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junction loss of l.OdB and the corresponding laser k-factor of
0.6. Fibre B cut-off wavelength was 45nm above the laser
source wavelength. The modal noise power penalty was in
excess of 8.0dB for each junction loss of 2.2dB and the laser
k-factor of 0.3. Both fibres showed negligible power penalty
for each junction losses less than 0-5dB.
The course of action to minimise the effects of modal noise
in computer applications, where signal delays must be kept
short and space is limited, differs from telephone plants, where
a combination of bends in fibre and use of minimum fibre
length is prescribed to avoid the effects of modal noise. For
short interconnections, where it is difficult to speCjfy the fibre
cut-off wavelength A,, to minimise modal noise power penalty
the splice/connector loss must be less than 0.5 dB if no severe
limitation is placed on laser k-factor. A junction loss of 1,OdB
can be tolerated if the laser k-factor is held to less than 0.3.
Acknowledgments: The authors thank L. Ulbricht and M. Wei
for helpful discussions and J. Reese and C. Grinnell for technical support.
V. S. SHUKLA
M. P. SINGH
ment and nitridation of the device surface. Vertical pnp transistors made by high energy ion implantation and having poor
electrical characteristics have been dramatically improved
after 30min annealing with this technique. The transistors
were fabricated on phosphorus doped (100) CZ wafers with a
resistivity of S 1 5 R c m and without any epitaxial layer. A
damage anneal was performed at 600°C after each implantation resulting in a junction. A 900nm thick field LOCOS
oxidation was followed by the implantation of the collector
junction and its contact area with doubly charged boron ions
at 500 keV. The emitter area and the intrinsic base region were
implanted with a high dose of boron at 40keV and phosphorus at SOOkeV, respectively. Prior to the deposition of a
0 . 5 ~ mthick BPSG layer, a l00nm thick APCVD SiO, was
deposited to prevent the out-diffusion of boron and phosphorus from the BPSG layer to the Si/SiO, interface. Fig. 1
shows the Gummel plot of the pnp transistor with a bad
ideality factor of 1.4 for the base current. this is because of the
10-2
12th February 1990
4
R. F. KEARNS
IO
GTE Laboratories
40 Syluan Road
Waltham, M A 02254, U S A
-6
IO
4.
D
References
I
-8
IO
R., RYCHNOVSKY, s., and HVSAIN, A.:‘Optical interconnsts for high speed computing’, Opt. Eng., 1986, 25, pp. 1 0 7 6
HAUGEN, P.
1085
CROW, J. D.: ‘Optoelectronic integrated circuits for high speed computer network’. OFF89, Houston, TX, USA, 1989, paper WJ3
HARTMAN, D. H.: ‘Digital high speed interconnects’, Opt. Eng., 1986,
1
U,pp. 1086-1102
ISSUI, A. R., mTI“EWsIU,I. E., HOPIAWORI, E. M., and WRLAND,
c.
1.: ‘Dependence of modal noise on cable repair section deployment
conditions for singlemode fibers’. OFC‘87, Reno, NV, USA, 1987,
paper WE3
DUFF, D. G., smw F. T., and wu, I.: ‘Measurement of modal noise
in single-mode lightwave systems’. OFC‘85, San Diego, CA, USA,
1985, paper Tu01
wn, L., LOWE, R. s., and SARAVAN% c.: ‘Practical upper limits to
cut-off wavelength for different single mode fiber design’, IEEE J .
Lightwave Technol., 1987, LT-5,pp. 1147-1155
NOVEL LOW TEMPERATURE RF PLASMA
ANNEALING USING NH,-N, GAS MIXTURE
do
-12
IO
200
340
480
620
Vb,. m v
760
900
Fig. 1 Characteristics of pnp transistor before annealing
large number of electrically active defect centres present in the
SiO, layer and at the Si/Si02 interface. The bipolar transistors were exposed to a plasma NH3-N, at the final stage of
the device fabrication in order to deactivate the surface states
at the SiOJSi interface. The experiments were carried out in a
Tempress PECVD hot-wall reactor with a multielectrode
system. The temperature in the reactor was set at 350”C, the
RF power and frequency were 300 W and 50 kHz, respectively.
An effective annealing could be achieved after 30min. The
base and collector currents are ideal over five decades after
annealing (Fig. 2). The Ic-Vb, and Ib-Vbr are parallel and the
current gain of the transistor is constant over four decades.
I
Indexing terms: Semiconductor devices and materials, Annealing
A novel low temperature (350°C)RF plasma technique using
a NH,-N, gas mixture was used to anneal bipolar strue
tures. Vertical pnp transistors made with high energy ion
implantation and possessing poor electrical characteristics
have been dramatically improved after 30min annealing with
this new technique. The value of the ideality factor of the
base current which was about 1.4 before annealing
approached the ideal value of 1.0after 30min annealing.
Radiation-damages introduced in MOS structures are usually
the generation of fixed and mobile charges in the oxide layer
and the increase of the surface states at the SiO,/Si interface.’.’ Molecular hydrogen has been used to passivate the
interface states at the SiO,/Si interface? Atomic hydrogen is
more effective for passivating defects. The conventional
thermal anneal does not affect all the charge centres in many
device structure^.^.^
We report results concerning a novel low-temperature RF
plasma annealing using an NH,-N, gas mixture. This process
combines the effects of hydrogenation, nitrogen ion bombard-
00
-~
___ _ _ ~ _ _ _ _ ~
0 4
06
V W V
08
IO
Is,slrl
Fig. 2 Characteristics of pnp transistor ajier annealing
Mmin NH,-N, RF plasma annealing
ELECTRONICS LE77ER.5 24th May 1990 Vol. 26 No. 11
__
02
___~__
733
The optical emission spectroscopy of the discharge was used
to provide information about the chemical and physical processes occurring in the plasma. Fig. 3 shows the optical emission spectrum of the NH,/N, glow discharge where the
I
NZ
L
I
C
7
material which inhibits the penetration of more hydrogen into
the device as well as its outdiffusion. Without this cappinglayer effect, hydrogen incorporation into the device could continue until saturation occurs and negative effects such as
creation of new defect centres could occur. This barrier effect
is confirmed by the fact that prolonged annealing exceeding
30min did not lead to any improvement nor degradation as
can be seen in Fig. 4. The nitrogen molecular ion bombardment enhances the creation of atomic hydrogen at the surface
of the BPSG film which is known to be hygroscopic. SiOH
builds up near the SO,-air interface. During N: ion impact
several reactions may occur at the surface
Si-OH
2SiO- + H
This increase of atomic hydrogen will enhance the main passivation reaction which occurs at the SO,/% interface
350
250
650
550
wavelength, nrn
L50
750
Fig. 3 Optical emission spectrum
NH,-N, discharge
presence of atomic hydrogen is characterised by emissive lines
corresponding to H,, H,, and H,. The dissociation reactions
for ammonia are as follows:
NH,+NH
NH,
+ NH,
+ H,
+H
This atomic hydrogen can also be ionised by electron impact
in the discharge. Hydrogen ions are accelerated to the substrate surface by the built-in potential in the dark space region
of the discharge. This hydrogen bombardment and the neutral
hydrogen atoms reaching the surface of the annealed device
enhance the process of passivation of the defects existing in
the oxide and at the Si/SiO, interface. Hydrogen diffusion in
SiO, is very fast. The diffusion constant of atomic hydrogen in
SiO, is 1 x
exp (0.18/kT) cm2s-1.5The diffusion length
will be 145pm after only lmin at a temperature of 350°C.
This length is much larger than the total SiO, thickness generally used, implying that the passivation occurs very deeply
in the device within a few pm. Fig. 4 shows that the ideality
factor improvement occurs mainly after the first 5 min of the
plasma annealing with a decrease of almost 75%.
1 5,
I
21
$ 4 31
0
5
IO
15
20
25
=Si.+H+
ZSi-H
In summary, bipolar transistors with poor electrical characteristics have been successfully annealed at 350°C in a R F
plasma using an NH,-N, gas mixture. The mechanism of this
annealing technique involves three effects at the surface,
hydrogenation, nitrogen ion bombardment, and nitridation.
The effect of this novel annealing technique on other types of
devices is the subject of investigation.
K. AITE
F. W. RAGAY
J. MIDDELHOEK
23rd February 19W
University of Twente, IC Technology and Electronics Laboratory
PO Box 217, 7500 AE, Enschede, The Netherlands
Refereaces
D.v., and KUSHNER,
R. A.: ‘Degradation of oxide films
1 MCCAUGHAN,
due to radiation effects in exposure to plasmas in sputter deposition and backsputtering’,Proc. IEEE, 1974.62, pp. 12361241
2 GDULA,
R. A.: ‘The effects of processing on radiation damage in
SiO,’. 1977 IEDM, Washington, DC, USA, Dec. 1977, pp.
148-1 50
3 AITKEN,
I. M.: ‘ l j m MOSFET VLSI technology: Part VIIIRadiation effects’, IEEE Trans., 1979, ED-26, (4), pp. 312-379
4 SCHULZ,
M.: ‘Interface states at the Si0,-Si interfaoe’, Surj Sci.,
1983,132, pp. 422455
5 GRISCOM,
D. L.: ‘Diffusion of radiolytic molecular hydrogen as a
mechanism for the post-irradiation buildup of interface states in
SO,-on-Si structures’, J. Appl. Phys., 1985,58, (7), pp. 25262532
6 AI% K., HOLLEMAN,
I., MIDDELHOEK. I., and KOEKOEK,
R.: ‘The
relationship between intrinsic stress of silicon nitride films and ion
generation in a 50kHz rf discharge’, Mat. Res. Soc. Symp. Proc.,
1989,130, pp. 347-353
30
time In NH3 - N 2 plasma, min
E
Fig. 4 Ideality factor ofbase current
Due to the presence of nitrogen in the N H J N , discharge, a
substantial number of N; ions are generated6 as can be seen
in Fig. 3. The bombardment of the device surface by these N,’
ions generates defects of which a majority are assumed to be
annealed at the annealing temperature which is 350°C. The
N
: ions which are energetic may be implanted at the surface
of the oxide layer and in combination with N H radicals and
silicon dangling bonds create a very thin silicon nitride film on
the top layer. This nitridation effect is confirmed by XPS
measurements on a Si surface exposed to a plasma NH,-N,
during 30min. The refractive index of the grown film measured by ellipsometry at a wavelength of 6324nm was 2.45
and the film thickness 6nm. This value of the refractive index
is somewhat higher than that of the stoichiometric Si,N4,
which implies that the film is Si-rich. When a reasonable
thickness of silicon nitride is reached, it will become a capping
134
SUPERIOR MICROWAVE PERFORMANCE
OF InGaAs JFETs GROWN BY MBE
Indexing terms: Microwave devices and components, Epitaxy
and epitaxial growth, Field efect transistors
The realisation of high performance InGaAs JFETs designed
for InP based optoelectronicintegrationis reported. An optimised MBE process for the gate layer growth is presented.
JFETs fabricated by using a self-aligning technique exhibit
very high cut-off frequencies of /T = 28GHz and ,/
=
38 GHz.
Introduction: The usability of optoelectronic integrated circuits (OEICs) for high bit rate communication systems will
depend on the development of integration compatible electronic devices with superior microwave characteristics. Recent
ELECTRONICS LE77ERS 24rh May 7990 Vol. 26 No. 1 1
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