drum is fixed above the side of the reflector, placed on the conveyor belt Workpieces to be washed; place a blower on one side of the conveyor belt and a vacuum cleaner on the other. The method includes continuously or intermittently transporting the substrate to be cleaned through the laser irradiation place, and irradiating the surface of the substrate with a watt-level nearly 266nm pulsed laser output by an all-solid-state laser; selecting appropriate laser intensity, pulse frequency and irradiation time to Effectively remove micron and submicron particles, organic matter, oil and other pollutants on the surface of the substrate; after cleaning, the adhesion of the coating and the yield of the product are greatly improved, and there is little warping and peeling of the coating.

Description

Watt-level all-solid-state ultraviolet laser cleaning machine and laser cleaning method

technical field

The invention belongs to the scope of light energy cleaning equipment, in particular to a tile-level all-solid-state ultraviolet laser cleaning machine and a laser cleaning method.

Background technique

The technology of cleaning pollutants with laser was created in the mid-1970s, but it was not really applied in industrial production until the early 1990s. This is because with the rapid development of the microelectronics industry, the integration of integrated circuits, especially large-scale and ultra-large-scale integrated circuits, is getting higher and higher, and the cleanliness requirements for microelectronic substrates such as silicon wafers are also getting higher and higher. . Traditional cleaning techniques, such as mechanical cleaning, wet chemical cleaning, ultrasonic cleaning, etc. are often powerless to remove micron and submicron pollution particles. In addition, the mechanical removal method is easy to cause damage to the substrate; the wet chemical method will cause environmental pollution, and the special working conditions of ultrasonic cleaning limit its application in integrated circuit cleaning. Compared with these traditional cleaning methods, laser cleaning can effectively remove micron and submicron pollution particles, organic matter, oil stains, etc., without damaging the workpiece being cleaned, causing no pollution to the environment, high efficiency, low cost, safe and reliable, and reliable. Harm the health of the operator, easy to realize cleaning automation control and so on.

At present, the lasers that have been applied to the cleaning of microelectronic substrates such as integrated circuits and liquid crystal display circuit boards mainly include excimer lasers, CO ₂ lasers and Nd:YAG lasers. Since the shorter the wavelength of the laser, the higher the photon energy, the stronger the corresponding cleaning ability and the better the cleaning effect, so the wavelength of the laser used for cleaning is also getting shorter and shorter, so that it can output high power (above watt level), ultraviolet (248nm), deep ultraviolet (193nm, 157nm) excimer lasers have been widely used in the field of microelectronic substrate cleaning. However, excimer lasers have the following disadvantages:

1. Expensive, an excimer laser with an output of 193nm, the price of Cymer Laser is 600,000 US dollars; 2. High maintenance costs, about 250,000 US dollars per year; 3. Short life, every 28 hours of work, it needs to be replaced once Working gas; 4. Low repetition frequency, the highest working frequency is 1kHz (product of Cymer Laser); 5. The working gas is poisonous and pollutes the environment; 6. Low electro-optical conversion efficiency; 7. Large geometric size.

Therefore, the use of excimer lasers for cleaning is not only expensive, troublesome to use, but also pollutes the environment. In comparison, all-solid-state lasers represented by Nd:YAG have the following advantages:

1. Low cost, generally no more than 60,000 US dollars per laser; 2. Long service life, the service life of the pump lamp can be more than 300 hours, and the service life of the LD pump can be more than 5000 hours; 3. Low maintenance costs, the annual cost of the lamp pump is less LD pumps are less than $5,000 per year; 4. High repetition rate: generally 1Hz to 100kHz, some can reach megahertz level; 5. The spectral line width is very narrow; 6. It can operate continuously, It can also output nanosecond, picosecond, and femtosecond pulses, with reliable performance; 7. It does not cause any pollution to the environment; 8. The geometric size is small, and it is easy to combine with other systems.

Although all-solid-state lasers such as Nd:YAG have many advantages, they were not very popular in the field of laser cleaning before, because the output wavelengths of such lasers are mostly in the infrared band. By using nonlinear optical crystals for frequency conversion, although laser output in the ultraviolet band can be obtained, the output power of ultraviolet light rarely exceeds 1 watt. For example, barium metaborate (β-BaB ₂ O ₄ , referred to as BBO) crystals are used as Nd:YAG The frequency of the 1064nm laser output by the laser is quadrupled to obtain a 266nm ultraviolet laser. But because the BBO crystal is prone to photorefractive damage under ultraviolet laser irradiation, if the average power of the 266nm laser output after quadrupling frequency exceeds 300mW, the BBO crystal will be destroyed. Since the output ultraviolet laser is below the watt level, the application of all solid-state lasers such as Nd:YAG in the field of high-precision microelectronic substrate cleaning is greatly restricted.

Recently, we have prepared a new type of nonlinear optical crystal cesium lithium borate (CsLiB ₆ O ₁₀ , CLBO for short), and used this crystal to quadruple the frequency of the 1064nm laser output by the Nd:YAG laser, and obtained a 28W 266nm ultraviolet laser output; In this context, we have completed the present invention and developed a tile-level ultraviolet laser cleaning machine. Some other novel ultraviolet nonlinear optical crystals currently being developed may also be used in the laser frequency conversion of the watt-level ultraviolet laser cleaning machine provided by the present invention.

Contents of the invention

The purpose of the present invention is to provide a tile-level all-solid-state ultraviolet laser cleaning machine and an ultraviolet laser cleaning method. The laser cleaning machine is composed of three parts: an ultraviolet laser, a laser scanning system and a dirt removal device. It is characterized in that the power supply 1 of the laser cleaning machine is connected to the laser resonator 2, and the pulse laser 3 emitted by the laser resonator 2 On the optical path, a double frequency nonlinear optical crystal 4, a quadruple frequency nonlinear optical crystal 6, a gate mirror 8 and a reflector 10 are arranged; a rotating mirror drum 13 is fixed above the side of the reflector 10, and a rotating mirror drum 13 is fixed between the reflector 10 and the rotating mirror. The workpiece 11 to be washed is placed on the conveyor belt 12 below the mirror drum 13; the blower 16 is placed on one side of the conveyor belt 12, and the vacuum cleaner 17 is placed on the other side.

The mirror drum 13 is formed into a drum shape by scanning mirrors on N surfaces, and the rotational angular velocity is ω radian/second, and the mirror drum rotates at a central angle φ=2π/N corresponding to a mirror surface, and the required time τ=2π/Nω seconds, that is, the time required to scan from one side of the conveyor belt 12 to the other side.

The laser generation method of this all-solid-state ultraviolet laser cleaning machine includes the following steps:

The power supply 1 provides excitation current to the laser resonator 2 to excite the semiconductor laser diode in the laser resonator 2. The pump light emitted by the laser diode is absorbed by the laser medium to generate laser oscillation. The laser generated in the laser cavity 2 undergoes high repetition frequency Electro-optic or acousto-optic Q-switch modulation becomes pulsed laser 3 output, the output pulsed laser 3 is frequency-doubled by double-frequency nonlinear optical crystal 4, and the emitted double-frequency light 5 passes through quadruple-frequency nonlinear optical crystal 6 Perform frequency quadrupling to obtain a 266nm ultraviolet laser 7; after the ultraviolet laser 7 passes through a gating mirror 8 that is highly reflective to double frequency light 5 and highly transparent to quadruple frequency light 7, watt-level pure quadruple frequency light 9 is obtained.

The cleaning method of the laser cleaning machine includes static cleaning and dynamic cleaning:

The static cleaning is that the watt-level pure quadruple frequency light 9 is reflected by the mirror 10 onto a certain mirror surface of the rotating mirror drum 11, and the angle of the mirror surface is adjusted so that the reflected light can vertically irradiate the workpiece to be cleaned placed on the conveyor belt 12 13, after the irradiation lasts for a period of time required for cleaning, the conveyor belt 12 is transferred to a distance equivalent to the size of the workpiece to be cleaned, so that the cleaned workpiece is removed, and the next workpiece to be cleaned is also moved to the laser irradiation. The irradiation area is cleaned by laser irradiation. This cleaning method is suitable for workpieces to be cleaned that are smaller in size than the diameter of the laser spot, and the number of workpieces is small. The conveyor belt 12 is intermittently conveyed step by step. Because the rotating mirror drum does not rotate, it is called static cleaning;

In the dynamic cleaning, if the size of the workpiece to be cleaned is larger than the laser spot, or the number of workpieces is huge, it is necessary to rotate the rotating mirror drum to scan the laser beam on the workpiece, and irradiate all the surfaces of the workpiece to be cleaned for cleaning. Pure quadruple-frequency light 9 is reflected to the intersection line A place between a certain pair of adjacent mirror surfaces I and II of the rotating mirror drum 11 through the reflector 10, at this position, as long as the rotating mirror drum 11 rotates clockwise one At a small angle, the mirror II immediately reflects the pure quadruple-frequency light 9, and the reflected light beam 14 reaches the position B of the conveyor belt 12, and the rotating mirror drum 11 continues to rotate clockwise. When the mirror II turns to a position close to the mirror I, this The reflected light beam 15 at the time is irradiated to the position C of the conveyor belt 12, and the central angle φ=2π/N corresponding to a mirror surface of the rotating mirror drum 11 is assumed, and N is the number of mirror surfaces, and the value of N is determined by the diameter of the rotating mirror drum, the rotation The distance between the mirror drum and the conveyor belt is determined by the width of the conveyor belt; the value range of N is 10 to 100. At this time, every time the mirror drum rotates by an angle of φ, that is, a mirror surface is turned, the laser beam is swept from the B end of the conveyor belt 12 to C end, the workpiece 13 is irradiated and cleaned; therefore, the power of the pure quadruple frequency light 9 is required to be large enough, and the power density of the laser is still large enough after the spot diameter increases, and the time for the laser to stay on the surface of the workpiece to be cleaned is relatively short. In this case, the conveyor belt can also be continuously conveyed at a certain speed. This kind of cleaning that requires the rotating mirror drum to scan the laser beam is called dynamic cleaning.

When the laser beam is scanned and cleaned, the spot size of the pure quadruple-frequency light 9 is on the order of centimeters, and it will stay on the order of seconds when the workpiece is irradiated for cleaning. The mirror drum rotates to cooperate. The conveyor belt is continuously conveyed at a certain speed.

The mirror drum 11 is formed into a drum shape by scanning mirrors on N surfaces, and the rotational angular velocity is ω radian/second. The mirror drum rotates at a central angle φ=2π/N corresponding to a mirror surface, and the required time τ=2π/Nω seconds, that is, the time required for scanning from one side of the conveyor belt 12 to the other side for cleaning the workpiece 13 .

The dust particles and dirt from the cleaning workpiece are blown up by the blower 16 and sucked in and removed by the vacuum cleaner 17, so as to achieve the dual purpose of cleaning the workpiece and protecting the environment.

The beneficial effects of the present invention are: 1. Low cost, generally no more than 60,000 US dollars per laser; 2. Long life, the service life of the pump lamp can be greater than 300 hours, and the service life of the LD pump can be greater than 5000 hours; 3. Maintenance costs Low, lamp pump is less than 6,000 US dollars per year; LD pump is less than 5,000 US dollars per year; 4. High repetition frequency: generally in the range of 1Hz to 100kHz, and some can reach megahertz level; 5. The spectral line width is very narrow; 6. It can operate in continuous wave, and can also output nanosecond, picosecond, and femtosecond pulses, with reliable performance; 7. It can effectively remove micron and micron-level pollution particles; 8. It does not cause any pollution to the environment; 9. .Small geometric size, easy to combine with other systems. 10. After cleaning, the coating has high adhesion, and there is little warping and peeling of the coating, and the product yield is high.

Description of drawings

Figure 1 is a block diagram of the UV laser source structure. Fig. 1 shows the ultraviolet laser block diagram that this laser cleaning machine is used:

Figure 2 is a diagram of the scanning principle and pollutant removal configuration of the dynamic laser cleaning machine.

Figure 3 is a schematic diagram of the scanning area of the laser beam on the continuous conveyor belt

Detailed ways

The present invention provides a tile-level all-solid-state ultraviolet laser cleaning machine and an ultraviolet laser cleaning method; the structure of the all-solid-state ultraviolet laser cleaning machine is shown in Figure 1 and Figure 2 . The structure and cleaning method of the laser cleaning machine provided by the present invention will be described in detail below in conjunction with the accompanying drawings and practical examples.

As shown in Figure 1, the current provided by the power supply 1 excites the semiconductor laser diode in the laser resonator 2, and the pumping light emitted by the laser diode is absorbed by the laser medium to generate laser oscillation. The laser medium can be Nd:YAG single crystal, or Nd:YVO ₄ , Nd:YLF and other laser crystals; it can be Nd:YAG transparent ceramics, or other laser crystals or transparent lasers that can emit high-power laser near 1064nm ceramics. The laser generated in the laser cavity 2 is modulated by a high-repetition-frequency electro-optic or acousto-optic Q-switch or a passive Q-switch to output a pulsed laser 3 , and the frequency is doubled by a frequency-doubling nonlinear optical crystal 4 . The emitted double frequency light 5 is frequency quadrupled by the quadruple frequency nonlinear optical crystal 6 to obtain an ultraviolet laser light 7 near 266nm. (Because different laser substrates have different lattice fields, the laser wavelength emitted by the same active ion in different substrates is slightly different. For example, in Nd:YAG, the laser wavelength emitted by Nd ions is 1064nm, while in Nd:YLF , the laser wavelength emitted by Nd ions is 1053nm. So the wavelength of ultraviolet light after quadruple frequency is not exactly the same, but nearly 266nm, and its range is 260～270nm.) After the gating mirror 8 with high frequency light reflection and high transparency to quadruple frequency light, watt-level pure quadruple frequency light 9 with a watt-level pure quadruple frequency near 266nm is obtained, and the laser power density is greater than 1W/cm ² .

The nonlinear optical crystals used for frequency doubling are KTP, LBO or BBO crystals, or other nonlinear optical materials that can be used for high-power frequency doubling, and the nonlinear optical crystals used for quadruple frequency are CLBO or other Nonlinear optical crystals with four-stage frequency doubling. The pure quadruple-frequency laser 9 reflects the watt-level pulsed laser to a certain mirror surface of the rotating mirror drum 11 through the reflector 10, and adjusts the angle of the mirror surface so that the reflected light can vertically irradiate the workpiece 13 placed on the conveyor belt 12 to be cleaned The surface can be cleaned as needed. The high-repetition electro-optic Q switch mentioned here is made of high-performance electro-optic crystal KTP. The half-wave voltage of this Q switch is lower than that of KDP, LN and other electro-optic switches. There is no ringing effect, and the repetition frequency can exceed MHz. Of course, the electro-optic Q-switch or acousto-optic Q-switch or passive Q-switch made of electro-optic crystals such as LN and KDP can also be used for the Q-switching of the present invention, but the performance is poor.

If the number of workpieces to be cleaned is small and the area to be cleaned is less than the laser spot area, the rotating mirror drum 11 can be stopped, and its nearly 266nm laser radiation can be radiated onto the workpiece 13 to be cleaned for cleaning. The cleaned workpiece can be manually removed from the cleaning position and replaced with the workpiece to be cleaned; the conveyor belt 12 can also be moved out of the cleaned workpiece one by one, and the cleaned workpiece can be moved to the cleaning position. The workpiece conveying system is composed of a conveyor belt and an adjustable speed motor. The conveying system can convey a certain distance at a certain speed (for example, within 1/10 second, convey a distance equivalent to the size of a workpiece), stop conveying for a period of time, and then continue conveying, so that it runs repeatedly; and the cleaned workpiece transported to the next process. This type of non-rotating cleaning of the rotating mirror drum is static cleaning.

If the number of workpieces to be cleaned is huge, or the area of the workpiece to be cleaned is much larger than the laser spot, the laser beam must be scanned. The conveyor belt can convey workpieces intermittently as mentioned above, and can also convey workpieces continuously. If the rotating mirror drum rotates at an angular velocity of ω (rad/second), the diameter of the laser beam spot 18 is D centimeter [seeing (18) in Fig. ), it can ensure that the scanning of the laser beam on the workpiece is neither repeated nor missed, as shown in Figure 3. The rotating cleaning of the rotating mirror drum is dynamic cleaning.

Below are just a few practical examples of using the watt-level near 266nm ultraviolet laser of the present invention to carry out static cleaning of microelectronic workpieces.

Practical example one

Using CLBO as a quadrupled Nd:YAG all-solid-state laser to output 1W 266nm laser, the pulse repetition frequency is 20.3Hz, and the spot diameter is 0.8cm to clean two thousand pieces of MH06LED substrates; the cleaning conditions are: constant temperature and humidity environment ( 25°C, 65%), Class 10,000 clean room, cleaning time for each component is 1.2 seconds. On the cleaned substrate, the LED chips are solidified and gold wire balls are soldered. Compared with the substrate without laser cleaning, the shear strength of the welded LED chip after cleaning is increased by 80%, the welding tension is increased by 120%, and the product yield is higher by 38%.

Practical example two

The Nd:YAG all-solid-state laser that uses CLBO as a quadruple frequency output 1W 266nm laser, the pulse repetition frequency is 20.3Hz, and the spot diameter is 0.8cm. It is used for the aluminum-based PCB board used in the LED backlight module for 26" liquid crystal displays. , cleaning treatment before electronic components welding, mainly cleaning the gold-plated pads on the aluminum-based PCB board, each PCB board has about 2000 pads. The cleaning conditions are: constant temperature and humidity environment (25°C, 65%), Class 10,000 clean room, cleaning time for each pad is 1.5 seconds. Use the cleaned PCB board for reflow soldering process, and solder the LED light source and related functional components on the PCB board. Compared with the PCB board without laser cleaning, The welding strength of the components welded after cleaning is greatly improved by about 80%, and the failure rate of component welding is reduced by about 90%, which reduces product repair links and greatly improves product reliability.

Practical example three

Using CLBO as a quadrupled Nd:YAG all-solid-state laser to output 1W 266nm laser, the pulse repetition frequency is 20.3Hz, and the spot diameter is 0.8cm. The ceramic substrate of the integrated package LED light source is cleaned. The ceramic substrate is AlN material. High thermal conductivity and silver plating on AlN for electrical pathways and pads. Clean the AlN plate with a laser before plating the silver layer. The cleaning conditions are: constant temperature and humidity environment (25°C, 65%), clean room with class 10,000, scan and clean the plate. Silver paste is plated on the cleaned AlN plate. Compared with the plate without laser cleaning, the adhesion of the plated silver layer after cleaning is increased by 120%, and there is little warping and peeling of the plated layer, and the product yield is higher by more than 40%.

The above practical examples only illustrate the present invention, but do not limit the present invention.

Claims (1)

1. A cleaning method of a watt-level all-solid-state ultraviolet laser cleaning machine, the cleaning method of the laser cleaning machine includes static cleaning and dynamic cleaning; The static cleaning is that the watt-level pure quadruple frequency light (9) is reflected to a certain mirror surface of the rotating mirror drum (11) through the reflector (10), and the angle of the mirror surface is adjusted so that the reflected light can be irradiated vertically to the After the surface of the workpiece (13) to be cleaned on the conveyor belt (12) is irradiated for a period of time required for cleaning, the conveyor belt (12) is transmitted to a distance equivalent to the size of the workpiece to be cleaned, so that the cleaned workpiece is removed, and at the same time Also move the next workpiece to be cleaned to the laser irradiation area for laser irradiation cleaning. This cleaning method is suitable for workpieces whose size is smaller than the diameter of the laser spot and has a small number of workpieces. The conveyor belt (12) is a stepping Continuous transmission, because the rotating mirror drum does not rotate, it is called static cleaning; In the dynamic cleaning, if the size of the workpiece to be cleaned is larger than the laser spot, or the number of workpieces is huge, it is necessary to rotate the rotating mirror drum to scan the laser beam on the workpiece, and irradiate all the surfaces of the workpiece to be cleaned for cleaning. Pure quadruple-frequency light (9) is reflected to the intersection line A place between a certain pair of adjacent mirror surfaces I and II of the rotating mirror drum (11) through the reflector (10), at this position, as long as the rotating mirror drum ( 11) Rotate a small angle clockwise, the mirror II will immediately reflect the pure quadruple frequency light (9), the reflected light beam (14) to the position B of the conveyor belt (12), and the rotating mirror drum (11) continues clockwise Direction rotates, and when mirror II turns to the position close to mirror I, the reflected light beam (15) at this moment is irradiated to the position C of conveyer belt (12), establishes the central angle φ= 2π/N, N is the number of mirrors, and the value of N is determined by the diameter of the mirror drum, the distance between the mirror drum and the conveyor belt, and the width of the conveyor belt; at this time, every time the mirror drum rotates by an angle of φ, it turns a mirror , the laser beam is swept from position B to position C of the conveyor belt (12), and the workpiece (13) is irradiated and cleaned; therefore, the power of the pure quadruple frequency light (9) needs to be large enough, and the power of the laser after the spot diameter increases The density is still high enough, and the laser stays on the surface of the workpiece to be cleaned for a short time. In this case, the conveyor belt is continuously conveyed at a certain speed. This kind of cleaning that requires the rotating mirror drum to scan the laser beam is called dynamic cleaning. Cleaning; If the rotating mirror drum rotates at an angular velocity of ω radians/second, the diameter of the laser spot is D centimeters, and the continuously conveyed conveyor belt continuously transmits the workpiece at a speed of transmission speed V=NωD/2πcm/sec; It is characterized in that the Nd:YAG all-solid-state laser that uses CLBO as a quadruple frequency output 1W 266nm laser, the pulse repetition frequency is 20.3Hz, and the spot diameter is 0.8cm, and statically cleans two thousand pieces of MH06LED substrates; the cleaning conditions are: Constant temperature of 25°C, 65% constant humidity environment, 10,000-class clean room, cleaning time of each component is 1.2 seconds, LED chips are bonded on the cleaned substrate, and gold wire ball welding is carried out, which is compatible with the substrate that has not been cleaned by laser. In comparison, the shear strength of the LED chip bonded after cleaning is increased by 80%, the welding tension is increased by 120%, and the product yield is higher by 38%.

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