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TEM techniques for two dimensional junction delineation in integrated circuits
Micron and Microscopica Acta, Vol. 20, No. 2, pp.151—152, 1989. Printed in Great Britain.
0739-6260189 $3.00 + 0.00 Pergamon Press plc
TEM TECHNIQUES FOR TWO DIMENSIONAL JUNCTION DELINEATION IN INTEGRATED CIRCUITS
A. Romano1’2, J. Vanhellemont’, J.R. Morante2, W. Vandervorst1 Interuniversitair Micro-Elektronica Centrum (1MEC), Kapeidreef75, B-3030 Leuven, Belgium
2
Càtedra dElectrônica, Facultatde FIsica, Universitat de Barcelona, Diagonal 645-647, E-08028 Barcelona, Spain
The packing density of integrated circuits is increasing continuously accompanied by a reduction of the effective size of its components (transistors, resistors and others), not only lateral but also in depth. In the case of the transistorsvery shallow junctions have to be fabricated. A tight control of the lateral spread of the junction is highly important, requiring the development of new sophisticated characterization techniques. In this paper first results are presented for two different approaches to obtain information on 2D junction profiles. 4 inch Czochralski (001) silicon wafers with a phosphorus content of 5 io’~cm-3 are used. Along one [1101 direction field oxide lines 10 ~,tmwide and with 12 ~.tmspacing are formed by oxidation at 950°Cfollowed by wet etching through a patterned Si 3N4 mask. Next a gate oxide of 40 nm thick is grown. Using the same mask but rotated 90°,i.e. oriented along the other [110] direction, polysilicon lines of the same width and separation are deposited. Next ion implantation is performed through the gate oxide, tilting the wafer 7°away of the (001) direction to avoid the channeling effect, and different annealing processes are used. Cross-section TEM specimens are prepared following a technique described extensively elsewhere [1]. After preparation the specimens are either chemically etched or irradiated for different times in the JEOL HVEM microscope operated at an acceleration voltage of 1000 kY. Qiemical etching As TEM does not allow to see a difference between n and p-type silicon, it is necessaiy to create an artificial contrast between the two types of materials. This can be achieved using a chemical etchant that attacks preferentially one type of material. The commonly used stains and etchants are based on the combined action of HF and HNO3 [2,3]. Figure la shows the result after etching3using the solution HF(40%):HNO3(65%)=l:300 at 5°Cduring 80 obtained by comparison with SRP (Spreading[3] Resistance Probe) measurements ib). level An idea the dopant seconds. The (figure delineated is of 6.1017 cm- concentration profilecan be obtained by studying thinner areas of the etched sample, where complex thickness fringe patterns can be observed, as shown in figure ic. As the etching velocity increases with increasing doping level [3], in the more heavily doped regions the thickness fringes will be more shifted related to their original position (obtained by extrapolating the thickness fringes profile). A continuous dopant profiling might be obtained by simulating the thickness fringe pattern. Irradiation During the observation of unetched cross-section specimens with the high voltage microscope the creation of radiation induced defects has been observed. These defects are generated preferentially in the n doped part of the sample and in the areas of the p doped part where the doping level is low. Figure 2a illustrates the result obtained after 30 mm irradiation. Three areas are easily distinguished: a) a region free of defects under the gate oxide,corresponding to doping levels higher than 5.1018 cm-3, obtained when comparing with SIMS (Secondary Ion Mass Spectroscopy) measurements and ICECREM simulations, but not with SUPREM 3 (figure 2b); b) a region (1) with a very high density ofdefects, corresponding to levels higher than 5.1017 cm-3 and c) a region with uniformly distributed defects (2), corresponding to the n type bulk material. This different behaviour of n and p type silicon can be explained by assuming that charged self-interstitials can be formed [4]. The assumption of an interstitial level in the middle of the bad gap implies that the number of charged and uncharged interstitials depends on the position of the Fermi level: for heavily doped p type the Fermi level is below the interstitial level and then these are positively charged and a repulsion exists, preventing from clustering, while when the Fermi level is above the defect level, interstitials are neutral and are able to cluster. Other studies [5] report that the radiation density is also strongly affected by the presence of an electrical field in the junction region and also close to the interface silicon-silicon oxide. Both effects can also be observed in the same figure under the field oxide (3) and under the gate oxide (4). Pre-existing defects (5) due to the implantation seem not to affect the defect generation mechanism. Further experiments are necessary to understand the fundamental mechanisms involved in the formation of the radiation induced defects and to explore the possibility of their use for junction delineation. 151
,
152
A. Romano et a!.
ACKNOWLEDGEMENTS The TEM work has been performed with the equipment of the University of Antwerpen (RUCA). Ann Dc Keersgieter is acknowledged for supplying the experimental material and the simulations of the dopant profiles. A.R. is indebted to the Spanish Ministry of Science and Education (MEC) for his fellowship. REFERENCES [1] [2] [3] [4]
A. Romano, J. Vanhellemont, H. Bender and J.R. Morante, accepted for publication in Ultraniicroscopy (1989) T.T. Sheng and R.B. Marcus, J. Electrochem. Soc. .]2~ (4), 881 (1981) M.C. Roberts, K.J. Yallup and G.R. Booker, Institute of Physics Conference Series j~,483 (1985) L.M. Brown and D. Fathy, Phil. Mag. B 4~(4), 715 (1981) S.N. Boldyrev, A.F. Vyatkin and V.N. Mordkovich, Radiation Effects J.Q~,155 (1988)
151
b
OXIDE
GATE
OXIDE
c
~JH ,,
—I —
~
‘~“~j,~
~
I
~.
~
,—~
Fig. 1: a) Cross-section specimen etched with HF(40%):HNO 3(65%)=1:300 [3] at 5°Cduring 80 seconds. b) SRP profile yielding the delineated concentration level. c) complex thickness fringe patterns observed in thin areas. M(’cu~3)
‘
ir2b
b
ties ~_supre.3 ———icecrem
1 a+is ~
I @E+18
last?
it
\~ \ \
, ~
0 8E+0i
5 ~-øl
,.,‘
,“ 1 it.0ø
•.i1 8 (au
Fig. 2: a) Specimen irradiated for 30 mm2) pindoped the HVEM 1000 kV. zones 1) field p doped zone 18cm3), zone
0739-6260189 $3.00 + 0.00 Pergamon Press plc
TEM TECHNIQUES FOR TWO DIMENSIONAL JUNCTION DELINEATION IN INTEGRATED CIRCUITS
A. Romano1’2, J. Vanhellemont’, J.R. Morante2, W. Vandervorst1 Interuniversitair Micro-Elektronica Centrum (1MEC), Kapeidreef75, B-3030 Leuven, Belgium
2
Càtedra dElectrônica, Facultatde FIsica, Universitat de Barcelona, Diagonal 645-647, E-08028 Barcelona, Spain
The packing density of integrated circuits is increasing continuously accompanied by a reduction of the effective size of its components (transistors, resistors and others), not only lateral but also in depth. In the case of the transistorsvery shallow junctions have to be fabricated. A tight control of the lateral spread of the junction is highly important, requiring the development of new sophisticated characterization techniques. In this paper first results are presented for two different approaches to obtain information on 2D junction profiles. 4 inch Czochralski (001) silicon wafers with a phosphorus content of 5 io’~cm-3 are used. Along one [1101 direction field oxide lines 10 ~,tmwide and with 12 ~.tmspacing are formed by oxidation at 950°Cfollowed by wet etching through a patterned Si 3N4 mask. Next a gate oxide of 40 nm thick is grown. Using the same mask but rotated 90°,i.e. oriented along the other [110] direction, polysilicon lines of the same width and separation are deposited. Next ion implantation is performed through the gate oxide, tilting the wafer 7°away of the (001) direction to avoid the channeling effect, and different annealing processes are used. Cross-section TEM specimens are prepared following a technique described extensively elsewhere [1]. After preparation the specimens are either chemically etched or irradiated for different times in the JEOL HVEM microscope operated at an acceleration voltage of 1000 kY. Qiemical etching As TEM does not allow to see a difference between n and p-type silicon, it is necessaiy to create an artificial contrast between the two types of materials. This can be achieved using a chemical etchant that attacks preferentially one type of material. The commonly used stains and etchants are based on the combined action of HF and HNO3 [2,3]. Figure la shows the result after etching3using the solution HF(40%):HNO3(65%)=l:300 at 5°Cduring 80 obtained by comparison with SRP (Spreading[3] Resistance Probe) measurements ib). level An idea the dopant seconds. The (figure delineated is of 6.1017 cm- concentration profilecan be obtained by studying thinner areas of the etched sample, where complex thickness fringe patterns can be observed, as shown in figure ic. As the etching velocity increases with increasing doping level [3], in the more heavily doped regions the thickness fringes will be more shifted related to their original position (obtained by extrapolating the thickness fringes profile). A continuous dopant profiling might be obtained by simulating the thickness fringe pattern. Irradiation During the observation of unetched cross-section specimens with the high voltage microscope the creation of radiation induced defects has been observed. These defects are generated preferentially in the n doped part of the sample and in the areas of the p doped part where the doping level is low. Figure 2a illustrates the result obtained after 30 mm irradiation. Three areas are easily distinguished: a) a region free of defects under the gate oxide,corresponding to doping levels higher than 5.1018 cm-3, obtained when comparing with SIMS (Secondary Ion Mass Spectroscopy) measurements and ICECREM simulations, but not with SUPREM 3 (figure 2b); b) a region (1) with a very high density ofdefects, corresponding to levels higher than 5.1017 cm-3 and c) a region with uniformly distributed defects (2), corresponding to the n type bulk material. This different behaviour of n and p type silicon can be explained by assuming that charged self-interstitials can be formed [4]. The assumption of an interstitial level in the middle of the bad gap implies that the number of charged and uncharged interstitials depends on the position of the Fermi level: for heavily doped p type the Fermi level is below the interstitial level and then these are positively charged and a repulsion exists, preventing from clustering, while when the Fermi level is above the defect level, interstitials are neutral and are able to cluster. Other studies [5] report that the radiation density is also strongly affected by the presence of an electrical field in the junction region and also close to the interface silicon-silicon oxide. Both effects can also be observed in the same figure under the field oxide (3) and under the gate oxide (4). Pre-existing defects (5) due to the implantation seem not to affect the defect generation mechanism. Further experiments are necessary to understand the fundamental mechanisms involved in the formation of the radiation induced defects and to explore the possibility of their use for junction delineation. 151
,
152
A. Romano et a!.
ACKNOWLEDGEMENTS The TEM work has been performed with the equipment of the University of Antwerpen (RUCA). Ann Dc Keersgieter is acknowledged for supplying the experimental material and the simulations of the dopant profiles. A.R. is indebted to the Spanish Ministry of Science and Education (MEC) for his fellowship. REFERENCES [1] [2] [3] [4]
A. Romano, J. Vanhellemont, H. Bender and J.R. Morante, accepted for publication in Ultraniicroscopy (1989) T.T. Sheng and R.B. Marcus, J. Electrochem. Soc. .]2~ (4), 881 (1981) M.C. Roberts, K.J. Yallup and G.R. Booker, Institute of Physics Conference Series j~,483 (1985) L.M. Brown and D. Fathy, Phil. Mag. B 4~(4), 715 (1981) S.N. Boldyrev, A.F. Vyatkin and V.N. Mordkovich, Radiation Effects J.Q~,155 (1988)
151
b
OXIDE
GATE
OXIDE
c
~JH ,,
—I —
~
‘~“~j,~
~
I
~.
~
,—~
Fig. 1: a) Cross-section specimen etched with HF(40%):HNO 3(65%)=1:300 [3] at 5°Cduring 80 seconds. b) SRP profile yielding the delineated concentration level. c) complex thickness fringe patterns observed in thin areas. M(’cu~3)
‘
ir2b
b
ties ~_supre.3 ———icecrem
1 a+is ~
I @E+18
last?
it
\~ \ \
, ~
0 8E+0i
5 ~-øl
,.,‘
,“ 1 it.0ø
•.i1 8 (au
Fig. 2: a) Specimen irradiated for 30 mm2) pindoped the HVEM 1000 kV. zones 1) field p doped zone 18cm3), zone