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Optimization of the epitaxial layer properties for low voltage capability VDMOS devices
Vacuum/volume39/numbers 7/8/pages 761 to 763/1989 Printed in Great Britain
0042-207X/89$3.00+.00
Pergamon Pressplc
Optimization of the epitaxial layer properties for low voltage capability V D M O S devices J Fernandez, F Berta, S Hidalgo, J Paredes, J Rebollo, J Mill&n and F S e r r a - M e s t r e s , Centro Nacional
de Microelectr6nica, CSIC-UAB, 08193, Bellaterra, Barcelona, Spain
The influence of the body doping level on the optimum epitaxial layer properties for low voltage capability VDMOS devices with minimum specific ON resistance is shown. The Poisson equation is analytically solved by approximating the Gaussian profile of the body region by a linear graded junction. The epitaxial properties obtained result in a reduction of the specific ON resistance around 20% for both interdigitated and cellular designs.
1. Introduction
Voltage controlled power devices are interesting as drivers for PICs (power integrated circuits) due to their facility to be controlled by a simple logic circuitry, which leads to size and cost savings. Within the power MOS devices, the VDMOS (vertical double-diffused MOS transistor) is the most used device for low voltage PICs because of the easy isolation between power and logic parts ~. In order to achieve a certain breakdown voltage and at the same time the smallest value of the VDMOS specific ON resistance, RoN × S, a suitable choice for the basic cell geometry as well as for the epitaxial layer properties (i.e. doping level and thickness) is needed 2. Moreover, adequate edge termination techniques have to be considered in order to approach the breakdown voltage to its plane value 3. Unlike medium and high voltage capability VDMOS devices, in which one can consider that the whole voltage drops across the drift region in the 'Off' state, an important percentage could be supported by the body region in the case of low voltage devices. As a consequence, a thinner epitaxial layer with a higher doping level can be used for fabricating the device, which can represent a remarkable reduction of RoN. This work is aimed at analysing the influence of the body doping level on the optimum epitaxial properties in order to minimize RoN × S for a given plane breakdown voltage, VBRp.In this way, the Poisson equation has been analytically solved approaching the Gaussian doping profile of the body diffusion by an effective linear graded junction and imposing a minimum for the associated series resistance. The results show a strong dependence of the minimum of the RoN × S on the body doping level for low voltage capability VDMOS devices.
Gaussian profile in the body region. In the following, we will assume that the depletion layer edge in the body region is within the linear approximation zone, according to the simulations carfled out for the doping levels usual in practical VDMOS devices. The unknown parameters are the doping level, ND, and the epitaxial thickness, WN, beyond the body diffusion. The solutions of the Poisson equation in the two diode regions are :
qNDx
E(x) -
Ep /3s qNgx 2 -k-Er.x q- C1 2e~
V(x) -
(1) (2)
in the N - region, and
qax 2 E(x) = ---Ep 2~s qax 3 V(x) + E p x q- C2 6~s
(3) (4)
in the body region, a being the slope of the linear region profile.
%
d I
..
,12
I
I h
P
Rbul k WN
2. Formulation
The cross-section of a VDMOS device is shown in Figure 1, where the different components of Ro• are indicated 4, i.e. the channel component, RCH, the distributed access resistance between cells, RACC, and the bulk epilayer resistance beyond the diffusion region, RBULK,this last component of RoN being the lowest in the case of low voltage capability devices. The doping profile, N(x) and the electric field distribution, E(x), in the P + N - N 2+ diode (dashed line in Figure 1) are shown in Figure 2, where one can see the linear approximation of the
N Epilayer
N "t-
L---,J
Figure 1. Cross-section of the VDMOS device showing the ON resistance
components. 761
J Fernandez et al:
Optimization of epitaxial layer properties 1017
p+
30
N~ b
c)
WN N(x) 1016
20
v
£
1015"
10
NA/Z~
c) NA
1074 b) 0
L--f -Wp
E(x)
VBRP (V)
Figure 3. Epitaxial layer properties vs the expected plane breakdown voltage for different body doping levels• (a) NA = 1 0 17 c m - 3 . ( b ) NA = 10 TM cm -3. (C) One-sided step junction.
The boundary conditions are V ( - W p ) = 0 and V(WN)= VBRp, the charge equality equation and the potential continuity at the origin. We must impose that the integral of the ionization coefficients to be equal to unity in order to relate the electric field peak at the breakdown, Ep, to the doping concentration. This ionization integral has two parts (one for each diode region) because of the electric field distribution within the body region. We have assumed an effective constant doping level for the doping profile in the linear region (by imposing the charge equality condition with the same Wp) in order to take into account analytically this region contribution to the ionization integral:
Using Fulop's approximation for the ionization coefficients 5 we can obtain the following expression for the peak electric field :
4 0 1 0 ~ ~ND,N,~AI~rl'/~ LND 4- N A J "
i0 500
400
Expected
Figure 2. Doping profile and electric field distribution of the P+N N 2+ diode. The linear approximation of the Gaussian profile in the body region is indicated.
gp =
2'00
the area under the field distribution curve when the electric field at the origin is Ep. Even though the variation of the electron mobility on the doping concentration could be taken into account using the C a u g h e y - T h o m a s approximation 6, it is worth assuming a constant mobility for simplicity, since carrier mobility scarcely varies for the considered doping ranges.
600 "
/"
400
? 200
(6) /
At this point, the optimum epitaxial layer properties can be obtained, for the required breakdown voltage, imposing a minim u m for the added series resistance per unit area :
/ 0
0
i
i
200
400
600
Expected VBRP (V)
R = p WN =
WN
q~Nv
(7)
constrained to the condition that the breakdown voltage is simply 762
Figure 4. Plane breakdown voltages obtained from numerical simulations
for a Guassian p-n junction using the epitaxial layer properties given by (a) the one-sided step junction approximation and (b) the proposed model for NA = 10 ~7 cm 3 vs the expected values.
J Fern,~ndez et al: Optimization of epitaxial layer properties
3. Results and discussion
The epitaxial properties vs the expected breakdown voltage for different body doping levels are plotted in Figure 3 together with the ones obtained in the case of a one-sided step junction 7. In this figure, one can observe that the epitaxial doping can be increased and its thickness decreased since a fraction of the applied voltage in the 'Off' state drops across the body diffusion. Considering the epitaxial doping levels and thicknesses for a given expected breakdown voltage obtained from Figure 3, we have carried out numerical simulations with the program BAMBI 8 solving the Poisson equation for a one-dimensional Guassian p - n junction. Breakdown voltages have been obtained by means of potential and electric field distributions throughout the structure and evaluated by calculating the ionization integral using the ionization coefficients given by Fulop. The results are shown in Figure 4, where we can note that the use of the epitaxial 30-
layer properties calculated considering a one-sided step junction (line a) determine a higher breakdown voltage than expected (dashed line), as a consequence of the voltage drop across the body region. Moreover, the breakdown voltage determined by the epitaxial properties obtained from the proposed model (line b) is also higher than expected, due to the compensation effect by the diffused impurity near the junction, but lower than the one obtained by the one-sided step junction approximation. Despite this higher breakdown voltage value it is worth using these epitaxial layer properties for fabricating the device due to edge termination effects, which often limit the breakdown voltage of practical devices to below the ideal plane value. The device specific ON resistance for these epitaxial properties has been calculated for interdigitated and square cell VDMOS designs 9. The minimum RON × S relative errors vs the breakdown voltage are shown in Figure 5 which are around 20% for low voltage capabilities, these being higher for cellular designs due to the loss of the active area outside the body diffusion region in the case of interdigitated designs.
3O
4. Conclusions 2O
A 2 o
b)
a)
m
]0
*= o
-3
26o
]0 ~
4bo
600
In this work we have shown the way in which the body doping level influences the optimum epitaxial layer properties for a certain breakdown voltage. The Gaussian profile has been approximated by a linear junction in order to solve analytically the Poisson equation. Results show a reduction around 20% of the VDMOS specific ON resistance for both linear and cellular designs and numerical simulations ratify the fact that an important fraction of the applied voltage drops across the body diffusion for low voltage devices. References
NA = 5 1017 cm-3 200
400
600
Expected 7BRp (7)
Figure 5. Minimum RONX S relative error, using the epitaxial layer properties calculated from the proposed model for different body doping levelsin respect to the ones obtained for a one-sided stepjunction approximation, vs the expected plane breakdown voltage for an interdigitated design. This relative error is also shown in the insert for the case of a square cellular design. Body depth, h = 5 /tin. (a) Body width, s = 25 /~m. (b) Body width, s = 40 #m.
] B J Baiiga, 1EEE Trans Electron Dev, ED-33, 1936 (1986). 2C Hu, M Chi and V M Patel, 1EEE Trans Electron Dev, ED-31, 1693 (1984). 3B J Baliga, l E E Proc, 129, 173 (1982). 4j Rebollo, J Mill~m and F Serra-Mestres, Proc Symp High Voltage Smart Power Devices, 87-13, 171~ Electrochemical Society Meeting, Philadelphia, p 157 (1987). 5W Fulop, Solid State Electron, 10, 39 (1967). 6D M Caughey and R E Thomas, Proc IEEE Lett, 55, 2192 (1967). 7C Hu, I E E E Power Electron Specialists Conf, p 358 (1979). 8A F Franz and G A Franz, IEEE Trans on Computer Aided-Design, CAD-4, 177 (1985). 9j Fernfindez, S Hidalgo, J Paredes, F Berta, J Rebollo, J Millfin and F Serra-Mestres, IEEE Electron Dev Lett. (to be published).
763
0042-207X/89$3.00+.00
Pergamon Pressplc
Optimization of the epitaxial layer properties for low voltage capability V D M O S devices J Fernandez, F Berta, S Hidalgo, J Paredes, J Rebollo, J Mill&n and F S e r r a - M e s t r e s , Centro Nacional
de Microelectr6nica, CSIC-UAB, 08193, Bellaterra, Barcelona, Spain
The influence of the body doping level on the optimum epitaxial layer properties for low voltage capability VDMOS devices with minimum specific ON resistance is shown. The Poisson equation is analytically solved by approximating the Gaussian profile of the body region by a linear graded junction. The epitaxial properties obtained result in a reduction of the specific ON resistance around 20% for both interdigitated and cellular designs.
1. Introduction
Voltage controlled power devices are interesting as drivers for PICs (power integrated circuits) due to their facility to be controlled by a simple logic circuitry, which leads to size and cost savings. Within the power MOS devices, the VDMOS (vertical double-diffused MOS transistor) is the most used device for low voltage PICs because of the easy isolation between power and logic parts ~. In order to achieve a certain breakdown voltage and at the same time the smallest value of the VDMOS specific ON resistance, RoN × S, a suitable choice for the basic cell geometry as well as for the epitaxial layer properties (i.e. doping level and thickness) is needed 2. Moreover, adequate edge termination techniques have to be considered in order to approach the breakdown voltage to its plane value 3. Unlike medium and high voltage capability VDMOS devices, in which one can consider that the whole voltage drops across the drift region in the 'Off' state, an important percentage could be supported by the body region in the case of low voltage devices. As a consequence, a thinner epitaxial layer with a higher doping level can be used for fabricating the device, which can represent a remarkable reduction of RoN. This work is aimed at analysing the influence of the body doping level on the optimum epitaxial properties in order to minimize RoN × S for a given plane breakdown voltage, VBRp.In this way, the Poisson equation has been analytically solved approaching the Gaussian doping profile of the body diffusion by an effective linear graded junction and imposing a minimum for the associated series resistance. The results show a strong dependence of the minimum of the RoN × S on the body doping level for low voltage capability VDMOS devices.
Gaussian profile in the body region. In the following, we will assume that the depletion layer edge in the body region is within the linear approximation zone, according to the simulations carfled out for the doping levels usual in practical VDMOS devices. The unknown parameters are the doping level, ND, and the epitaxial thickness, WN, beyond the body diffusion. The solutions of the Poisson equation in the two diode regions are :
qNDx
E(x) -
Ep /3s qNgx 2 -k-Er.x q- C1 2e~
V(x) -
(1) (2)
in the N - region, and
qax 2 E(x) = ---Ep 2~s qax 3 V(x) + E p x q- C2 6~s
(3) (4)
in the body region, a being the slope of the linear region profile.
%
d I
..
,12
I
I h
P
Rbul k WN
2. Formulation
The cross-section of a VDMOS device is shown in Figure 1, where the different components of Ro• are indicated 4, i.e. the channel component, RCH, the distributed access resistance between cells, RACC, and the bulk epilayer resistance beyond the diffusion region, RBULK,this last component of RoN being the lowest in the case of low voltage capability devices. The doping profile, N(x) and the electric field distribution, E(x), in the P + N - N 2+ diode (dashed line in Figure 1) are shown in Figure 2, where one can see the linear approximation of the
N Epilayer
N "t-
L---,J
Figure 1. Cross-section of the VDMOS device showing the ON resistance
components. 761
J Fernandez et al:
Optimization of epitaxial layer properties 1017
p+
30
N~ b
c)
WN N(x) 1016
20
v
£
1015"
10
NA/Z~
c) NA
1074 b) 0
L--f -Wp
E(x)
VBRP (V)
Figure 3. Epitaxial layer properties vs the expected plane breakdown voltage for different body doping levels• (a) NA = 1 0 17 c m - 3 . ( b ) NA = 10 TM cm -3. (C) One-sided step junction.
The boundary conditions are V ( - W p ) = 0 and V(WN)= VBRp, the charge equality equation and the potential continuity at the origin. We must impose that the integral of the ionization coefficients to be equal to unity in order to relate the electric field peak at the breakdown, Ep, to the doping concentration. This ionization integral has two parts (one for each diode region) because of the electric field distribution within the body region. We have assumed an effective constant doping level for the doping profile in the linear region (by imposing the charge equality condition with the same Wp) in order to take into account analytically this region contribution to the ionization integral:
Using Fulop's approximation for the ionization coefficients 5 we can obtain the following expression for the peak electric field :
4 0 1 0 ~ ~ND,N,~AI~rl'/~ LND 4- N A J "
i0 500
400
Expected
Figure 2. Doping profile and electric field distribution of the P+N N 2+ diode. The linear approximation of the Gaussian profile in the body region is indicated.
gp =
2'00
the area under the field distribution curve when the electric field at the origin is Ep. Even though the variation of the electron mobility on the doping concentration could be taken into account using the C a u g h e y - T h o m a s approximation 6, it is worth assuming a constant mobility for simplicity, since carrier mobility scarcely varies for the considered doping ranges.
600 "
/"
400
? 200
(6) /
At this point, the optimum epitaxial layer properties can be obtained, for the required breakdown voltage, imposing a minim u m for the added series resistance per unit area :
/ 0
0
i
i
200
400
600
Expected VBRP (V)
R = p WN =
WN
q~Nv
(7)
constrained to the condition that the breakdown voltage is simply 762
Figure 4. Plane breakdown voltages obtained from numerical simulations
for a Guassian p-n junction using the epitaxial layer properties given by (a) the one-sided step junction approximation and (b) the proposed model for NA = 10 ~7 cm 3 vs the expected values.
J Fern,~ndez et al: Optimization of epitaxial layer properties
3. Results and discussion
The epitaxial properties vs the expected breakdown voltage for different body doping levels are plotted in Figure 3 together with the ones obtained in the case of a one-sided step junction 7. In this figure, one can observe that the epitaxial doping can be increased and its thickness decreased since a fraction of the applied voltage in the 'Off' state drops across the body diffusion. Considering the epitaxial doping levels and thicknesses for a given expected breakdown voltage obtained from Figure 3, we have carried out numerical simulations with the program BAMBI 8 solving the Poisson equation for a one-dimensional Guassian p - n junction. Breakdown voltages have been obtained by means of potential and electric field distributions throughout the structure and evaluated by calculating the ionization integral using the ionization coefficients given by Fulop. The results are shown in Figure 4, where we can note that the use of the epitaxial 30-
layer properties calculated considering a one-sided step junction (line a) determine a higher breakdown voltage than expected (dashed line), as a consequence of the voltage drop across the body region. Moreover, the breakdown voltage determined by the epitaxial properties obtained from the proposed model (line b) is also higher than expected, due to the compensation effect by the diffused impurity near the junction, but lower than the one obtained by the one-sided step junction approximation. Despite this higher breakdown voltage value it is worth using these epitaxial layer properties for fabricating the device due to edge termination effects, which often limit the breakdown voltage of practical devices to below the ideal plane value. The device specific ON resistance for these epitaxial properties has been calculated for interdigitated and square cell VDMOS designs 9. The minimum RON × S relative errors vs the breakdown voltage are shown in Figure 5 which are around 20% for low voltage capabilities, these being higher for cellular designs due to the loss of the active area outside the body diffusion region in the case of interdigitated designs.
3O
4. Conclusions 2O
A 2 o
b)
a)
m
]0
*= o
-3
26o
]0 ~
4bo
600
In this work we have shown the way in which the body doping level influences the optimum epitaxial layer properties for a certain breakdown voltage. The Gaussian profile has been approximated by a linear junction in order to solve analytically the Poisson equation. Results show a reduction around 20% of the VDMOS specific ON resistance for both linear and cellular designs and numerical simulations ratify the fact that an important fraction of the applied voltage drops across the body diffusion for low voltage devices. References
NA = 5 1017 cm-3 200
400
600
Expected 7BRp (7)
Figure 5. Minimum RONX S relative error, using the epitaxial layer properties calculated from the proposed model for different body doping levelsin respect to the ones obtained for a one-sided stepjunction approximation, vs the expected plane breakdown voltage for an interdigitated design. This relative error is also shown in the insert for the case of a square cellular design. Body depth, h = 5 /tin. (a) Body width, s = 25 /~m. (b) Body width, s = 40 #m.
] B J Baiiga, 1EEE Trans Electron Dev, ED-33, 1936 (1986). 2C Hu, M Chi and V M Patel, 1EEE Trans Electron Dev, ED-31, 1693 (1984). 3B J Baliga, l E E Proc, 129, 173 (1982). 4j Rebollo, J Mill~m and F Serra-Mestres, Proc Symp High Voltage Smart Power Devices, 87-13, 171~ Electrochemical Society Meeting, Philadelphia, p 157 (1987). 5W Fulop, Solid State Electron, 10, 39 (1967). 6D M Caughey and R E Thomas, Proc IEEE Lett, 55, 2192 (1967). 7C Hu, I E E E Power Electron Specialists Conf, p 358 (1979). 8A F Franz and G A Franz, IEEE Trans on Computer Aided-Design, CAD-4, 177 (1985). 9j Fernfindez, S Hidalgo, J Paredes, F Berta, J Rebollo, J Millfin and F Serra-Mestres, IEEE Electron Dev Lett. (to be published).
763