Laser scribing on-line monitoring device adopting all-optical method

Abstract

The present invention provides an on-line monitoring device for laser scribing in an all-optical manner, which is used for in-situ real-time observation of the full dynamic process of laser scribing, and includes: a frequency doubling crystal and a half-reflecting mirror, the frequency doubling crystal doubles the detection light and then reflects it to the wafer to be measured through the half-reflecting mirror; a dichroic mirror, which is located on the optical path between the half-reflecting mirror and the wafer to be measured, and is used to reflect the pumping light / scribing light to be combined with the detection light; a dichroic objective lens, which is located on the optical path between the dichroic mirror and the wafer to be measured, and is used to focus the pumping light / scribing light and the detection light, and the focal point is placed inside the wafer to be measured; a first detector, which is located on the transmission optical path of the half-reflecting mirror on one side of the wafer to be measured and is farther away from the wafer to be measured than the half-reflecting mirror, and is used to record the reflection information of the detection light; a second detector, which is located on the other side of the wafer to be measured and is arranged on the extension of the optical path from the dichroic objective lens to the wafer to be measured, and is used to record the transmission information of the detection light.

Description

Technical Field

[0001] The present invention relates to the monitoring of laser scribing, and particularly to an on-line monitoring device for laser scribing adopting an all-optical method. Background Art

[0002] In semiconductor manufacturing, wafers are the core upstream of the semiconductor industrial chain, and materials and equipment are the cornerstones of the entire semiconductor industry. Larger wafers, smaller die (chips), and more functions are the future development trends of the semiconductor industry. The semiconductor production process consists of wafer manufacturing, wafer testing, chip packaging, and testing. Among them, wafer scribing is an important link in the semiconductor chip packaging process, which is the process of dividing a whole wafer into individual die along a pre-set cutting lane. This link has a direct and important impact on the quality and lifespan of the chips. The quality standards of wafer scribing consist of multiple stringent conditions such as chip integrity, no micro-cracks, no peeling at the interface, no burrs at the chip edge, no chipping at the back boundary, no shedding at the corners, etc. These conditions are all measured at the micron scale. As the die pitch gets smaller and smaller, the space reserved for the scribing cutting lane becomes narrower and narrower. Therefore, how to further reduce the size of the chips while enriching their functions, how to further improve the production rate without reducing the chip yield, and how to cut complex integrated circuit chips without causing defects are all closely related to the innovation of wafer scribing technology.

[0003] Mechanical scribing uses the high-speed rotation of a grinding wheel or blade to strongly cut the wafer. The mechanical scribing process is mature, with a typical scribing speed of 100 mm / s and a cutting lane width of 60 - 80 μm. However, mechanical scribing also has obvious drawbacks. For example, mechanical scribing is a wet scribing process that requires coolant flushing and cooling; the downward pressure applied by the blade will cause mechanical deformation of the wafer; the scribing cutting lane is wide and the speed is slow, and there are also problems such as sputtering, residue, chipping, film layer peeling, and curling defects. In particular, for high-hardness brittle materials such as SiC, mechanical scribing must make a compromise between scribing quality and scribing speed.

[0004] The technological iteration of semiconductor manufacturing processes has placed increasingly high demands on the precision of laser processing. When multiple factors such as the width of the cutting lane, the heat-affected zone, and the chipping size must be controlled below a few micrometers, ultrashort pulses have become an effective means of laser precision machining. Therefore, combining ultrashort pulses with laser scribing technology has become an important breakthrough point for improving the yield and quality of wafer scribing. Ultrashort-pulse stealth cutting is quite different from nanosecond-pulse scribing, manifested as the transient (femtosecond / picosecond) time effects introduced by high-peak-power ultrashort pulses, the nanoscale spatial effects (nonlinearity and heat transport) generated under tightly focused conditions, and so on. When an ultrashort pulse in the near-infrared band is focused on a semiconductor material, multiphoton ionization and avalanche ionization are dominated by nonlinear absorption; when the plasma density is higher than a certain critical value, it will strongly absorb the laser and heat up violently like a metal; further, when the strong Coulomb repulsive force between positive ions in the region exceeds the Rayleigh instability limit value, a strong Coulomb explosion will occur. Understanding these ultrafast dynamic mechanisms is crucial for improving the performance of semiconductor devices.

[0005] Recently, people have begun to attempt to use ultrashort pulses for stealth cutting of traditional substrate materials such as Si and Al2O3. During the on-site verification process in the production line, the cutting speed was set at 600 mm / s, and an Al2O3 substrate LED wafer with a thickness of 110 μm was used as the scribing object. By optimizing laser parameters such as the average power, pulse energy, number of sub-pulses within a pulse train, and beam quality of picosecond pulses, a yield of more than 99.5% was obtained at the highest, and the yield of the wafer before and after scribing hardly decreased. However, for SiC wafers, the development of both mechanical scribing and laser scribing has been relatively slow because there are many problems to be solved in the growth of SiC materials from ingots to wafer processing. For example: First, SiC has multiple allotropes, and the number of atoms contained in the unit cell of the allotrope is large, the calculation work is complex, and the theoretical understanding of the band structures of these allotropes is not sufficient; second, the growth conditions of SiC single crystals are harsh and the growth cycle is long; third, since SiC belongs to a hard and brittle material (Mohs hardness reaches 9.5, exceeding Al2O3 and second only to diamond), it is extremely difficult to cut the ingot and to grind, thin, and polish the wafer; fourth, due to the great processing difficulty, the SiC substrate (350 - 500 μm thickness) is thicker than the Al2O3 and Si substrates (100 - 200 μm thickness), which leads to the key problem of optical transmittance restricting stealth cutting; fifth, the bandgap width of SiC is about 3.4 eV, which is 3 times that of Si and 1 / 3 of that of Al2O3, and the laser scribing process of Al2O3 and Si substrates cannot be directly adopted.

[0006] However, no matter what, the effect of laser scribing needs to be judged after laser scribing. Usually, the effect of laser scribing is judged by means of visual inspection and electrical performance testing after laser chipping. At present, there is still a lack of in-situ real-time monitoring means for laser scribing of ultra-hard wafers, and the transient recording and analysis of the formation process of modified points and modified layers inside the wafers have not been realized. Summary of the Invention

[0007] The object of the present invention is to provide an on-line monitoring device for laser scribing, which is used for in-situ real-time observation of the full dynamic process of laser scribing.

[0008] To achieve the above object, the technical solution of the present invention is as follows:

[0009] Provide an on-line monitoring device for laser scribing adopting a full-optical method, including:

[0010] A frequency doubling crystal and a semi-reflecting mirror, the frequency doubling crystal doubles the frequency of the probe light and reflects it to the wafer to be measured through the semi-reflecting mirror;

[0011] A dichroic mirror, which is located on the optical path between the semi-reflecting mirror and the wafer to be measured, and is used for reflecting the pump light / scribing light to be combined with the probe light;

[0012] A dichroic objective lens, which is located on the optical path between the dichroic mirror and the wafer to be measured, and is used for focusing the pump light / scribing light and the probe light, and the focal point is placed inside the wafer to be measured;

[0013] A first detector, which is located on the transmission optical path of the semi-reflecting mirror on one side of the wafer to be measured and is farther away from the wafer to be measured than the semi-reflecting mirror, and is used for recording the reflection information of the probe light;

[0014] A second detector, which is located on the other side of the wafer to be measured and is arranged on the extended optical path from the dichroic objective lens to the wafer to be measured, and is used for recording the transmission information of the probe light.

[0015] Furthermore, it further includes a delay line, and the probe light enters the frequency doubling crystal after passing through the delay line.

[0016] The delay line includes a plurality of total reflection mirrors, and a displacement mechanism is also provided for realizing the adjustable distance between these total reflection mirrors.

[0017] Furthermore, a first focusing lens and a second focusing lens are respectively arranged on the optical paths on the input side and the output side of the frequency doubling crystal. The first focusing lens focuses the probe light to the frequency doubling crystal, and the second focusing lens restores the probe light.

[0018] A first half-wave plate is arranged on the optical path on the side of the first focusing lens away from the frequency doubling crystal to adjust the polarization of the light, and a second half-wave plate is arranged on the optical path on the side of the second focusing lens away from the frequency doubling crystal for secondary polarization adjustment.

[0019] It further includes a third half-wave plate, and the pump light / scribing light is transmitted to the dichroic mirror only after polarization adjustment by the third half-wave plate.

[0020] Furthermore, it also includes a chopper. The probe light is pulse-modulated by the chopper first and then enters the frequency doubling crystal. The outputs of the first detector and the second detector are connected to a lock-in amplifier for demodulation.

[0021] Furthermore, a filter is provided on the optical path between the second detector and the wafer to be measured.

[0022] Furthermore, it also includes a highly nonlinear fiber pulse amplifier. The probe light is first amplified in power and spectrally broadened by the highly nonlinear fiber pulse amplifier and then enters the frequency doubling crystal.

[0023] It also includes a low-nonlinearity fiber amplifier. The pump light / scribing light is transmitted to the dichroic mirror only after the net dispersion and pump power management by the low-nonlinearity fiber amplifier.

[0024] Furthermore, the probe light is a femtosecond pulse. The scribing light / pump light is a femtosecond pulse or a picosecond pulse with adjustable pulse width.

[0025] In the present invention, the wafer is laser scribed with the scribing light / pump light, and the modification process at the focus of the scribing laser is detected with the frequency-doubled probe light. Then, by respectively recording the information of the reflection and transmission of the probe light in different directions, the full dynamic process from the start to the end of the interaction between the pump light / scribing light and the wafer can be obtained, so as to analyze the transient laws of ultrafast physical phenomena such as the starting point of nonlinear absorption, the initial stage of plasma formation, the intermediate states of isochoric heating and adiabatic expansion, and the generation of cracks caused by thermal shock waves, realizing in-situ real-time observation and analysis of laser scribing, and providing a feasible solution for effectively controlling the time and material costs of chip manufacturing.

[0026] The above description is only an overview of the technical solution of the present invention. In order to be able to understand the technical means of the present invention more clearly, it can be implemented according to the content of the specification. And in order to make the above and other purposes, features and advantages of the present invention more obvious and understandable, the following specifically illustrates the specific embodiments of the present invention. Brief Description of the Drawings

[0027] By reading the detailed description of the preferred embodiments below, various other advantages and benefits will become clear to those of ordinary skill in the art. The drawings are only for the purpose of showing the preferred embodiments and are not considered to be a limitation of the present invention. And throughout the drawings, the same reference numerals are used to represent the same components. In the drawings:

[0028] Figure 1 A schematic structural diagram of the dual-wavelength ultrashort pulse laser light source of the present invention is shown;

[0029] Figure 2 The structural schematic diagram of the dual-wavelength pump-probe wafer dicing device of the present invention is shown. Detailed implementation manners

[0030] Exemplary embodiments of the present disclosure will be described in more detail below with reference to the accompanying drawings. Although the exemplary embodiments of the present disclosure are shown in the drawings, it should be understood that the present disclosure can be implemented in various forms and should not be limited by the embodiments set forth herein. On the contrary, these embodiments are provided so that the present disclosure can be more thoroughly understood and the scope of the present disclosure can be completely conveyed to those skilled in the art.

[0031] The laser dicing on-line monitoring device of this embodiment consists of Figure 1 as shown in the Figure 2 dual-wavelength ultrashort pulse laser light source and

[0032] as shown in the

[0033] dual-wavelength pump-probe wafer dicing device.

[0034] As Figure 1 shown, the picosecond or sub-picosecond pulses output by the ultrashort pulse seed source 1 are split into two paths by the beam splitter 2. One path is amplified in power and spectrally broadened by the highly nonlinear fiber pulse amplifier 31, and then the femtosecond pulse output is obtained through the first pulse compressor 32. This path of femtosecond pulse serves as the probe light. The other path passes through the low-nonlinear fiber amplifier 41. In the low-nonlinear fiber amplifier 41, by managing the net dispersion of its fiber link and the pump power of each stage of the amplifier, the contrast or signal-to-noise ratio of the pulse in the time-frequency domain is optimized. After optimization, the amplified pulse output by the low-nonlinear fiber amplifier 41 is sent to the second pulse compressor 42 for compression to obtain the femtosecond pulse output. This path of femtosecond pulse serves as the pump light / dicing light.

[0035] Since, when wafers of different types, sizes, and structures are subjected to laser scribing, the average power, peak power, pulse width, and pulse time required by the laser are quite different to achieve the optimal scribing effect. Therefore, for the pump light / scribing light, in order to be able to scribe multiple types of wafers without replacing the laser, a first total reflection mirror 431, a second total reflection mirror 432, a third total reflection mirror 433, and a fourth total reflection mirror 434 are provided in the dual-wavelength ultrashort pulse laser light source. Specifically, the first total reflection mirror 431 is disposed on the optical path at the input side of the second pulse compressor 42, the second total reflection mirror 432 is disposed on the optical path at the output side of the second pulse compressor 42, and the bases of the first total reflection mirror 431 and the second total reflection mirror 432 are both provided with a rotating structure to facilitate their rotation. The third total reflection mirror 433 and the fourth total reflection mirror 434 are disposed beside the optical path where the second pulse compressor 42 is located and are respectively adjacent to the first total reflection mirror 431 and the second total reflection mirror 432. During use, if the pump light / scribing light is required to output picosecond pulses, the angles of the first total reflection mirror 431 and the second total reflection mirror 432 are adjusted so that the first total reflection mirror 431 reflects the optical path to the third total reflection mirror 433, and then propagates successively along the third total reflection mirror 433, the fourth total reflection mirror 434, and the second total reflection mirror 432. The second total reflection mirror 432 adjusts the optical path back to its original position. During this process, the second pulse compressor 42 is placed beside. The amplified pulse remains picosecond output without being compressed. When the pump light / scribing light needs to output femtosecond pulses, the angles of the first total reflection mirror 431 and the second total reflection mirror 432 are adjusted until the mirror surfaces are parallel to the optical path, then the optical path still passes through the second pulse compressor 42 and is compressed by it to form femtosecond pulses, realizing the mutual switching between picosecond / femtosecond pulses.

[0036] As Figure 2 shown, the dual-wavelength pump-probe wafer scribing device includes a fifth total reflection mirror 331, a sixth total reflection mirror 332, a seventh total reflection mirror 333, an eighth total reflection mirror 334, a chopper 34, a first half-wave plate 35, a first focusing lens 36, a frequency doubling crystal 37, a second focusing lens 38, a second half-wave plate 39, a third half-wave plate 44, a first detector 51, a third focusing lens 52, a half-reflection mirror 53, a dichroic mirror 54, a dichroic objective lens 55, a filter 57, a fourth focusing lens 58, and a second detector 59.

[0037] The fifth total reflection mirror 331, the sixth total reflection mirror 332, the seventh total reflection mirror 333, and the eighth total reflection mirror 334 form a delay line and are placed on the propagation optical path of the probe light, and the distance between them can be adjusted mutually through a displacement mechanism, so as to conveniently control the time delay and realize the time coincidence of the probe light and the pump light at the focus of the SiC wafer 56.

[0038] After passing through the delay line, the probe light enters the chopper 34 for pulse modulation. After the polarization of the light is adjusted by the first half-wave plate 35, the light beam is focused into the frequency doubling crystal 37 by the first focusing lens 36 for frequency doubling, converting the near-infrared band pulse into the visible band. Then, after being reversed into a parallel beam by the second focusing lens 38, the light undergoes secondary polarization adjustment by the second half-wave plate 39, and then is reflected from top to bottom by the semi-reflecting mirror 53 to the SiC wafer 56 located on the workbench.

[0039] The dichroic mirror 54 is located on the optical path between the semi-reflecting mirror 53 and the SiC wafer 56. After the pump light / dicing light passes through the third half-wave plate 44 for polarization adjustment, it is combined with the probe light through the dichroic mirror 54.

[0040] The dichroic objective lens 55 is located on the optical path between the dichroic mirror 54 and the SiC wafer 56, and is used to focus the pump light / dicing light and the probe light, with the focus placed inside the SiC wafer 56.

[0041] The third focusing lens 52 and the first detector 51 are located on one side of the SiC wafer 56 (vertically above in the embodiment), and are sequentially arranged farther away from the SiC wafer 56 than the semi-reflecting mirror 53, and are both located on the transmitted light path of the semi-reflecting mirror 53. The third focusing lens 52 plays a role in focusing the light beam, and the first detector 51 is used to record information such as the reflection intensity, position, and divergence angle of the probe light.

[0042] The filter 57, the fourth focusing lens 58, and the second detector 59 are located on the other side of the SiC wafer 56 (vertically below in the embodiment), and are sequentially arranged on the extended optical path from the dichroic objective lens 55 to the SiC wafer 56. After the probe light passes through the SiC wafer 56, it is filtered and focused, and then collected by the second detector 59, which is used to record information such as the transmission intensity, position, and divergence angle of the probe light.

[0043] It should be noted that both the reflected light and the transmitted light can reflect the changes in the material under laser irradiation, but the detected images will be different. Since the dicing pulse is incident from above the material, the plasma will be ejected upward from the modified point, thus forming a modified area. Among them, the shape of the plasma is elliptical, and the major axis is along the vertical direction. The probe light is used to detect and record the formation process of the plasma, and the transmitted light and the reflected light can reflect different information of the plasma region.

[0044] After the first detector 51 and the second detector 59 collect the optical signals, they are converted into electrical signals, and the electrical signals are sent to the lock-in amplifier for demodulation. Since the probe light is added with a modulation frequency, while the noise does not have the characteristics of the modulation frequency, therefore, after the electrical signals are demodulated by the lock-in amplifier, accurate detection signals without noise interference can be obtained.

[0045] By analyzing the detection signal, information such as the absorption intensity, position, and divergence angle of the detection light is obtained, and the transient interaction effect between the pumping light / scribing light and the wafer is calculated by inversion. By further adjusting the delay line for time scanning, the full dynamic process of the pumping light / scribing light from the start of interaction with the wafer to the end of interaction can be obtained to analyze the transient laws of ultrafast physical phenomena such as the starting point of nonlinear absorption, the initial stage of plasma formation, the intermediate states of isochoric heating and adiabatic expansion, and the generation of cracks caused by thermal shock waves.

[0046] In this embodiment, by regulating multiple variables of the dual-wavelength ultrafast laser, a series of transient processes such as multi-photon absorption and Coulomb explosion are controlled to optimize the yield and efficiency of laser scribing. Specifically, a femtosecond or picosecond pulse with an adjustable pulse width is used as the scribing light / pumping light, and a femtosecond frequency-doubled pulse is used as the detection light. By optimizing multiple parameters of the scribing light / pumping light (such as pulse wavelength, pulse width, repetition frequency, sub-pulse interval), and detecting and recording the modification process at the focus of the scribing laser through the femtosecond frequency-doubled pulse, in-situ real-time observation and analysis of laser scribing are carried out, providing a feasible solution for effectively controlling the time and material costs of chip manufacturing.

[0047] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit the protection scope of the present invention. Although the present invention has been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that the technical solutions of the present invention can be modified or equivalently replaced without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. An on-line monitoring device for laser scribing using an all-optical method, characterized in that, Comprising: A frequency doubling crystal and a semi-reflecting mirror, where the frequency doubling crystal doubles the frequency of the probe light and reflects it to the wafer to be measured through the semi-reflecting mirror; A dichroic mirror, which is located on the optical path between the semi-reflecting mirror and the wafer to be measured and is used to reflect the pump light / dicing light to be combined with the probe light; A dichroic objective lens, which is located on the optical path between the dichroic mirror and the wafer to be measured and is used to focus the pump light / dicing light and the probe light, with the focal point placed inside the wafer to be measured; A first detector, which is located on the transmitted optical path of the semi-reflecting mirror on one side of the wafer to be measured and is farther from the wafer to be measured than the semi-reflecting mirror, and is used to record the reflection information of the probe light; the reflection information of the light includes at least one of the reflection intensity, position, and divergence angle of the light; A second detector, which is located on the other side of the wafer to be measured and is arranged on the extension of the optical path from the dichroic objective lens to the wafer to be measured, and is used to record the transmission information of the probe light; the transmission information of the light includes at least one of the transmission intensity, position, and divergence angle of the light; The probe light is a femtosecond pulse, the pump light / dicing light is a femtosecond or picosecond pulse with adjustable pulse width, and the output signals of the first detector and the second detector are connected to a lock-in amplifier, which is used to demodulate the reflection and transmission information to inversely calculate the transient interaction effect between the pump light / dicing light and the wafer, so as to analyze the transient laws of ultrafast physical phenomena such as the starting point of non-linear absorption, the initial stage of plasma formation, the intermediate states of isochoric heating and adiabatic expansion, and crack generation caused by thermal shock waves.

2. The online monitoring device for laser scribing using an all-optical method as described in claim 1, wherein: It further includes a delay line, and the probe light enters the frequency doubling crystal only after passing through the delay line.

3. The online monitoring device for laser scribing using an all-optical method according to claim 2, characterized in that: The delay line includes a plurality of total reflection mirrors, and a shifting mechanism is also provided to realize adjustable distance between these total reflection mirrors.

4. The online monitoring device for laser scribing using an all-optical method as described in claim 1, wherein: On the optical paths on the input side and the output side of the frequency doubling crystal, a first focusing lens and a second focusing lens are respectively provided. The first focusing lens focuses the probe light onto the frequency doubling crystal, and the second focusing lens restores the probe light.

5. The on-line monitoring device for laser scribing adopting an all-optical method as described in claim 4, characterized in that: On the optical path on the side of the first focusing lens away from the frequency doubling crystal, a first half-wave plate is provided to adjust the polarization of the light, and on the optical path on the side of the second focusing lens away from the frequency doubling crystal, a second half-wave plate is provided for secondary polarization adjustment.

6. The online monitoring device for laser scribing using an all-optical method as described in claim 5, characterized in that: It further includes a third half-wave plate, and the pump light / dicing light is transmitted to the dichroic mirror only after polarization adjustment through the third half-wave plate.

7. The on-line monitoring device for laser scribing adopting an all-optical method as described in claim 1, characterized in that: It further includes a chopper, and the probe light enters the frequency doubling crystal only after being pulse-modulated by the chopper.

8. The online monitoring device for laser scribing using an all-optical method according to claim 1, characterized in that: A filter is provided on the optical path between the second detector and the wafer to be measured.

9. The laser dicing on-line monitoring device adopting an all-optical method according to claim 1, characterized in that: It further includes a highly nonlinear fiber pulse amplifier, and the probe light enters the frequency doubling crystal only after being boosted in power and spectrally broadened by the highly nonlinear fiber pulse amplifier; and / or It further includes a low-nonlinearity fiber amplifier, and the pump light / dicing light is transmitted to the dichroic mirror only after being subjected to net dispersion and pump power management by the low-nonlinearity fiber amplifier.

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