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Irradiation of semiconductor devices using a 10 MeV travelling wave electron linear accelerator
Nuclear Instruments and Methods in Physics Research B 174 (2001) 194±198
www.elsevier.nl/locate/nimb
Irradiation of semiconductor devices using a 10 MeV travelling wave electron linear accelerator Li Quanfeng a
a,*
, Yan Huiyong a, Du Taibin a, Wang Peiqing
b
Department of Engineering Physics, Tsinghua University, Beijing 100084, People's Republic of China b INET of Tsinghua University, Beijing 100084, People's Republic of China Received 6 June 2000; received in revised form 1 August 2000
Abstract A 10 MeV travelling wave electron linear accelerator, previously used for teaching, was adapted to irradiate semiconductor devices. Theoretical formulas and the EGS4 programs were used to validate the feasibility and superiority of using the 10 MeV electron linear accelerator for irradiating the semiconductor devices. Electron energies should be greater than 7 MeV to irradiate packaged high-power thyristors. Irradiation increased diodes' breakdown voltage and decreased their reverse recovery time when accelerated electrons irradiated packaged diodes. Controllable high-power packaged power thyristors are also irradiated to shorten the minority carrier lifetime and turn-o time. The resulting product quality reached expected requirements. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Electron irradiation; Semiconductor devices; EGS4; Packaged power thyristor
1. Introduction In all fast power semiconductor devices, from simple fast recovery diodes to more sophisticated GTOs and power MOS transistors, the switching times and the highest possible voltages and currents are strictly related to the minority carrier lifetime. The carrier lifetime is strongly aected by the presence of deep levels (induced by the presence of impurity atoms and/or lattice defects inside
* Corresponding author. Tel.: +86-10-6278-4542; fax: +8610-6278-8900. E-mail address: [email protected] (L. Quanfeng).
the Si/Ge crystal) which act as recombination centers [1]. Lifetime control could be achieved by three dierent techniques: thermal diusion of heavy metal impurities such as gold or platinum into Si/Ge; gamma irradiation; and electron irradiation using electron accelerators. The ®rst two procedures have some drawbacks and cost much. Electron irradiation is a relatively new technique, using high-energy electron beams to irradiate cores of power devices made of silicon or germanium to move the atoms in the crystal lattice to create crystal holes and spacing atoms. These holes and spacing atoms interact with other impurities, spacing atoms and holes, forming spacing pairs such as oxygen spacing pairs, phosphor spacing pairs, double spacing pairs, etc. These spacing
0168-583X/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 0 ) 0 0 4 3 8 - 9
Quanfeng et al. / Nucl. Instr. and Meth. in Phys. Res. B 174 (2001) 194±198
pairs (which form deep energy levels) control the lifetime of minority carriers in semiconductor. Electron irradiation is eective for controlling lifetime in both p-type and n-type silicon [2]. Selective electron irradiation at high temperature can produce high dynamic dv=dt fast thyristors. This method has the advantage of being easily controllable, highly reproducible, quick and produces high electrical performance devices [3]. According to deep level transient spectroscopy tests in annealing experiments, energy levels which control minority carrier lifetime are stable [4]. If the maximum working temperature is below 300°C, long-term tests have shown that the performance of irradiated semiconductor devices is stable, even though they work under large current pulse conditions.
2. Main parameters of the adapted accelerator A scanning magnet and scanning box are added behind the accelerator. Electrons are emitted out from a titanium window. A sample holder for components to be irradiated is placed behind the window, see Fig. 1. Main parameters: electron energy: E 10 MeV, average current near the holder: I 32 lA, holder size (area with average beam current): 60 16 cm2 .
195
ionization, radiation and multi-dispersion. Therefore, energy losses of electrons passing through packages of the semiconductor devices must be analyzed to show that ®nal electron energies are above some limit to meet the irradiation requirements when they arrive at the surface of the inner layer of silicon or germanium material. 3.1. Energy losses in irradiation process Ionization and radiation are the primary energy losses mechanisms for electrons passing through materials. The electron ionization loss rate in material of density q is expressed as [5] dE 2pre c2 NA qZ ÿ dx ion b2 A " # s 2 s 2 ln F s ÿ d ; 2 2 I=m0 c2 1 where F s 1 ÿ b2
s2 =8 ÿ ln 2 2s 1 s 1 2
;
Irradiation of packaged semiconductor devices diers from direct irradiation of unpackaged chips. Electrons enter material and interact with atoms in the material, producing energy losses, such as
where q is the material density; d the material density correction factor; m0 the electron rest mass; s E=m0 c2 the ratio of kinetic energy to rest electron mass; b m=c the ratio of electron velocity to velocity of light; re 2:818 10ÿ15 m the electron radius; NA the Avogadro's constant; A, Z the material atomic weight and material atomic number, respectively; and I is the average inspiring energy of material atoms. The electron radiation loss rate could be calculated as [5] dE NEZ Z 1e4 2E 4 4 ln ; ÿ ÿ dx rad m 0 c2 3 137m20 c4
Fig. 1. The sketch of the adapted accelerator.
where e is the electron charge; E the electron kinetic energy; and N is the number of atoms per unit volume of material. Electrons strike the back surfaces of packaged power thyristors, then pass through their outer shells and substrates before arriving at the inner
3. 10 MeV electrons irradiating packaged power thyristors
2
196
Quanfeng et al. / Nucl. Instr. and Meth. in Phys. Res. B 174 (2001) 194±198
layer of silicon material. Generally, high-power KK and KG series silicon devices have outer shells made of copper whose thickness is d1 0:2 mm and substrates made of molybdenum whose thickness is d2 2:9 mm. The sketch of a packaged power thyristor is shown in Fig. 2. The calculated energy loss for a 10 MeV electron passing through the outer shell is dE dE ÿ ÿ d1 dE1 dx ion dx rad 0:36 MeV: So, when the electron arrives at the substrate surface, its energy is
semiconductor devices are stable. Therefore, large power packaged power thyristors can be feasibly irradiated using 10 MeV electrons. 3.2. Simulation using EGS4 Monte Carlo simulator EGS4 was used to simulate electron transportation in copper and molybdenum materials to calculate electron energies when they arrive at the surface of the inner layer of silicon material. The simulation sampled 100,000 electrons with initial energies of 10 MeV. The electrons arriving at the inner layer surface had an average energy of
E1 Ei ÿ dE1 9:66 MeV;
Ef 4:08 MeV:
where Ei is the initial energy of the incident electron, 10 MeV. The electron energy loss in the substrate is dE dE ÿ ÿ d2 dE2 dx ion dx rad
This result meets the above theoretic result very well, that proves the feasibility to irradiate chips or packaged power thyristors using 10 MeV electrons. EGS4 code was also used to simulate the motion of incident electrons with various accelerated energies to determine the ®nal energy and probability of electrons passing through the shell and substrate, Figs. 3 and 4. The initial energy is the energy of the electrons leaving the accelerator and the ®nal energy is the energy of electrons arriving at the surface of the inner layer of Si material. Fig. 3 shows that the ®nal energy Ef is linearly related to the initial energy Ei . Both Figs. 3 and 4 indicate that for high-power packaged power thyristors: (1)
6:40 MeV: So electron energy arriving at the surface of the inner layer is Ef Ei ÿ dE1 ÿ dE2 3:24 MeV: Both literature data and factory experience indicate that, if the electron energies when they arrive at the chip (without packaging) surfaces are above 2 MeV, the performance of the irradiated
Fig. 2. The sketch of a packaged power thyristor.
Fig. 3. Average electron energy Ef of electrons passing through the shell and substrate as they arrive at the inner Si material surface, plotted as a function of the initial energy Ei of the incident electrons.
Quanfeng et al. / Nucl. Instr. and Meth. in Phys. Res. B 174 (2001) 194±198
197
1=ms 1=s0 KUe ;
4
where s0 , s are the minority carrier lifetimes before and after the irradiation and K is the irradiation damage constant. Ue is the electron ¯uence e=cm2 , which is given by Ue 6:25 1012 FIT =S;
Fig. 4. Probability P in electrons being able to pass through the shell and substrate, plotted as a function of the initial energy Ei of the incident electrons.
electrons can hardly pass through shells (including the substrate) if their energies are below 5 MeV; (2) for energies above 5 MeV but below 7 MeV, the probability of electrons passing through shells is very small and their ®nal energies are below 2 MeV, so the irradiation eciency is poor. Therefore, when irradiated packaged power thyristors must have initial electron energies above 7 MeV. Since irradiation damage constant increases as electron energies increase, higher energies are suggested. However, for electron energies above 10 MeV, irradiation damage constant changes little, and higher energy may cause photon±neutron problems and residual radiation problems. Generally, electron energies for irradiation are not higher than 12 MeV. When we choose electron energies, all of the factors should be considered and 10 MeV is a good choice. 3.3. Dosage for irradiating semiconductors The turn-o time tq for high-power packaged power thyristors is related to minority carrier lifetime as tq s ln
Ii ; Ih
3
where s is the minority carrier lifetime; Ii the forward current; and Ih is the holding current. The minority carrier lifetime s is related to electron ¯uence Ue as
5
where F is the coecient of utilization of electron beam current; I the electron beam average current lA; T the irradiation time (s); and S is the area to be irradiated cm2 . Eqs. (4) and (5) can be used to calculate the proper electron dose for the irradiation.
4. Experimental results 4.1. Turn-o speed The packaged power thyristors models, KK200 and KK500, have long turn-o times. Irradiation was used to reduce the turn-o times to less than 25 ls and 30 ls, respectively. The electron ¯uences were calculated using Eqs. (3) and (4). The turn-o times after irradiation are listed in Table 1. In Table 1, tq0 and tq are the turn-o times before and after irradiation. 4.2. Reverse recovery time Irradiated by high-energy electron beams, both the turn-o time and the reverse recovery time of diodes shorten, but their threshold voltages rise. The electron ¯uence should be chosen to obtain the best trade-o, which is a compromise between the static and dynamic characteristics. Irradiation tests were made about many kinds of diodes, and Table 1 Turn-o times for devices before and after irradiation No.
Type
1 2 3 4
200 200 500 500
A/1000 A/1200 A/1000 A/1200
V V V V
tq0 ls
tq ls
59 43 44 32
20 21 20 21
198
Quanfeng et al. / Nucl. Instr. and Meth. in Phys. Res. B 174 (2001) 194±198
Table 2 Diode parameters before and after irradiation No.
trr0 ls
trr ls
VF V
Ue 1013 e=cm2
1 2 3 4
5.82 4.86 4.10 7.22
1.41 0.75 0.48 0.27
0.92 1.03 1.10 1.45
3.85 7.69 12.31 23.08
satisfactory results were achieved. Irradiation results for SZ1-04 diodes are shown in Table 2. In Table 2, trr0 and trr are reverse recovery times of diodes before and after irradiation, respectively. VF is the forward voltage drop after irradiation. The forward voltage drop before irradiation was about 0.87 V.
5. Conclusions Both theoretical analysis and experimental results indicate that the minority carrier lifetime and switching speed can be controlled by using 10 MeV
electrons to irradiate semiconductors (bare chips or packaged devices). The electron energies should be greater than 7 MeV to irradiate packaged high-power thyristors. The high-energy electron irradiation process is simple, quick, has good reproducibility and much expensive metal such as gold and platinum could be saved. The electrical characteristics of devices irradiated by high-energy electrons are stable. With further research and development, the irradiation technique using highenergy electron beams will provide greater improvements in semiconductor production. References [1] P.G. Fuochi, Radiat. Phys. Chem. 44 (4) (1994) 431. [2] S. Daliento, A. Sanseverino, P. Spritio, L. Zeni, IEEE Trans. Power Electronics 14 (1999) 117. [3] E. Iliescu, V. Banu, Nucl. Instr. and Meth. B 113 (1±4) (1996) 103. [4] A.Y. Usenko, IEEE Trans. Electron Devices 41 (4) (1994) 1055. [5] Y. Xiaozhong, L. Guogang et al., Nuclear Radiation Physics, China Atomic Energy Press, 1986, p. 209 (in Chinese).
www.elsevier.nl/locate/nimb
Irradiation of semiconductor devices using a 10 MeV travelling wave electron linear accelerator Li Quanfeng a
a,*
, Yan Huiyong a, Du Taibin a, Wang Peiqing
b
Department of Engineering Physics, Tsinghua University, Beijing 100084, People's Republic of China b INET of Tsinghua University, Beijing 100084, People's Republic of China Received 6 June 2000; received in revised form 1 August 2000
Abstract A 10 MeV travelling wave electron linear accelerator, previously used for teaching, was adapted to irradiate semiconductor devices. Theoretical formulas and the EGS4 programs were used to validate the feasibility and superiority of using the 10 MeV electron linear accelerator for irradiating the semiconductor devices. Electron energies should be greater than 7 MeV to irradiate packaged high-power thyristors. Irradiation increased diodes' breakdown voltage and decreased their reverse recovery time when accelerated electrons irradiated packaged diodes. Controllable high-power packaged power thyristors are also irradiated to shorten the minority carrier lifetime and turn-o time. The resulting product quality reached expected requirements. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Electron irradiation; Semiconductor devices; EGS4; Packaged power thyristor
1. Introduction In all fast power semiconductor devices, from simple fast recovery diodes to more sophisticated GTOs and power MOS transistors, the switching times and the highest possible voltages and currents are strictly related to the minority carrier lifetime. The carrier lifetime is strongly aected by the presence of deep levels (induced by the presence of impurity atoms and/or lattice defects inside
* Corresponding author. Tel.: +86-10-6278-4542; fax: +8610-6278-8900. E-mail address: [email protected] (L. Quanfeng).
the Si/Ge crystal) which act as recombination centers [1]. Lifetime control could be achieved by three dierent techniques: thermal diusion of heavy metal impurities such as gold or platinum into Si/Ge; gamma irradiation; and electron irradiation using electron accelerators. The ®rst two procedures have some drawbacks and cost much. Electron irradiation is a relatively new technique, using high-energy electron beams to irradiate cores of power devices made of silicon or germanium to move the atoms in the crystal lattice to create crystal holes and spacing atoms. These holes and spacing atoms interact with other impurities, spacing atoms and holes, forming spacing pairs such as oxygen spacing pairs, phosphor spacing pairs, double spacing pairs, etc. These spacing
0168-583X/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 0 ) 0 0 4 3 8 - 9
Quanfeng et al. / Nucl. Instr. and Meth. in Phys. Res. B 174 (2001) 194±198
pairs (which form deep energy levels) control the lifetime of minority carriers in semiconductor. Electron irradiation is eective for controlling lifetime in both p-type and n-type silicon [2]. Selective electron irradiation at high temperature can produce high dynamic dv=dt fast thyristors. This method has the advantage of being easily controllable, highly reproducible, quick and produces high electrical performance devices [3]. According to deep level transient spectroscopy tests in annealing experiments, energy levels which control minority carrier lifetime are stable [4]. If the maximum working temperature is below 300°C, long-term tests have shown that the performance of irradiated semiconductor devices is stable, even though they work under large current pulse conditions.
2. Main parameters of the adapted accelerator A scanning magnet and scanning box are added behind the accelerator. Electrons are emitted out from a titanium window. A sample holder for components to be irradiated is placed behind the window, see Fig. 1. Main parameters: electron energy: E 10 MeV, average current near the holder: I 32 lA, holder size (area with average beam current): 60 16 cm2 .
195
ionization, radiation and multi-dispersion. Therefore, energy losses of electrons passing through packages of the semiconductor devices must be analyzed to show that ®nal electron energies are above some limit to meet the irradiation requirements when they arrive at the surface of the inner layer of silicon or germanium material. 3.1. Energy losses in irradiation process Ionization and radiation are the primary energy losses mechanisms for electrons passing through materials. The electron ionization loss rate in material of density q is expressed as [5] dE 2pre c2 NA qZ ÿ dx ion b2 A " # s 2 s 2 ln F s ÿ d ; 2 2 I=m0 c2 1 where F s 1 ÿ b2
s2 =8 ÿ ln 2 2s 1 s 1 2
;
Irradiation of packaged semiconductor devices diers from direct irradiation of unpackaged chips. Electrons enter material and interact with atoms in the material, producing energy losses, such as
where q is the material density; d the material density correction factor; m0 the electron rest mass; s E=m0 c2 the ratio of kinetic energy to rest electron mass; b m=c the ratio of electron velocity to velocity of light; re 2:818 10ÿ15 m the electron radius; NA the Avogadro's constant; A, Z the material atomic weight and material atomic number, respectively; and I is the average inspiring energy of material atoms. The electron radiation loss rate could be calculated as [5] dE NEZ Z 1e4 2E 4 4 ln ; ÿ ÿ dx rad m 0 c2 3 137m20 c4
Fig. 1. The sketch of the adapted accelerator.
where e is the electron charge; E the electron kinetic energy; and N is the number of atoms per unit volume of material. Electrons strike the back surfaces of packaged power thyristors, then pass through their outer shells and substrates before arriving at the inner
3. 10 MeV electrons irradiating packaged power thyristors
2
196
Quanfeng et al. / Nucl. Instr. and Meth. in Phys. Res. B 174 (2001) 194±198
layer of silicon material. Generally, high-power KK and KG series silicon devices have outer shells made of copper whose thickness is d1 0:2 mm and substrates made of molybdenum whose thickness is d2 2:9 mm. The sketch of a packaged power thyristor is shown in Fig. 2. The calculated energy loss for a 10 MeV electron passing through the outer shell is dE dE ÿ ÿ d1 dE1 dx ion dx rad 0:36 MeV: So, when the electron arrives at the substrate surface, its energy is
semiconductor devices are stable. Therefore, large power packaged power thyristors can be feasibly irradiated using 10 MeV electrons. 3.2. Simulation using EGS4 Monte Carlo simulator EGS4 was used to simulate electron transportation in copper and molybdenum materials to calculate electron energies when they arrive at the surface of the inner layer of silicon material. The simulation sampled 100,000 electrons with initial energies of 10 MeV. The electrons arriving at the inner layer surface had an average energy of
E1 Ei ÿ dE1 9:66 MeV;
Ef 4:08 MeV:
where Ei is the initial energy of the incident electron, 10 MeV. The electron energy loss in the substrate is dE dE ÿ ÿ d2 dE2 dx ion dx rad
This result meets the above theoretic result very well, that proves the feasibility to irradiate chips or packaged power thyristors using 10 MeV electrons. EGS4 code was also used to simulate the motion of incident electrons with various accelerated energies to determine the ®nal energy and probability of electrons passing through the shell and substrate, Figs. 3 and 4. The initial energy is the energy of the electrons leaving the accelerator and the ®nal energy is the energy of electrons arriving at the surface of the inner layer of Si material. Fig. 3 shows that the ®nal energy Ef is linearly related to the initial energy Ei . Both Figs. 3 and 4 indicate that for high-power packaged power thyristors: (1)
6:40 MeV: So electron energy arriving at the surface of the inner layer is Ef Ei ÿ dE1 ÿ dE2 3:24 MeV: Both literature data and factory experience indicate that, if the electron energies when they arrive at the chip (without packaging) surfaces are above 2 MeV, the performance of the irradiated
Fig. 2. The sketch of a packaged power thyristor.
Fig. 3. Average electron energy Ef of electrons passing through the shell and substrate as they arrive at the inner Si material surface, plotted as a function of the initial energy Ei of the incident electrons.
Quanfeng et al. / Nucl. Instr. and Meth. in Phys. Res. B 174 (2001) 194±198
197
1=ms 1=s0 KUe ;
4
where s0 , s are the minority carrier lifetimes before and after the irradiation and K is the irradiation damage constant. Ue is the electron ¯uence e=cm2 , which is given by Ue 6:25 1012 FIT =S;
Fig. 4. Probability P in electrons being able to pass through the shell and substrate, plotted as a function of the initial energy Ei of the incident electrons.
electrons can hardly pass through shells (including the substrate) if their energies are below 5 MeV; (2) for energies above 5 MeV but below 7 MeV, the probability of electrons passing through shells is very small and their ®nal energies are below 2 MeV, so the irradiation eciency is poor. Therefore, when irradiated packaged power thyristors must have initial electron energies above 7 MeV. Since irradiation damage constant increases as electron energies increase, higher energies are suggested. However, for electron energies above 10 MeV, irradiation damage constant changes little, and higher energy may cause photon±neutron problems and residual radiation problems. Generally, electron energies for irradiation are not higher than 12 MeV. When we choose electron energies, all of the factors should be considered and 10 MeV is a good choice. 3.3. Dosage for irradiating semiconductors The turn-o time tq for high-power packaged power thyristors is related to minority carrier lifetime as tq s ln
Ii ; Ih
3
where s is the minority carrier lifetime; Ii the forward current; and Ih is the holding current. The minority carrier lifetime s is related to electron ¯uence Ue as
5
where F is the coecient of utilization of electron beam current; I the electron beam average current lA; T the irradiation time (s); and S is the area to be irradiated cm2 . Eqs. (4) and (5) can be used to calculate the proper electron dose for the irradiation.
4. Experimental results 4.1. Turn-o speed The packaged power thyristors models, KK200 and KK500, have long turn-o times. Irradiation was used to reduce the turn-o times to less than 25 ls and 30 ls, respectively. The electron ¯uences were calculated using Eqs. (3) and (4). The turn-o times after irradiation are listed in Table 1. In Table 1, tq0 and tq are the turn-o times before and after irradiation. 4.2. Reverse recovery time Irradiated by high-energy electron beams, both the turn-o time and the reverse recovery time of diodes shorten, but their threshold voltages rise. The electron ¯uence should be chosen to obtain the best trade-o, which is a compromise between the static and dynamic characteristics. Irradiation tests were made about many kinds of diodes, and Table 1 Turn-o times for devices before and after irradiation No.
Type
1 2 3 4
200 200 500 500
A/1000 A/1200 A/1000 A/1200
V V V V
tq0 ls
tq ls
59 43 44 32
20 21 20 21
198
Quanfeng et al. / Nucl. Instr. and Meth. in Phys. Res. B 174 (2001) 194±198
Table 2 Diode parameters before and after irradiation No.
trr0 ls
trr ls
VF V
Ue 1013 e=cm2
1 2 3 4
5.82 4.86 4.10 7.22
1.41 0.75 0.48 0.27
0.92 1.03 1.10 1.45
3.85 7.69 12.31 23.08
satisfactory results were achieved. Irradiation results for SZ1-04 diodes are shown in Table 2. In Table 2, trr0 and trr are reverse recovery times of diodes before and after irradiation, respectively. VF is the forward voltage drop after irradiation. The forward voltage drop before irradiation was about 0.87 V.
5. Conclusions Both theoretical analysis and experimental results indicate that the minority carrier lifetime and switching speed can be controlled by using 10 MeV
electrons to irradiate semiconductors (bare chips or packaged devices). The electron energies should be greater than 7 MeV to irradiate packaged high-power thyristors. The high-energy electron irradiation process is simple, quick, has good reproducibility and much expensive metal such as gold and platinum could be saved. The electrical characteristics of devices irradiated by high-energy electrons are stable. With further research and development, the irradiation technique using highenergy electron beams will provide greater improvements in semiconductor production. References [1] P.G. Fuochi, Radiat. Phys. Chem. 44 (4) (1994) 431. [2] S. Daliento, A. Sanseverino, P. Spritio, L. Zeni, IEEE Trans. Power Electronics 14 (1999) 117. [3] E. Iliescu, V. Banu, Nucl. Instr. and Meth. B 113 (1±4) (1996) 103. [4] A.Y. Usenko, IEEE Trans. Electron Devices 41 (4) (1994) 1055. [5] Y. Xiaozhong, L. Guogang et al., Nuclear Radiation Physics, China Atomic Energy Press, 1986, p. 209 (in Chinese).