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Laser assisted particle removal ‘dry’ cleaning of critical surfaces
ELSEVIER
Materials Science and Engineering B49 (1997) 85-88
Laser assisted particle removal ‘dry’ cleaning of critical surfaces S.D. Allen a,*, A.S. Miller b, S.J. Lee c ’ Department
n 217 W’estcott Building, Florida State Unicersity, Tallahassee, FL 323061330, USA ’ XonTech Inc., 6862 Hayrenhurst Ave., Van Nuys, CA 91406, USA of Mathematics and Science Education, National Pingturrg Teachers College, 1 Lin-Sen Road,
Pingrung,
Taiwan
Abstract Laser assisted particle removal (LAPR) is a technique for removing minute particles from surfaces using an energy transfer medium condensed under and around the particles. The laser energy is absorbed in the energy transfer medium, either directly or via conduction from the substrate, explosively evaporating the condensed medium and propelling the particles off the substrate. The effect of the amount of energy transfer medium, water in this case, has been studied and a technique developed for monitoring the condensation and amount of water condensed on the substrate. 6 1997 Elsevier Science %A. Keywords:
Laser processing; Silicon processing; Surface contamination
1. Contamination control
controlling particulate contamination on the order of tens of nanometers is imperative.
Minimizing contamination of semiconductor-surfaces is a primary concern in microelectronics fabrication, and a great deal of effort and resourcesare dedicated to this task. One type of contaminant which is problematic in the current generation of ultralarge scale integration (ULSI) chips is particulates. If, for example, a particle rests where a metal line is to be deposited, it could disrupt the conductive path. This may render the circuit useless. The destructive capacity of dust particles is dependent on the size of the particles relative to the size of the electronic devices. A good rule of thumb is that particles down to one-tenth of the minimum definable feature size of the electronic circuitry must be kept from contaminating the substrate. Otherwise, the result is a substantial reduction in the yield of functioning circuits. Feature sizes, which are typically defined as the length of the gate in a MOS transistor, in currently manufactured commercial chips are on the order of 1 pm. For example, the PowerPC microprocessor has a minimum feature size of 0.65 pm. It is expected that the next generation of dynamic random access memory (DRAM) chips will have a capacity of 256 Mbits, and a minimum feature size of 0.25 pm. This means that
2. Laser assistedparticle removal We have demonstrated in previous work a process for removing particles from a Si substrate [l]. In this process, we use a small amount of water condensed on the substrate’s surface to absorb energy from a laser pulse. The laser energy is deposited very quickly and causes the water to evaporate explosively from the substrate, carrying with it any particles on the surface. For particle sizes on the order of 1 pm, this laser assisted particle removal (LAPR) process effectively removes more than 90% of particulate contamination with only a single laser pulse. If one repeats the process Removal
1
Efficiency
3
2
Number
Fig.
0921-5107/97/$17.00 0 1997 Elsevier P’ISD92J -5107(96)01984-3
I. LAPR
is capable
of approaching
repetitions of the dose-and-irradiate
* Corresponding author. Science
%A. All
rights
reserved.
vs Number
of Laser Pulses
4
5
6
of Laser Pulses
100% efficiency
sequence.
within
a few
86
S.D.
Men
ei ni. /Materids
-
S&me
si
photodiode
Fig. 2. Experimental setup of LAPR.
of condensing water on the substrate and irradiating with the laser, one may achieve near 100% particle removal efficiency typically within five to eight repetitions. The situation is depicted in Fig. 1. Zapka et al. have demonstrated effective particle removal using a similar process for particle sizes down to 0.2 urn [2]. There are significant advantages to LAPR over conventional cleaning techniques such as ultrasonic baths. First, the force imposed on the particle by LAPR scales with its cross-sectional area A (as projected onto the substrate). Conventional removal processes yield a force on the particle which is proportional to its volume I? This tends to be an advantage for sealing the removal technique to smaller particles. To understand this, consider the case of a spherical particle. The cross-sectional area and volume are proportional to the square and cube, respectively, of the radius r, which means that the forces are given by the proportionalities F LAPR-A”1.2
(1)
F COnY- Y--P3
(2)
This means that each of the removal forces decrease rapidly with decreasing particle size, but that the force imparted by LAPR falls more slowly than the conventional forces. The second advantage of LAPR over conventional techniques is that it does not require immersing the substrate in a liquid bath. So even though there is some small-amount of condensed water necessary for LAPR, it may still be considered a ‘dry’ process.
3. Experimental
rind Engineering
B49 (1997)
85-58
the sample, we use a pair of zinc selenide (&Se) Brewster windows, which allows only a certain polarization of the CO, beam to pass. Since the CO1 beam is almost linearly polarized when it is emitted from the laser, rotating the Brewster windows allows us to vary the transmission from about 10% to 90% of full power. We have placed a salt (NaCl) window in the path of the beam to split off a portion of the beam, which we use to monitor the energy in each pulse. We then focus the beam onto the Si substrate using a ZnSe lens whose focal length is 10 in. Water is condensed onto the substrate using a dosing apparatus, illustrated in Fig. 3. A beaker is partially filled with deionized water, and is heated to roughly 37°C. Through a rubber stopper in the top of the beaker, we have placed a bubbler. The bubbler is a glass tube with a dispersive element at the bottom, which is porous and allows air to pass through small holes. We immerse the bubbler in the warm water, and blow nitrogen through it. This forces bubbles up through the water, and vapor out of the beaker and through the tube which is directed toward the sample. The water vapor then condenses in small droplets on the substrate, with a preference for condensing around imperfections such as particulate contamination. We have successfully removed particles between 1 and 9 ,um in size using energy fluences well below the threshold at which the Si substrate is damaged. For a variety of particle sizes, the amount of energy per area required from the laser is consistently about 2 J cm -2. We do not observe damage in the Si until the energy fluence is raised to approximately 30 or 40 J cm-l 131. We therefore have a range of roughly one order of magnitude over which particle removal may be accomplished safely.
4. Process monitoring The Si photodiode shown in Fig. 2 is positioned to monitor the specular reflection of the HeNe beam from the substrate. By specular, we mean the rectilinear nitrogen
input
-
particle removal
Our experimental setup is shown in Fig. 2. We use a CO, transverse electric atmospheric (TEA) laser which emits pulses of infrared radiation. The light has a wavelength of 10.6 urn, and the duration of the pulse is approximately 200 ns. Because the CO, beam is invisible, we have aligned it with a red beam from a HeNe laser. In order to vary the amount of energy irradiating
Fig. 3. Dosing arrangement for sending water vapor to substrate.
S.D.
Allen
et al. /Materials
Science
HRWOPY
2 s 1 .2 v
'I DC 1 DC E.RBE v 18.0 "S < pulse
0 STOPKD 1 ks/s
Fig. 4. Variation of specular reflection after dosing.
reflection from a smooth surface. A rough surface tends to scatter incident light somewhat randomly, thereby lowering the specular reflection. The water which condenses onto our substrate has the effect of lowering the specular reflectance (and increasing the diffuse reflectance). So, prior to dosing the substrate with water vapor, its specular reflectivity is high, and the voltage output of the photodiode is correspondingly high. As water condenses on the substrate, the HeNe beam is scattered, and the amount of light reaching the photodiode is reduced. The corresponding voltage drop can be observed on an oscilloscope, as shown in Fig. 4. The upper trace in Fig. 4 is the photodiode voltage corresponding to the fluctuation in specular reflectance of the substrate, and the lower trace shows the pulse which controls the valve to the doser. When the voltage to the valve increases, the valve opens, blowing nitrogen through the bubbler, and forcing water vapor toward the substrate. As the water condenses, the specular reflectance in the upper trace drops. The time scale (as can be seen in the boxes at the upper left of the oscilloscope readout) is 2 s div- ‘. So, it took roughly eight seconds after the doser valve was closed for the condensation to evaporate from the substrate (as shown by the return to high specular reflectance). This response time is dependent upon ambient conditions, which in our case were approximately 20°C and 50% relative humidity, and upon the amount of dosing.
and
Engineering
B49
(1997)
85-88
87
which resemble water spots on dirty dishes. The critical variable is the amount of water condensed on the substrate at the time of irradiation by the laser. If one waits too long after the dosing has stopped, water spots appear, meaning that LAPR is ineffective. This result implies that there is some minimum amount of water necessary for the process to be effective. We have found that ensuring the efficacy of LAPR is not difficult. Provided that the laser irradiates the substrate while the specular reflectance is still low, LAPR is consistently effective. Again referring to the time scale in the oscilloscope readout of Fig. 4 we see that we have a window of as much as 4 s after the dosing valve is closed in which to irradiate the sample. We stress here that the actual duration of this window is dependent upon the rate at which the water evaporates from the substrate, and thus upon the ambient conditions. In Fig. 5, we show the readout of specular reflectance from the photodiode when the CO, laser is fired at an appropriate time for effective LAPR, in this case after a delay of about one and a half seconds after the dosing valve is closed. We can see exactly when the laser is fired, because the laser pulse causes the water to evaporate instantaneously (for the time scale depicted here). This causes the specular reflectance to return abruptly to its high value, indicating that the water droplets have evaporated. By monitoring the specular reflectance in this fashion, we have a means of ensuring that particle removal is effective and no water spots remain.
6. Conclusion
We have demonstrated a process which removes particulate contamination from a silicon substrate using radiation from a pulsed carbon dioxide laser and water msmf
2 II
25 2.m v 7 a3 v
5. Optimization and process control
This leads to the important issue of defining optimum conditions for effective particle removal in LAPR. We have observed repeatedly that under certain conditions, LAPR is thoroughly effective at cleaning the substrate, and under others, we see particle remnants
2 s 1 ,2 v
DC
1 DC8.888v la.O "5 (
pulse
0 STOPPED 1 ks/s
Fig. 5. Laser pulse for LAPR causes specular reflectance to return abruptly to its original high value.
88
S.D. Allen
et al. ,‘Materials
Science
as a medium for transferring the laser energy to offending particles. Our results have shown that the process is both efficient, approaching 100% for a small number of repetitions of the process, and safe for the silicon substrate. In future work, we expect to compare our results from the CO, laser with LAPR using two other lasers, an erbium doped yttrium aluminum garnet (Er:YAG), whose wavelength is 2.94 pm, and a krypton fluoride (KrF) excimer laser, whose wavelength is 0.248 urn. We will also measure the amount of water condensed on the substrate surface in order to gain a better quantitative understanding of the energy transfer from
and Engineering
B49 (1997)
85-88
the laser beam to the particulates. This should allow us to optimize the process for a variety of substrates and contaminant particles.
References [l] K. Imen, %I. Lee and S.D. Allen, Appl. Phys. Let;., 58 (2) (1991) 203; U.S. Patent 429,026 (1991). 121 W. Zapka, W. Ziemlich and A.C. Tam, Appl. Phys. Letf., 58 (20) (1991) 2217. [3] S.J. Lee, K. Zmen and S.D. Allen, Appl. Plys. Left., 61 (19) (1992) 2314.
Materials Science and Engineering B49 (1997) 85-88
Laser assisted particle removal ‘dry’ cleaning of critical surfaces S.D. Allen a,*, A.S. Miller b, S.J. Lee c ’ Department
n 217 W’estcott Building, Florida State Unicersity, Tallahassee, FL 323061330, USA ’ XonTech Inc., 6862 Hayrenhurst Ave., Van Nuys, CA 91406, USA of Mathematics and Science Education, National Pingturrg Teachers College, 1 Lin-Sen Road,
Pingrung,
Taiwan
Abstract Laser assisted particle removal (LAPR) is a technique for removing minute particles from surfaces using an energy transfer medium condensed under and around the particles. The laser energy is absorbed in the energy transfer medium, either directly or via conduction from the substrate, explosively evaporating the condensed medium and propelling the particles off the substrate. The effect of the amount of energy transfer medium, water in this case, has been studied and a technique developed for monitoring the condensation and amount of water condensed on the substrate. 6 1997 Elsevier Science %A. Keywords:
Laser processing; Silicon processing; Surface contamination
1. Contamination control
controlling particulate contamination on the order of tens of nanometers is imperative.
Minimizing contamination of semiconductor-surfaces is a primary concern in microelectronics fabrication, and a great deal of effort and resourcesare dedicated to this task. One type of contaminant which is problematic in the current generation of ultralarge scale integration (ULSI) chips is particulates. If, for example, a particle rests where a metal line is to be deposited, it could disrupt the conductive path. This may render the circuit useless. The destructive capacity of dust particles is dependent on the size of the particles relative to the size of the electronic devices. A good rule of thumb is that particles down to one-tenth of the minimum definable feature size of the electronic circuitry must be kept from contaminating the substrate. Otherwise, the result is a substantial reduction in the yield of functioning circuits. Feature sizes, which are typically defined as the length of the gate in a MOS transistor, in currently manufactured commercial chips are on the order of 1 pm. For example, the PowerPC microprocessor has a minimum feature size of 0.65 pm. It is expected that the next generation of dynamic random access memory (DRAM) chips will have a capacity of 256 Mbits, and a minimum feature size of 0.25 pm. This means that
2. Laser assistedparticle removal We have demonstrated in previous work a process for removing particles from a Si substrate [l]. In this process, we use a small amount of water condensed on the substrate’s surface to absorb energy from a laser pulse. The laser energy is deposited very quickly and causes the water to evaporate explosively from the substrate, carrying with it any particles on the surface. For particle sizes on the order of 1 pm, this laser assisted particle removal (LAPR) process effectively removes more than 90% of particulate contamination with only a single laser pulse. If one repeats the process Removal
1
Efficiency
3
2
Number
Fig.
0921-5107/97/$17.00 0 1997 Elsevier P’ISD92J -5107(96)01984-3
I. LAPR
is capable
of approaching
repetitions of the dose-and-irradiate
* Corresponding author. Science
%A. All
rights
reserved.
vs Number
of Laser Pulses
4
5
6
of Laser Pulses
100% efficiency
sequence.
within
a few
86
S.D.
Men
ei ni. /Materids
-
S&me
si
photodiode
Fig. 2. Experimental setup of LAPR.
of condensing water on the substrate and irradiating with the laser, one may achieve near 100% particle removal efficiency typically within five to eight repetitions. The situation is depicted in Fig. 1. Zapka et al. have demonstrated effective particle removal using a similar process for particle sizes down to 0.2 urn [2]. There are significant advantages to LAPR over conventional cleaning techniques such as ultrasonic baths. First, the force imposed on the particle by LAPR scales with its cross-sectional area A (as projected onto the substrate). Conventional removal processes yield a force on the particle which is proportional to its volume I? This tends to be an advantage for sealing the removal technique to smaller particles. To understand this, consider the case of a spherical particle. The cross-sectional area and volume are proportional to the square and cube, respectively, of the radius r, which means that the forces are given by the proportionalities F LAPR-A”1.2
(1)
F COnY- Y--P3
(2)
This means that each of the removal forces decrease rapidly with decreasing particle size, but that the force imparted by LAPR falls more slowly than the conventional forces. The second advantage of LAPR over conventional techniques is that it does not require immersing the substrate in a liquid bath. So even though there is some small-amount of condensed water necessary for LAPR, it may still be considered a ‘dry’ process.
3. Experimental
rind Engineering
B49 (1997)
85-58
the sample, we use a pair of zinc selenide (&Se) Brewster windows, which allows only a certain polarization of the CO, beam to pass. Since the CO1 beam is almost linearly polarized when it is emitted from the laser, rotating the Brewster windows allows us to vary the transmission from about 10% to 90% of full power. We have placed a salt (NaCl) window in the path of the beam to split off a portion of the beam, which we use to monitor the energy in each pulse. We then focus the beam onto the Si substrate using a ZnSe lens whose focal length is 10 in. Water is condensed onto the substrate using a dosing apparatus, illustrated in Fig. 3. A beaker is partially filled with deionized water, and is heated to roughly 37°C. Through a rubber stopper in the top of the beaker, we have placed a bubbler. The bubbler is a glass tube with a dispersive element at the bottom, which is porous and allows air to pass through small holes. We immerse the bubbler in the warm water, and blow nitrogen through it. This forces bubbles up through the water, and vapor out of the beaker and through the tube which is directed toward the sample. The water vapor then condenses in small droplets on the substrate, with a preference for condensing around imperfections such as particulate contamination. We have successfully removed particles between 1 and 9 ,um in size using energy fluences well below the threshold at which the Si substrate is damaged. For a variety of particle sizes, the amount of energy per area required from the laser is consistently about 2 J cm -2. We do not observe damage in the Si until the energy fluence is raised to approximately 30 or 40 J cm-l 131. We therefore have a range of roughly one order of magnitude over which particle removal may be accomplished safely.
4. Process monitoring The Si photodiode shown in Fig. 2 is positioned to monitor the specular reflection of the HeNe beam from the substrate. By specular, we mean the rectilinear nitrogen
input
-
particle removal
Our experimental setup is shown in Fig. 2. We use a CO, transverse electric atmospheric (TEA) laser which emits pulses of infrared radiation. The light has a wavelength of 10.6 urn, and the duration of the pulse is approximately 200 ns. Because the CO, beam is invisible, we have aligned it with a red beam from a HeNe laser. In order to vary the amount of energy irradiating
Fig. 3. Dosing arrangement for sending water vapor to substrate.
S.D.
Allen
et al. /Materials
Science
HRWOPY
2 s 1 .2 v
'I DC 1 DC E.RBE v 18.0 "S < pulse
0 STOPKD 1 ks/s
Fig. 4. Variation of specular reflection after dosing.
reflection from a smooth surface. A rough surface tends to scatter incident light somewhat randomly, thereby lowering the specular reflection. The water which condenses onto our substrate has the effect of lowering the specular reflectance (and increasing the diffuse reflectance). So, prior to dosing the substrate with water vapor, its specular reflectivity is high, and the voltage output of the photodiode is correspondingly high. As water condenses on the substrate, the HeNe beam is scattered, and the amount of light reaching the photodiode is reduced. The corresponding voltage drop can be observed on an oscilloscope, as shown in Fig. 4. The upper trace in Fig. 4 is the photodiode voltage corresponding to the fluctuation in specular reflectance of the substrate, and the lower trace shows the pulse which controls the valve to the doser. When the voltage to the valve increases, the valve opens, blowing nitrogen through the bubbler, and forcing water vapor toward the substrate. As the water condenses, the specular reflectance in the upper trace drops. The time scale (as can be seen in the boxes at the upper left of the oscilloscope readout) is 2 s div- ‘. So, it took roughly eight seconds after the doser valve was closed for the condensation to evaporate from the substrate (as shown by the return to high specular reflectance). This response time is dependent upon ambient conditions, which in our case were approximately 20°C and 50% relative humidity, and upon the amount of dosing.
and
Engineering
B49
(1997)
85-88
87
which resemble water spots on dirty dishes. The critical variable is the amount of water condensed on the substrate at the time of irradiation by the laser. If one waits too long after the dosing has stopped, water spots appear, meaning that LAPR is ineffective. This result implies that there is some minimum amount of water necessary for the process to be effective. We have found that ensuring the efficacy of LAPR is not difficult. Provided that the laser irradiates the substrate while the specular reflectance is still low, LAPR is consistently effective. Again referring to the time scale in the oscilloscope readout of Fig. 4 we see that we have a window of as much as 4 s after the dosing valve is closed in which to irradiate the sample. We stress here that the actual duration of this window is dependent upon the rate at which the water evaporates from the substrate, and thus upon the ambient conditions. In Fig. 5, we show the readout of specular reflectance from the photodiode when the CO, laser is fired at an appropriate time for effective LAPR, in this case after a delay of about one and a half seconds after the dosing valve is closed. We can see exactly when the laser is fired, because the laser pulse causes the water to evaporate instantaneously (for the time scale depicted here). This causes the specular reflectance to return abruptly to its high value, indicating that the water droplets have evaporated. By monitoring the specular reflectance in this fashion, we have a means of ensuring that particle removal is effective and no water spots remain.
6. Conclusion
We have demonstrated a process which removes particulate contamination from a silicon substrate using radiation from a pulsed carbon dioxide laser and water msmf
2 II
25 2.m v 7 a3 v
5. Optimization and process control
This leads to the important issue of defining optimum conditions for effective particle removal in LAPR. We have observed repeatedly that under certain conditions, LAPR is thoroughly effective at cleaning the substrate, and under others, we see particle remnants
2 s 1 ,2 v
DC
1 DC8.888v la.O "5 (
pulse
0 STOPPED 1 ks/s
Fig. 5. Laser pulse for LAPR causes specular reflectance to return abruptly to its original high value.
88
S.D. Allen
et al. ,‘Materials
Science
as a medium for transferring the laser energy to offending particles. Our results have shown that the process is both efficient, approaching 100% for a small number of repetitions of the process, and safe for the silicon substrate. In future work, we expect to compare our results from the CO, laser with LAPR using two other lasers, an erbium doped yttrium aluminum garnet (Er:YAG), whose wavelength is 2.94 pm, and a krypton fluoride (KrF) excimer laser, whose wavelength is 0.248 urn. We will also measure the amount of water condensed on the substrate surface in order to gain a better quantitative understanding of the energy transfer from
and Engineering
B49 (1997)
85-88
the laser beam to the particulates. This should allow us to optimize the process for a variety of substrates and contaminant particles.
References [l] K. Imen, %I. Lee and S.D. Allen, Appl. Phys. Let;., 58 (2) (1991) 203; U.S. Patent 429,026 (1991). 121 W. Zapka, W. Ziemlich and A.C. Tam, Appl. Phys. Letf., 58 (20) (1991) 2217. [3] S.J. Lee, K. Zmen and S.D. Allen, Appl. Plys. Left., 61 (19) (1992) 2314.