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Interface states under LOCOS bird's beak region
Solid-State Electronics Vol. 30, No. 7, pp. 145753,
$3.00 + 0.00 0038-I lOI/ Copyright 0 1987 Pergamon JournalsLtd
1987
Printed in Great Britain. All rights reserved
INTERFACE
STATES UNDER LOCOS BIRD’S BEAK REGION
J.-C. MARCHETAUX, B. S. JIOYLE and A. BOUDOU Bull S.A., 78340 Les Clayes-sous-Bois, France (Received
12 September
1986; revised 7 January
1987)
Abstract-A method has been developed which allows the extraction of the density of electrically active defects in the bird’s beak region of LOCOS isolation of a MOS structure. The measurements were performed on a special structure with enhanced edge effects using quasi-static techniques. The experimental results are compared with simulations using a model which allows for the inclusion of surface states in the bird’s beak region. It is found that the best fit is obtained for a uniform energy distribution with N, = lE12 cm-2 eV-‘, while the states are situated in the bird’s beak region up to 18OOA from the gate oxide edge.
NOTATION
area of the elementary capacitor number i (cm*) capacitance of the elementary capacitor number i (F/cm’) silicon oxide capacitance of the elementary capacitor number i (F/cm3 capacitance of the elementary capacitor number i due to the variation of charge in silicon (F/cm3 capacitance of the elementary capacitor number i due to the variation of charge trap at the interface (F/cm*) minimum capacitance of the quasistatic C/V curve (F/cm*) silicon oxide capacitance (F/cm3 capacitance measured at the voltage V (F/cm*) conduction band edge, valence band edge, respectively (eV) field oxide thickness and gate oxide thickness, respectively (cm) silicon oxide thickness (cm) Boltzman constant (1.38E-23 J/K) channel length of MOS transistors
(cm) total length of bird’s beak (cm) distance of the mask e&e from the edge of the field oxide (cm) distance of the mask edge from the edge of the thin oxide (cm) distance from the thin oxide edge in which the surface states can be detected (cm) number bf klementary capacitors in the bird’s beak reeion concentration in &con (cme3) number of cells in test structure concentrations in silicon under the field oxide, under the thin oxide, respectively (cm-‘) intrinsic density (cm-‘) total density of traps at the interface (cm-* eV-I) acceptor and donor densities of traps, respectively, at the interface (cm-z l=V-‘1
el&;on
charge (1 .602E-1g C)
charge in the silicon (C/cm3 charge at the interface (C/cm2) ratio of the mask edge-thin oxide edge to the bird’s beak length temperature (300 K in this paper) T reference voltage where the capacitor V, is in accumulation (V) flatband voltage (V) Vfi flatband voltage at x from the thin V, (BB, x) edge under the bird’s beak region flatband voltages of field capacitor and Vi&-ox), v, 004 gate capacitor respectively (V) V gate voltage (V) gti”(fox), V&,(tox) potential of minimum capacitances for field test reference and gate test reference respectively (V) potential drop across the silicon oxide VOX (v) channel width of MOS transistors (cm) W range of the Gaussian variation of -5 concentration under the bird’s beak region (cm) permittivity in vacuum (8.854 E-l4 F/ cm) tan-’ {(% - .%X2*&,)) Si02 and Si dielectric constants respectively energies of, respectively, acceptor and donor localized traps at the interface (eV) standard deviation of the Gaussian (I variation of concentration under the bird’s beak region (cm) Fermi potential deep in the substrate @b (v) yap,,6 surface potential (v) s ,r, surface potential of the elementary TM, capacitor number i (V)
1. INTRODUCTION
In recent years, advances in silicon fabrication technology have led to a continuing scaling down of MOSFET dimensions and to improvements of electrical performance. At the present time, MOSFET gate lengths approach micron and submicron sizes. It 745
J.-C.
746
MARCHETAUXet
is well known that at these dimensions parasitic effects arise, which are not important for long devices. These effects are, for example, Short Channel Effects (SCE)[l], Narrow Width Effects (NWE)[Z] and Hot Electron Effects[3-51. While the first two problems can be controlled by an appropriate choice of threshold implantation conditions, the third, which arises due to high fields in the channel region, is much harder to control and (if not mastered) can lead to circuit degradation and eventual circuit failure. Consequently much research has been carried out into the design of such structures as Lightly Doped Drain (LDD) and Double Diffused Drain (DDD) to modify the channel area near the drain edge. In contrast to the above, although interface state measurements in device boundaries by differing methods are reported in the literature[6,7], the influence of the reduction of gate width has not, to our knowledge, been the subject of research in terms of intrinsic defects and hot electron damage. With a ratio w 11of about 3, and gate lengths approaching the 1 pm size, the bird’s beak region between gate and field oxide occupies a nonnegligible area of the total surface of the MOS transistor. This area is the site of constraints coming mostly from the silicon nitride mask used in the LOCOS process, and can be responsible for the generation of interface states[8]. Other process steps, such as field implantation, etching, etc. are also possible sources of interface damages in the bird’s beak region. With the reduction in gate size, the amount of current which flows in this region becomes greater, and this can cause a degradation in performance of the MOS, either intrinsically, due to electrically active sites, or extrinsically, due to hot injection which can activate defects in this part of the device. The aim of this article is to describe a method which enables the comparison of simulation with the measurement of the capacitance in the bird’s beak region, to ascertain whether interface states exist in this region, and to estimate their density and spatial repartition. Section 2 describes the measurement system as well as the test circuit used. In Section 3, the method of simulation is described for the test structure equivalent circuit, which allows for the inclusion of the effect of interface states in the bird’s beak region. The results of both simulation and experiment are described in Section 4 for a gate oxide thickness of 400 A. Section 5 discusses the results followed by conclusions in Section 6.
al.
luminescent diode placed above the sample to allow for the illumination of the sample before the voltage sweep, and a HP 98163 microcomputer which controls the system, stores the data as well as performing the simulations. It has not proved possible to use the Berglund method[lO] for these measurements as the surface potential cannot be estimated when both the oxide thickness and the substrate doping varies. This point will be developed in Section 3. For the measurement, the voltage is swept from inversion to accumulation, using the diode to illuminate initially the capacitor to allow the system to reach full inversion-for these samples, the low generation rate of electron/hole pairs means that an external means is necessary to provide the minority carriers needed. 2.2. Test structure Most of the capacitance measurement methods suffer from the disadvantage that they require large area devices, typically (lOO*100) pm2, and cannot thus be carried out on transistors. In our particular study, however, it is not so much the area (a large area is still needed), but the ratio of the bird’s beak to the thin and field oxides that is important. In the case of test structures consisting of a large area thin oxide surrounded by the bird’s beak and thick oxide, the bird’s beak occupies typically OS-l% of the surface. It is for this reason that we have designated a test structure in which the area of the edges is optimized over the area of the thin oxide with an overall capacitance of the order of 120pF, and a bird’s beak area of about 30% of the total area. The bird’s beak capacitance test structure is made up of basic cells each of which is approximately
Si,N, -
2. EXPERIMENTAL
2.1. Measurement
MASK POLY
system
The method used here for the detection of surface states is the quasi-static capacitive method[9]. The measurement system consists of a HP 4140B pA meter/DC Voltage Source for the voltage ramp and to measure the displacement current, an electro-
Fig. 1. View of an elementary cell of
the bird’s beak mosaic structure. The lower view is a cross-section of the upper view along AA line.
Interface states under locos bird’s beak region
747
Table 1. Area of field oxide, gate oxide, bird’s beak oxide and total capacitors for a standard structure and the mosaic test structure.. Ratio of bird’s beak area to total area is also presented. E,,,, = 400 A; I$,, = 5500 A; bb = 1.3pm; Rb = 0.5; N, = 6400 (in cm-s)
Surface tox
Surface bb
Surface fox
surface total
bb surface Total surface
Structure examined Standard structure
9.78B-4 6.42E-3
1.77E-3 4.26E-5
3.59E-3 0
6.43E-3 6.46E-3
27.5% 0.7%
equivalent in area to a MOS transistor of gate length of several microns. The cell itself consists of a square of thin oxide of dimensions (5*5) pm2 surrounded by the field oxide. The sides of the cell are 10 pm long. This is shown in Fig. 1 with a cross-sectional cut of this symmetrical cell. To obtain the sensitivity necessary for the measurements, the number of basic cells is simply increased, thus increasing the capacitance while keeping constant the ratio of edge to bulk. In the structure used here, the capacitance consisted of a mosaic of 80 by 80 unit cells in parallel. For the calculation of the areas and capacities of the different parts of this basic cell, the cell is divided into three regions, the field oxide, the gate oxide, and the bird’s beak. Given in Table 3 are the definitions of the units used. These can also be seen in Fig. 1. Tables 1 and 2 give the values of area and oxide capacitance respectively for the optimized bird’s beak structure and a standard MOS capacitance with equivalent area. It can be seen that the relative fraction of the bird’s beak area is multiplied by 40 and by a factor of 175 for the capacitance compared with an equivalent thin-oxide surface. 2.3. Samples The samples used came from a 2-pm NMOS process with a dry oxide and a P-type substrate with a resistivity of 6.5 f 1.5 ohm cm and have no threshold voltage implant. The samples had a nitride mask deposited just before the field oxidation step (used for the LDCOS process) and the field implantation. This was followed by the growth of the gate oxide, using a dry process, giving an oxide of thickness 400 A. The polysilicon gate electrode was then deposited using LPCVD doped N+ and etched by RIE. After the passivation, etching of contacts and deposition of conductors, the final anneal was carried out in an
atmosphere of 15% hydrogen-85% 450°C for 30min.
(in PF) Structure examined Standard structure
121 791
Capacitance bb 30 0.8
In the quasi-static capacitance measurement method, the determination of the interface state density is obtained by the Berglund technique[lO], which involves the determination of the surface potential in the definition of the space charge region. Berglund showed that the surface potential can be obtained from the experimental C-V curves by:
Y,(V) =
c ’
[I -C(V,)/C,,,]*dV,+K
J v., where V is the applied voltage to the gate electrode, V, is the voltage taken as reference where the capacitance is in accumulation, C(V) is the capacitance measured at the voltage V, and K is a constant determined experimentally, [K = Y,( V,)]. This method is not applicable as such to the structure described above, as the surface potential is not the same under the various oxide regions, the gate, the field, and the bird’s beak region (where !Fs varies continuously with distance from the gate oxide edge) and the values of the different capacitances cannot be directly extracted from the overall capacitance. It is thus necessary to use another method. For this, an equivalent circuit is necessary in which each separate contribution can be separated out and treated numerically. It is this approach that we have adopted here. 3.1. Equivalent circuit Each basic cell has been divided into three zones (Fig. 2). The: Tox (E,,, , gate oxide thickness) and Fox 1 for oxide capacitance Capacitance fox 23 -
Capacitance total 174 792
bb capacitance Total capacitance 17.5% 0.1%
Table 3. Parameter values necessary for simulation b Erw 4, Lbb % & e 4ooA 1.2~ 0.5 0.3 p 0.05p 1OP 0.55~ a-side mask of the square of thin oxide (a = 5 pm; Fig. 1); b-side mask of the cell of test structure (Fig. 1); &-total length of bird’s beak (Fig. 1); &-distance of the mask edge from the edge of the thin oxide (Fig. I); R,,-ratio of the mask edge-thin oxide edge to the bird’s beak length (Fig. I); X,,, a-parameters for the Gaussian variation of concentration under the bird’s beak region.
Values used
a-2&, 3.05 p
at
3. MODELLIZATION
Table 2. The same as Table Capacitance tax
nitrogen
J.-C. MARCHETAUX et al.
748
I
$
tox
Fox zone
.I#“.
.
: .
.
.
.
Bird’s
.
.
.
.
.
.
beak
zone
(
t
1
TO, zone
_--_j------I-----A x
Lbb
L bl
0
Fig. 2. Modelization of the bird’s beak region. (Er,,,, field oxide thickness) zones can immediately be treated separately, and constitute two planar capacitors. However, the bird’s beak region itself must be sectioned into: n separate capacitors, each of which must be sufficiently small to be regarded as a planar capacitance, each slice having a surface potential different from its neighbour [Fig. 3(a)]. Each of these capacitors C, is made up of the three individual capacitances: Ci_ = oxide capacitance of this elementary capacitor in series with Ci, (YS,), due to the variation of trap charge at the interface, and C, (!I’,), due to the variation of charge in the semiconductor. This is shown in Fig. 3(b). The determination of these capacitances is extracted by first determining the different surface potentials. 3.2. The dz$erent steps of the calculation The calculation parts:
can be divided into 12 separate
(1) Dividing the capacitance up into n + 2 different capacitances, as defined above. For each of these. (2) Determination of the area Ai. (3) Determination of the concentration of dopants in the substrate under each capacitor section. This is necessary as the standard cell has an insulation implantation under the field oxide. (4) Determination of the interface charge. (5) Determination of the surface potential as a function of the gate voltage. (6) Determination of the capacitance per unit surface, C,,(Y&
Fig. 3. Equivalent circuit of: (a) an elementary cell of the bird’s beak structure; (b) a “slice” of the bird’s beak region of an elementary cell.
(7) Addition of the capacitance C&P,) coming from the supposed concentration of N,. (8‘) Determination of the total capacitance per unit surface. (9) From (5), C,(V,) can immediately be deduced. (10) The total capacitance of the cell Ai * C,( V,) is then added to the capacitances already determined. (I 1) If all n + 2 capacitances have not been calculated, we go back to step (2) for the next capacitance (12) The total capacitance obtained is multiplied, for each of the values of V,, by the number of cells in the structure, N,. This is done for each value of I’.., the sweep of the gate voltage being chosen so that all the regions of the structure pass from accumulation to inversion: The non-trivial steps (3), (4), (5), (6) and (7) are described below. Step (5): Relation between surface potential and gate voltage
We have VP- Vfb= VOX + YS where V,, is the potential drop across the oxide, V,, is the flat-band voltage, Yy,is the surface potential and V,,, = -(Q,+ Q,)/C,, with Q,= charge in the semiconductor; Q, = charge at the interface. It can be deduced [ 1l] that: B*(V,-V,,)=X-Sgn(X)*A * F(X ni/N) * .5,x -
Q&m
(1)
Interface
under locos bird’s beak region
states
with
(b) and in the capacitance with C,$(Y,).
B = qlkT X=/?*Yu,
3.3. Determination
Sgn(X) = 1 if X > 0 = -1
749 as CJY,)
of Qss(Y,)
According to [12], the total traps is given by
ifX
A = fi/(rcO * t,,) * ,,/m
icO* t0
is in parallel
charge
of interface
QssV'u,)= 4 *
F(X ni/N) =
[exp(-X)+X-
l] + (n,/fV)2 * [exp(X) -X
x fo(5, - ‘4 - y’,) * P’:sK) + N:s(L)l
- I].
This exact analytical relation of V, =fn(~,) is transformed into a numerical relation: Yy,=fi( v,) by iteration and second order development of Yy, to obtain the different Vg’s necessary. Step (3): Determination of the dopant concentration the substrate under the oxide
in
The following simplification has been used: The concentration in the substrate under the thin oxide is constant (there is no threshold implant). The same constant concentration assumption is taken for the field oxide as the field implant has diffused sufficiently that it can be considered constant. Under the bird’s beak, for each capacitance slice, the concentration is assumed to be constant into the substrate, but there is a Gaussian variation of concentration from one slice to the next, according to equation:
where fO is the Fermi-Dirac
3.4. Determination
- Nsubrc)* exp] - (x - &)‘/(2a
Step (6): Determination
we can obtain
[12]:
surface, C,*( y, ) Once the surface potential is known, the surface capacitance coming from charge variations in the semiconductor, is given by [l 11: CJX)
=
s W?
Cs,(y,) =
-4
*
d5, *
m5, - 6 - yu,)
h/q
3.5. Distribution
per unit
have the usual meaning.
From
')I + Nsubst.
of the capacitance
*x)1-’
of C,(Y,)
*
N = (&,
function:
J)(x) = [l + l/2 * exp(p and the other symbols
(2)
D’i’s(5,) + Ws(t,)l.
(3b)
in energy
To simpify the calculation, two theoretical cases on Nss distribution in energy are simulated, states localized in energy and states uniformly distributed in energy in the forbidden gap: 3.5.1. Case 1. Traps localized in energy at the interface. Taking
T d
* [l - ew( --Xl +
h/W2 * (exp@‘)- 111
F(X n,lN) and
where 6 is the S-function, and 5, the energy of traps. The donor and acceptor states used in the simulation have energies: E, + 0.85 eV and E, + 0.4 eV respectively (chosen arbitrarily). From eqn (2) we obtain:
Q,,(ys, = q * [W’s, * (1 -f&C’ - @I,- YJ)
Ld-JF
- N:s,*.6(5: - % - YJI where
the symbols
have
the same
notation
as in
eqn (1). Step (4) and (7): Determination
These interface states intervene in two ways: (a) in the determination of the surface potential [eqn
(l)l,
and from eqn (3b):
Cs,(Y,) = q * P * {N:s,,*AAt: of interface states
(44
* 11-f,(r_:‘+ N&*f,(<;* [1 -fo(TY
@b -
- Qi, - yu,) Yu,)l
%-- @b-
Ys)
Yu,)lI.
Cab)
750
J.-C. MARCHETAUX etal.
be noted It should employed [ 121:
that
the
expression
usually (5)
C,(Y,)
= q * N,(YY,)
with N,,( Yy,) = Nts(Y,) + Nis(YJ is no longer usable. In effect, eqn (5) is obtained using the hypothesis that N,,(Y’,) remains constant in the energy range A[ = kT/q around Y,, which is manifestly not true in the case being considered (see definition of Nzs and Nfs above). 3.5.2. Case 2. Interface traps distributed uniformly in the forbidden gap. Taking and
N$(5,) = N&
K(L)
= KS0
so N,,(&) = N,,, = N& + N& and using eqns (2) and (3a), gives:
Q,,VJ = -s/B * N,,, * lnMYs)I Ec- Ev
+ N,, * /I * -
4
+ WR (Y,))
(64
11
with R(y
)
s
=.hdEc/q - @b- ys> “w”lq - @b- Yu,)
C,,(y,) = -4
*
Nsso* U&%/q - @I,-
- h(Evlq - @b- u’s)].
‘J’s) (6b)
If we suppose that the Fermi level comes no closer than a few kTq to the band edges, then: fo(Wq
- @b - YJ = 0
fo(-Wq
-
and @b
-
yu,)
=
1
and the well known relation (5). However, there are problems in the determination of R(YJ and thus Q,s(Y,), which means that the full relation must be conserved.
4.RESULTS 4. I. Parameter values necessary for simulation Electron microscope photos were used of crosssections of the test structure to evaluate the various parameters used in the simulation. The mean values of the various parameters are such as L,, , R,, E,,, , E fOx,A’,,, r~ are shown in Table 3. Figure 4 is a typical example, obtained by electron microscopy, of a cross-section of an elementary cell of the test structure with E,,, = 400 A, showing the details of bird’s beak and particularly the encroachment near the middle of bird’s beak which marks out the position of Si3N4 mask edge and allows for the determination of L,, and Rb. Two important parameters are however missing, the flat band voltages Vrb(tox) and V,,(fox) for the thin and thick oxides respectively. These were determined on thin oxide and thick oxide test structures on the same chip, giving values of Vfb of -0.925 and -0.7 V for the thin and thick oxides respectively. From these values, the values of Vfb at any point in the bird’s beak structure can be estimated using the linear variation: I’r,, (BB, x) = [Vn,(fox) - Vrh(tox)] x .r/L,, + Vri,(tox). From the point of view of oxide thickness at any one point X along the bird’s beak, the following
Si Substrate
Fig. 4. Cross-sectional
view by MEB of the bird’s beak region.
751
Interface. states under locos bird’s beak region equation
was used:
5 * J%,(x) ‘feEtox+
F*x’r(3-2x,&)
(7)
bb
0.6
this being found to be more accurate than the linear form usually taken. The average concentrations NSubstand N,, are obtained from the ratio C,,/C,, taken at 1 MHz. The values used below are
0
z o
0.7
C ::
0.6
Q
0.5
N rubs,= 2.OEl5 cmm3 N,, = 2SE16 cmm3. Given the time needed to perform the simulation, the number of bird’s beak capacitor slices needs to be chosen to be the minimum required to describe the experimental reality. From experience it has been found that n = 100 represents a good compromise between time of simulation and accuracy, as for a greater number nonnoticeable modification of simulations is observed.
Gate Volta@(V)
c (V, 1 / c,,, 1.00
4.2. Experimental
and simulation results as9
Figures 5 give the experimental results and simulations for the bird’s beak structure. Due to the detail in these curves, they have been split into two: Fig. 5(a) refer to the “thin oxide response region”, for gate voltages between - 3 and + 3 V, while Fig. 5(b) refer to the full voltage range of - 5 to + 15 V, but with the capacitance axis expanded in order to see the small changes in capacitance involved. In these figures, the results obtained by simulation are shown by curves 1, 2 and 3, while the experimental results are shown by curve 4. The three full curves shown in each simulation are: Curve 1 corresponds to the simulation with zero density of N,,. Curve 2 corresponds to a density of states at the interface with the following characteristics (taken arbitrarily): Nz = 3.5Ell cm-‘eV_’ {: = E, + 0.4 eV Ng = 6.5E11 crnm2 eV-I
In comparing simulation and experiment, it is assumed in these simulations, firstly that there are no
am 0)
0 C
z
OS7
i
0.96
s o
0.95
z ‘;, E & z
B .;po
I
.. P
b *
.
\ * \ .
.I
cl94 a93 0.92 0.91 0.60
0
I
1 -5
: iI
’
”
lb8
’
__-
,, ‘0
.. . . . .
Localized
-
Uniform
0 0 0
Experiment
’
65
’
0
’
N,, N,
’
’
’
15
Gate voltage (Vl
Fig. 5. Experimental and simulated curves for a 400 A thin oxide. Curve 1 (---) simulation without defect; curve 2 (. . .) simulation with localized distribution of defects (see text); curve 3 (-) simulation with uniform distribution of defects; curve 4 (0 0 0) experimental results. (a) For V, = [ - 3 V; + 3 V] without expanding of normalized capacitance axis. (b) For Vg= [- 5V; + 15V] with expanding of normalized capacitance axis.
interface states, neither in the thin oxide nor in the thick oxide (this was confirmed experimentally on the test structures by quasi-static measurements) and secondly that the surface states in the bird’s beak are distributed uniformly in length between field and gate oxides. Comparing the results of simulation and experiment on Fig. 5, it can be seen that the simulations show good agreement with the measured curve, if we fit simulated results with N, = lE12 crne2 eV_‘. The only outstanding difference is in the peak which appears in the experimental data between V, = 9 and 10 V. This point will be discussed in more detail later.
152
J.-C. MARCHETAUXet al.
The most interesting aspects of the comparison are:
C(V,)
/C”,,
(i) the C,, dip which appears between - 1.5 and OV [Fig. 5(a)], (ii) the “plateau” which extends from 0 to 11 V, followed by an increase, saturating at V, = 14 V [Fig.
WN.
,
Comparing results and simulated curves, it can be seen that, with N, = 0 (curve 1) there is a large difference between the experimental curve (lozenges) and the simulation. This situation is improved by taking N,, to be localized and of concentration lE12cm-* eV-’ (curve 2). It can be seen, however, 0.5 that this does not exactly match the minimum of I the experimental C-V curve. Furthermore, there is 1 0.4 , , I I 1 a departure from the experimental curve at Vg = 3 0 -3 - 1.8 V, due to the traps situated in the forbidden gap Gate voltage (V ) at E, + 0.4eV. It can be seen that the best fit is Fig. 6. Simulated and experimental curves for a 400 A thin obtained for the case of a uniform distribution of oxide. Curve 1: simulation with N, = 0; curve 2: simulation states in energy and of concentration lE12 cmm2 eV-’ with N, = IEll cm-2eV-‘; curve 3: simulation with (curve 3), which gives good agreement not only N, = lE12 c16 eV-‘; curve 4: experimental results. around C,,, but also over the full voltage range shown here. The region around C,, corresponds to the region section. The point at which there is no divergence of influence of the gate oxide-the quasi-static C-V with regard to simulated curve with uniform N,, curve minimum would have a value of 0.18 for 400 A marked in this figure by an arrow, marks the limit of of thin oxide alone, due to the low substrate doping. the effect of bird’s beak interface states in the comIt would thus be expected that Cminbe lower for the parison of simulation and experiment. Taking as a experimental curve than is seen here. However, the first approximation that there is a linear relationship value of the minimum is strongly influenced by the between the gate voltage of minimum capacitance for field oxide, which is itself in strong accumulation at each capacitor slice in the bird’s beak section, and the these voltages. This capacitance in parallel with the position of that particular capacitor section in going gate oxide has the effect of raising the minimum to from thin to thick oxide [13], we can estimate the a value of 0.38 (this value was obtained from the length of the bird’s beak region that plays a role in simulation of a structure containing thick and thin the response of the C-V curve. If we call Vmin(limit) oxides, and no bird’s beak, i.e. L,, = 0). the voltage for which the simulated and experimental Figure 6 shows the simulated curves obtained for curves coincide [arrow on Fig. 5(b)], we can infer, interface state density assumed to be. uniformly dis- from the equation below, that the distance lo in the tributed with respect to energy, from: 0 to lEl3 cm-* bird’s beak from the gate oxide edge in which we can eV_‘, with the experimental curve of Fig. 5(a) for detect defects is: comparison. It can be seen that the method is sensi1 = V,. (limit) - Vti, (tox) * Lbb= 1800A tive to concentrations of states at least: lEl1 cn-* 0 V~/,i,(fOX) - Vmin(tOX) eV-‘, and that the best fit to measurements is where obtained with N, = lE12 cmm2 eV-I. The comparison of simulation and measurement V,,(limit) = 1.5 V are shown in Fig. S(b) for the gate voltages ranging V,,(tox) = -0.29 V from - 5 to 15 V on an enlarged y-axis, and for the best fit in N, of the previous region. Vmin(fox)= 11.9v It can be seen that the simulation and experiment fit well the high positive gate voltage region ( VBfrom and 11 to 15 V). This indicates that there is agreement in L,, = 1.2 I*m. the field gate oxide region, (i.e. where the thick oxide passes from depletion into inversion) within the It is found that the limit 1, from the gate oxide edge in which the surface states in the bird’s beak region incertainty of the measurement. This incertainty arises due to the inaccuracy in the estimation of V,, play a role is 1800A. There are two possibilities as to why the part of for the thick oxide. In the “plateau” region between 0 and 11 V, the the bird’s beak beyond the limit 1, = 1800 A has an apparent zero density of states compared to that part comparison of simulation and experiment show that close to the thin oxide. The first is that it is the latter there is a greater and greater divergence as more and region that corresponds to the region under the more interface states are added to the bird’s beak
753
Interface states under locos bird’s beak region
itself. In our case, it is difficult to reconcile the effect of sweep rate mentioned above with such a model. CONCLUSIONS
0
Gate voltage (V 1 Fig. 7. Normalized experimental C-V curves for a 400 A thin oxide with two sweep rates: -, lOmV/s; ---, 100 mV/s.
Quasi-static measurements have been carried out on a special MOS capacitor test structure which enhances the ratio of the bird’s beak area and the total capacitor area. A simulation program has been developed for this special structure, and in order to match simulation to experiment it is found necessary to add a continuum of surface states to the bird’s beak region with concentrations of the order of lE12 cmW2 eV_‘. For the best fit these states need to be situated to within 1800 A of the thin oxide edge. For large distances it has not been possible to draw definitive conclusions. In addition a peak in the C-V curves has been found at voltages near the field oxide threshold voltage, which has not yet been explained, but which appears not to be due to interface states. Acknowledgements-The
authors would like to thank P.
nitride mask, which is more susceptible to the cre- Chantraine and his team for preparation of devices. This ation of defects during oxidation because of the work is supported in part by the E.E.C. “SPECTRE” project. nitride mask itself [8]. The second possibility is the sensitivity of the measurement in this zone. The REFERENCES current measured coming from each capacitor slice is proportional to the capacitance which is, in turn, 1. R. H. Dennard, F. H. Gaensslen, H.-N. Yu, V. L. inversely proportional to the oxide thickness. The Rideout, E. Bassous and A. R. LeBlanc, IEEE J. Solid-St. Circuits SC-9, 256 (1974). capacitance of each “slice” decreases with distance 2. P. P. Wang, IEEE Trans. Electron Dev. ED-U, 779 from the gate oxide edge [see eqn (7)], and so the (1978). current resulting from charge exchange of these states 3. S. A. Abbas and R. C. Dockerty, Appl. Phys. Lett. 27, near the field oxide edge becomes a negligible part 147 (1975). 4. T. H. Ning, C. M. Osburn and H. N. Yu, J. Electron. of the overall current, and is thus not seen on the Muter. 6, 65 (1977). experimental curve. For the moment, it has not been 5. P. E. Cotrell, R. R. Troutman and T. H. Ning, IEEE possible to distinguish between the two effects. Any Trans. Electron Dev. ED-24, 520 (1979). distinction would require a more sensitive measure6. R. G. Penning, Insulating Films on Semiconductors ment system than is possible with this instrument. (Edited by J. F. Verweij and D. R. Wolters), p. 171. North Holland, Amsterdam (1983). Returning to the anomalous peak seen in Fig. 5(b) 7. X. Q. Zheng, K. Hofmann, M. Schulz and L. Risch, just before field oxide inversion (at approx. lOV), J. uppl. Phys. 53, 9146 (1982). Fig. 7 shows two measurements made on the mosaic 8. J. Hui, P. V. Voorde and J. Moll, ZEDM 85 14.7, 392 structure at different sweep rates. The first is that of (1985). Fig. S(b), with a ramp sweep rate of 10 mV/s, whereas 9. M. Kuhn, Solid-St. Electron. 13, 873 (1970). 10. C. N. Berglund, IEEE Trans. Electron Dev. ED-13, 701 the other is made under the same conditions, but with (1966). a ramp speed of lOOmV/s. It can be seen that there 11. S. M. Sze, Physics of Semiconductor Devices, 2nd edn, is an increase in the size of the peak for the faster rate D. 367. Wiley-Interscience, New York (1981). of sweep, indicating that this might be due to the 12. E. H. Nicolhan and J. R. Brews, MOS (Metal Oxide Semi-conductor) Physics and Technology, p. 323. measurement system. It should be noted that this type Wiley-Interscience, New York (1982). -. _ of peak in the quasi-static response has also been seen 13. S. M. Sze, Physics of Semiconductor Deuices, 2nd edn, after electron injection in S-rich SO2 capacitors [ 141. pp. 375, 378. Wiley-Interscience, New York (1981). The explanation given here is a current due to the 14 C. Falcony and F. H. Salas, J. uppl. Phys. 59, 3787 neutralization of the positive charge in the oxide (1986).
$3.00 + 0.00 0038-I lOI/ Copyright 0 1987 Pergamon JournalsLtd
1987
Printed in Great Britain. All rights reserved
INTERFACE
STATES UNDER LOCOS BIRD’S BEAK REGION
J.-C. MARCHETAUX, B. S. JIOYLE and A. BOUDOU Bull S.A., 78340 Les Clayes-sous-Bois, France (Received
12 September
1986; revised 7 January
1987)
Abstract-A method has been developed which allows the extraction of the density of electrically active defects in the bird’s beak region of LOCOS isolation of a MOS structure. The measurements were performed on a special structure with enhanced edge effects using quasi-static techniques. The experimental results are compared with simulations using a model which allows for the inclusion of surface states in the bird’s beak region. It is found that the best fit is obtained for a uniform energy distribution with N, = lE12 cm-2 eV-‘, while the states are situated in the bird’s beak region up to 18OOA from the gate oxide edge.
NOTATION
area of the elementary capacitor number i (cm*) capacitance of the elementary capacitor number i (F/cm’) silicon oxide capacitance of the elementary capacitor number i (F/cm3 capacitance of the elementary capacitor number i due to the variation of charge in silicon (F/cm3 capacitance of the elementary capacitor number i due to the variation of charge trap at the interface (F/cm*) minimum capacitance of the quasistatic C/V curve (F/cm*) silicon oxide capacitance (F/cm3 capacitance measured at the voltage V (F/cm*) conduction band edge, valence band edge, respectively (eV) field oxide thickness and gate oxide thickness, respectively (cm) silicon oxide thickness (cm) Boltzman constant (1.38E-23 J/K) channel length of MOS transistors
(cm) total length of bird’s beak (cm) distance of the mask e&e from the edge of the field oxide (cm) distance of the mask edge from the edge of the thin oxide (cm) distance from the thin oxide edge in which the surface states can be detected (cm) number bf klementary capacitors in the bird’s beak reeion concentration in &con (cme3) number of cells in test structure concentrations in silicon under the field oxide, under the thin oxide, respectively (cm-‘) intrinsic density (cm-‘) total density of traps at the interface (cm-* eV-I) acceptor and donor densities of traps, respectively, at the interface (cm-z l=V-‘1
el&;on
charge (1 .602E-1g C)
charge in the silicon (C/cm3 charge at the interface (C/cm2) ratio of the mask edge-thin oxide edge to the bird’s beak length temperature (300 K in this paper) T reference voltage where the capacitor V, is in accumulation (V) flatband voltage (V) Vfi flatband voltage at x from the thin V, (BB, x) edge under the bird’s beak region flatband voltages of field capacitor and Vi&-ox), v, 004 gate capacitor respectively (V) V gate voltage (V) gti”(fox), V&,(tox) potential of minimum capacitances for field test reference and gate test reference respectively (V) potential drop across the silicon oxide VOX (v) channel width of MOS transistors (cm) W range of the Gaussian variation of -5 concentration under the bird’s beak region (cm) permittivity in vacuum (8.854 E-l4 F/ cm) tan-’ {(% - .%X2*&,)) Si02 and Si dielectric constants respectively energies of, respectively, acceptor and donor localized traps at the interface (eV) standard deviation of the Gaussian (I variation of concentration under the bird’s beak region (cm) Fermi potential deep in the substrate @b (v) yap,,6 surface potential (v) s ,r, surface potential of the elementary TM, capacitor number i (V)
1. INTRODUCTION
In recent years, advances in silicon fabrication technology have led to a continuing scaling down of MOSFET dimensions and to improvements of electrical performance. At the present time, MOSFET gate lengths approach micron and submicron sizes. It 745
J.-C.
746
MARCHETAUXet
is well known that at these dimensions parasitic effects arise, which are not important for long devices. These effects are, for example, Short Channel Effects (SCE)[l], Narrow Width Effects (NWE)[Z] and Hot Electron Effects[3-51. While the first two problems can be controlled by an appropriate choice of threshold implantation conditions, the third, which arises due to high fields in the channel region, is much harder to control and (if not mastered) can lead to circuit degradation and eventual circuit failure. Consequently much research has been carried out into the design of such structures as Lightly Doped Drain (LDD) and Double Diffused Drain (DDD) to modify the channel area near the drain edge. In contrast to the above, although interface state measurements in device boundaries by differing methods are reported in the literature[6,7], the influence of the reduction of gate width has not, to our knowledge, been the subject of research in terms of intrinsic defects and hot electron damage. With a ratio w 11of about 3, and gate lengths approaching the 1 pm size, the bird’s beak region between gate and field oxide occupies a nonnegligible area of the total surface of the MOS transistor. This area is the site of constraints coming mostly from the silicon nitride mask used in the LOCOS process, and can be responsible for the generation of interface states[8]. Other process steps, such as field implantation, etching, etc. are also possible sources of interface damages in the bird’s beak region. With the reduction in gate size, the amount of current which flows in this region becomes greater, and this can cause a degradation in performance of the MOS, either intrinsically, due to electrically active sites, or extrinsically, due to hot injection which can activate defects in this part of the device. The aim of this article is to describe a method which enables the comparison of simulation with the measurement of the capacitance in the bird’s beak region, to ascertain whether interface states exist in this region, and to estimate their density and spatial repartition. Section 2 describes the measurement system as well as the test circuit used. In Section 3, the method of simulation is described for the test structure equivalent circuit, which allows for the inclusion of the effect of interface states in the bird’s beak region. The results of both simulation and experiment are described in Section 4 for a gate oxide thickness of 400 A. Section 5 discusses the results followed by conclusions in Section 6.
al.
luminescent diode placed above the sample to allow for the illumination of the sample before the voltage sweep, and a HP 98163 microcomputer which controls the system, stores the data as well as performing the simulations. It has not proved possible to use the Berglund method[lO] for these measurements as the surface potential cannot be estimated when both the oxide thickness and the substrate doping varies. This point will be developed in Section 3. For the measurement, the voltage is swept from inversion to accumulation, using the diode to illuminate initially the capacitor to allow the system to reach full inversion-for these samples, the low generation rate of electron/hole pairs means that an external means is necessary to provide the minority carriers needed. 2.2. Test structure Most of the capacitance measurement methods suffer from the disadvantage that they require large area devices, typically (lOO*100) pm2, and cannot thus be carried out on transistors. In our particular study, however, it is not so much the area (a large area is still needed), but the ratio of the bird’s beak to the thin and field oxides that is important. In the case of test structures consisting of a large area thin oxide surrounded by the bird’s beak and thick oxide, the bird’s beak occupies typically OS-l% of the surface. It is for this reason that we have designated a test structure in which the area of the edges is optimized over the area of the thin oxide with an overall capacitance of the order of 120pF, and a bird’s beak area of about 30% of the total area. The bird’s beak capacitance test structure is made up of basic cells each of which is approximately
Si,N, -
2. EXPERIMENTAL
2.1. Measurement
MASK POLY
system
The method used here for the detection of surface states is the quasi-static capacitive method[9]. The measurement system consists of a HP 4140B pA meter/DC Voltage Source for the voltage ramp and to measure the displacement current, an electro-
Fig. 1. View of an elementary cell of
the bird’s beak mosaic structure. The lower view is a cross-section of the upper view along AA line.
Interface states under locos bird’s beak region
747
Table 1. Area of field oxide, gate oxide, bird’s beak oxide and total capacitors for a standard structure and the mosaic test structure.. Ratio of bird’s beak area to total area is also presented. E,,,, = 400 A; I$,, = 5500 A; bb = 1.3pm; Rb = 0.5; N, = 6400 (in cm-s)
Surface tox
Surface bb
Surface fox
surface total
bb surface Total surface
Structure examined Standard structure
9.78B-4 6.42E-3
1.77E-3 4.26E-5
3.59E-3 0
6.43E-3 6.46E-3
27.5% 0.7%
equivalent in area to a MOS transistor of gate length of several microns. The cell itself consists of a square of thin oxide of dimensions (5*5) pm2 surrounded by the field oxide. The sides of the cell are 10 pm long. This is shown in Fig. 1 with a cross-sectional cut of this symmetrical cell. To obtain the sensitivity necessary for the measurements, the number of basic cells is simply increased, thus increasing the capacitance while keeping constant the ratio of edge to bulk. In the structure used here, the capacitance consisted of a mosaic of 80 by 80 unit cells in parallel. For the calculation of the areas and capacities of the different parts of this basic cell, the cell is divided into three regions, the field oxide, the gate oxide, and the bird’s beak. Given in Table 3 are the definitions of the units used. These can also be seen in Fig. 1. Tables 1 and 2 give the values of area and oxide capacitance respectively for the optimized bird’s beak structure and a standard MOS capacitance with equivalent area. It can be seen that the relative fraction of the bird’s beak area is multiplied by 40 and by a factor of 175 for the capacitance compared with an equivalent thin-oxide surface. 2.3. Samples The samples used came from a 2-pm NMOS process with a dry oxide and a P-type substrate with a resistivity of 6.5 f 1.5 ohm cm and have no threshold voltage implant. The samples had a nitride mask deposited just before the field oxidation step (used for the LDCOS process) and the field implantation. This was followed by the growth of the gate oxide, using a dry process, giving an oxide of thickness 400 A. The polysilicon gate electrode was then deposited using LPCVD doped N+ and etched by RIE. After the passivation, etching of contacts and deposition of conductors, the final anneal was carried out in an
atmosphere of 15% hydrogen-85% 450°C for 30min.
(in PF) Structure examined Standard structure
121 791
Capacitance bb 30 0.8
In the quasi-static capacitance measurement method, the determination of the interface state density is obtained by the Berglund technique[lO], which involves the determination of the surface potential in the definition of the space charge region. Berglund showed that the surface potential can be obtained from the experimental C-V curves by:
Y,(V) =
c ’
[I -C(V,)/C,,,]*dV,+K
J v., where V is the applied voltage to the gate electrode, V, is the voltage taken as reference where the capacitance is in accumulation, C(V) is the capacitance measured at the voltage V, and K is a constant determined experimentally, [K = Y,( V,)]. This method is not applicable as such to the structure described above, as the surface potential is not the same under the various oxide regions, the gate, the field, and the bird’s beak region (where !Fs varies continuously with distance from the gate oxide edge) and the values of the different capacitances cannot be directly extracted from the overall capacitance. It is thus necessary to use another method. For this, an equivalent circuit is necessary in which each separate contribution can be separated out and treated numerically. It is this approach that we have adopted here. 3.1. Equivalent circuit Each basic cell has been divided into three zones (Fig. 2). The: Tox (E,,, , gate oxide thickness) and Fox 1 for oxide capacitance Capacitance fox 23 -
Capacitance total 174 792
bb capacitance Total capacitance 17.5% 0.1%
Table 3. Parameter values necessary for simulation b Erw 4, Lbb % & e 4ooA 1.2~ 0.5 0.3 p 0.05p 1OP 0.55~ a-side mask of the square of thin oxide (a = 5 pm; Fig. 1); b-side mask of the cell of test structure (Fig. 1); &-total length of bird’s beak (Fig. 1); &-distance of the mask edge from the edge of the thin oxide (Fig. I); R,,-ratio of the mask edge-thin oxide edge to the bird’s beak length (Fig. I); X,,, a-parameters for the Gaussian variation of concentration under the bird’s beak region.
Values used
a-2&, 3.05 p
at
3. MODELLIZATION
Table 2. The same as Table Capacitance tax
nitrogen
J.-C. MARCHETAUX et al.
748
I
$
tox
Fox zone
.I#“.
.
: .
.
.
.
Bird’s
.
.
.
.
.
.
beak
zone
(
t
1
TO, zone
_--_j------I-----A x
Lbb
L bl
0
Fig. 2. Modelization of the bird’s beak region. (Er,,,, field oxide thickness) zones can immediately be treated separately, and constitute two planar capacitors. However, the bird’s beak region itself must be sectioned into: n separate capacitors, each of which must be sufficiently small to be regarded as a planar capacitance, each slice having a surface potential different from its neighbour [Fig. 3(a)]. Each of these capacitors C, is made up of the three individual capacitances: Ci_ = oxide capacitance of this elementary capacitor in series with Ci, (YS,), due to the variation of trap charge at the interface, and C, (!I’,), due to the variation of charge in the semiconductor. This is shown in Fig. 3(b). The determination of these capacitances is extracted by first determining the different surface potentials. 3.2. The dz$erent steps of the calculation The calculation parts:
can be divided into 12 separate
(1) Dividing the capacitance up into n + 2 different capacitances, as defined above. For each of these. (2) Determination of the area Ai. (3) Determination of the concentration of dopants in the substrate under each capacitor section. This is necessary as the standard cell has an insulation implantation under the field oxide. (4) Determination of the interface charge. (5) Determination of the surface potential as a function of the gate voltage. (6) Determination of the capacitance per unit surface, C,,(Y&
Fig. 3. Equivalent circuit of: (a) an elementary cell of the bird’s beak structure; (b) a “slice” of the bird’s beak region of an elementary cell.
(7) Addition of the capacitance C&P,) coming from the supposed concentration of N,. (8‘) Determination of the total capacitance per unit surface. (9) From (5), C,(V,) can immediately be deduced. (10) The total capacitance of the cell Ai * C,( V,) is then added to the capacitances already determined. (I 1) If all n + 2 capacitances have not been calculated, we go back to step (2) for the next capacitance (12) The total capacitance obtained is multiplied, for each of the values of V,, by the number of cells in the structure, N,. This is done for each value of I’.., the sweep of the gate voltage being chosen so that all the regions of the structure pass from accumulation to inversion: The non-trivial steps (3), (4), (5), (6) and (7) are described below. Step (5): Relation between surface potential and gate voltage
We have VP- Vfb= VOX + YS where V,, is the potential drop across the oxide, V,, is the flat-band voltage, Yy,is the surface potential and V,,, = -(Q,+ Q,)/C,, with Q,= charge in the semiconductor; Q, = charge at the interface. It can be deduced [ 1l] that: B*(V,-V,,)=X-Sgn(X)*A * F(X ni/N) * .5,x -
Q&m
(1)
Interface
under locos bird’s beak region
states
with
(b) and in the capacitance with C,$(Y,).
B = qlkT X=/?*Yu,
3.3. Determination
Sgn(X) = 1 if X > 0 = -1
749 as CJY,)
of Qss(Y,)
According to [12], the total traps is given by
ifX
A = fi/(rcO * t,,) * ,,/m
icO* t0
is in parallel
charge
of interface
QssV'u,)= 4 *
F(X ni/N) =
[exp(-X)+X-
l] + (n,/fV)2 * [exp(X) -X
x fo(5, - ‘4 - y’,) * P’:sK) + N:s(L)l
- I].
This exact analytical relation of V, =fn(~,) is transformed into a numerical relation: Yy,=fi( v,) by iteration and second order development of Yy, to obtain the different Vg’s necessary. Step (3): Determination of the dopant concentration the substrate under the oxide
in
The following simplification has been used: The concentration in the substrate under the thin oxide is constant (there is no threshold implant). The same constant concentration assumption is taken for the field oxide as the field implant has diffused sufficiently that it can be considered constant. Under the bird’s beak, for each capacitance slice, the concentration is assumed to be constant into the substrate, but there is a Gaussian variation of concentration from one slice to the next, according to equation:
where fO is the Fermi-Dirac
3.4. Determination
- Nsubrc)* exp] - (x - &)‘/(2a
Step (6): Determination
we can obtain
[12]:
surface, C,*( y, ) Once the surface potential is known, the surface capacitance coming from charge variations in the semiconductor, is given by [l 11: CJX)
=
s W?
Cs,(y,) =
-4
*
d5, *
m5, - 6 - yu,)
h/q
3.5. Distribution
per unit
have the usual meaning.
From
')I + Nsubst.
of the capacitance
*x)1-’
of C,(Y,)
*
N = (&,
function:
J)(x) = [l + l/2 * exp(p and the other symbols
(2)
D’i’s(5,) + Ws(t,)l.
(3b)
in energy
To simpify the calculation, two theoretical cases on Nss distribution in energy are simulated, states localized in energy and states uniformly distributed in energy in the forbidden gap: 3.5.1. Case 1. Traps localized in energy at the interface. Taking
T d
* [l - ew( --Xl +
h/W2 * (exp@‘)- 111
F(X n,lN) and
where 6 is the S-function, and 5, the energy of traps. The donor and acceptor states used in the simulation have energies: E, + 0.85 eV and E, + 0.4 eV respectively (chosen arbitrarily). From eqn (2) we obtain:
Q,,(ys, = q * [W’s, * (1 -f&C’ - @I,- YJ)
Ld-JF
- N:s,*.6(5: - % - YJI where
the symbols
have
the same
notation
as in
eqn (1). Step (4) and (7): Determination
These interface states intervene in two ways: (a) in the determination of the surface potential [eqn
(l)l,
and from eqn (3b):
Cs,(Y,) = q * P * {N:s,,*AAt: of interface states
(44
* 11-f,(r_:‘+ N&*f,(<;* [1 -fo(TY
@b -
- Qi, - yu,) Yu,)l
%-- @b-
Ys)
Yu,)lI.
Cab)
750
J.-C. MARCHETAUX etal.
be noted It should employed [ 121:
that
the
expression
usually (5)
C,(Y,)
= q * N,(YY,)
with N,,( Yy,) = Nts(Y,) + Nis(YJ is no longer usable. In effect, eqn (5) is obtained using the hypothesis that N,,(Y’,) remains constant in the energy range A[ = kT/q around Y,, which is manifestly not true in the case being considered (see definition of Nzs and Nfs above). 3.5.2. Case 2. Interface traps distributed uniformly in the forbidden gap. Taking and
N$(5,) = N&
K(L)
= KS0
so N,,(&) = N,,, = N& + N& and using eqns (2) and (3a), gives:
Q,,VJ = -s/B * N,,, * lnMYs)I Ec- Ev
+ N,, * /I * -
4
+ WR (Y,))
(64
11
with R(y
)
s
=.hdEc/q - @b- ys> “w”lq - @b- Yu,)
C,,(y,) = -4
*
Nsso* U&%/q - @I,-
- h(Evlq - @b- u’s)].
‘J’s) (6b)
If we suppose that the Fermi level comes no closer than a few kTq to the band edges, then: fo(Wq
- @b - YJ = 0
fo(-Wq
-
and @b
-
yu,)
=
1
and the well known relation (5). However, there are problems in the determination of R(YJ and thus Q,s(Y,), which means that the full relation must be conserved.
4.RESULTS 4. I. Parameter values necessary for simulation Electron microscope photos were used of crosssections of the test structure to evaluate the various parameters used in the simulation. The mean values of the various parameters are such as L,, , R,, E,,, , E fOx,A’,,, r~ are shown in Table 3. Figure 4 is a typical example, obtained by electron microscopy, of a cross-section of an elementary cell of the test structure with E,,, = 400 A, showing the details of bird’s beak and particularly the encroachment near the middle of bird’s beak which marks out the position of Si3N4 mask edge and allows for the determination of L,, and Rb. Two important parameters are however missing, the flat band voltages Vrb(tox) and V,,(fox) for the thin and thick oxides respectively. These were determined on thin oxide and thick oxide test structures on the same chip, giving values of Vfb of -0.925 and -0.7 V for the thin and thick oxides respectively. From these values, the values of Vfb at any point in the bird’s beak structure can be estimated using the linear variation: I’r,, (BB, x) = [Vn,(fox) - Vrh(tox)] x .r/L,, + Vri,(tox). From the point of view of oxide thickness at any one point X along the bird’s beak, the following
Si Substrate
Fig. 4. Cross-sectional
view by MEB of the bird’s beak region.
751
Interface. states under locos bird’s beak region equation
was used:
5 * J%,(x) ‘feEtox+
F*x’r(3-2x,&)
(7)
bb
0.6
this being found to be more accurate than the linear form usually taken. The average concentrations NSubstand N,, are obtained from the ratio C,,/C,, taken at 1 MHz. The values used below are
0
z o
0.7
C ::
0.6
Q
0.5
N rubs,= 2.OEl5 cmm3 N,, = 2SE16 cmm3. Given the time needed to perform the simulation, the number of bird’s beak capacitor slices needs to be chosen to be the minimum required to describe the experimental reality. From experience it has been found that n = 100 represents a good compromise between time of simulation and accuracy, as for a greater number nonnoticeable modification of simulations is observed.
Gate Volta@(V)
c (V, 1 / c,,, 1.00
4.2. Experimental
and simulation results as9
Figures 5 give the experimental results and simulations for the bird’s beak structure. Due to the detail in these curves, they have been split into two: Fig. 5(a) refer to the “thin oxide response region”, for gate voltages between - 3 and + 3 V, while Fig. 5(b) refer to the full voltage range of - 5 to + 15 V, but with the capacitance axis expanded in order to see the small changes in capacitance involved. In these figures, the results obtained by simulation are shown by curves 1, 2 and 3, while the experimental results are shown by curve 4. The three full curves shown in each simulation are: Curve 1 corresponds to the simulation with zero density of N,,. Curve 2 corresponds to a density of states at the interface with the following characteristics (taken arbitrarily): Nz = 3.5Ell cm-‘eV_’ {: = E, + 0.4 eV Ng = 6.5E11 crnm2 eV-I
In comparing simulation and experiment, it is assumed in these simulations, firstly that there are no
am 0)
0 C
z
OS7
i
0.96
s o
0.95
z ‘;, E & z
B .;po
I
.. P
b *
.
\ * \ .
.I
cl94 a93 0.92 0.91 0.60
0
I
1 -5
: iI
’
”
lb8
’
__-
,, ‘0
.. . . . .
Localized
-
Uniform
0 0 0
Experiment
’
65
’
0
’
N,, N,
’
’
’
15
Gate voltage (Vl
Fig. 5. Experimental and simulated curves for a 400 A thin oxide. Curve 1 (---) simulation without defect; curve 2 (. . .) simulation with localized distribution of defects (see text); curve 3 (-) simulation with uniform distribution of defects; curve 4 (0 0 0) experimental results. (a) For V, = [ - 3 V; + 3 V] without expanding of normalized capacitance axis. (b) For Vg= [- 5V; + 15V] with expanding of normalized capacitance axis.
interface states, neither in the thin oxide nor in the thick oxide (this was confirmed experimentally on the test structures by quasi-static measurements) and secondly that the surface states in the bird’s beak are distributed uniformly in length between field and gate oxides. Comparing the results of simulation and experiment on Fig. 5, it can be seen that the simulations show good agreement with the measured curve, if we fit simulated results with N, = lE12 crne2 eV_‘. The only outstanding difference is in the peak which appears in the experimental data between V, = 9 and 10 V. This point will be discussed in more detail later.
152
J.-C. MARCHETAUXet al.
The most interesting aspects of the comparison are:
C(V,)
/C”,,
(i) the C,, dip which appears between - 1.5 and OV [Fig. 5(a)], (ii) the “plateau” which extends from 0 to 11 V, followed by an increase, saturating at V, = 14 V [Fig.
WN.
,
Comparing results and simulated curves, it can be seen that, with N, = 0 (curve 1) there is a large difference between the experimental curve (lozenges) and the simulation. This situation is improved by taking N,, to be localized and of concentration lE12cm-* eV-’ (curve 2). It can be seen, however, 0.5 that this does not exactly match the minimum of I the experimental C-V curve. Furthermore, there is 1 0.4 , , I I 1 a departure from the experimental curve at Vg = 3 0 -3 - 1.8 V, due to the traps situated in the forbidden gap Gate voltage (V ) at E, + 0.4eV. It can be seen that the best fit is Fig. 6. Simulated and experimental curves for a 400 A thin obtained for the case of a uniform distribution of oxide. Curve 1: simulation with N, = 0; curve 2: simulation states in energy and of concentration lE12 cmm2 eV-’ with N, = IEll cm-2eV-‘; curve 3: simulation with (curve 3), which gives good agreement not only N, = lE12 c16 eV-‘; curve 4: experimental results. around C,,, but also over the full voltage range shown here. The region around C,, corresponds to the region section. The point at which there is no divergence of influence of the gate oxide-the quasi-static C-V with regard to simulated curve with uniform N,, curve minimum would have a value of 0.18 for 400 A marked in this figure by an arrow, marks the limit of of thin oxide alone, due to the low substrate doping. the effect of bird’s beak interface states in the comIt would thus be expected that Cminbe lower for the parison of simulation and experiment. Taking as a experimental curve than is seen here. However, the first approximation that there is a linear relationship value of the minimum is strongly influenced by the between the gate voltage of minimum capacitance for field oxide, which is itself in strong accumulation at each capacitor slice in the bird’s beak section, and the these voltages. This capacitance in parallel with the position of that particular capacitor section in going gate oxide has the effect of raising the minimum to from thin to thick oxide [13], we can estimate the a value of 0.38 (this value was obtained from the length of the bird’s beak region that plays a role in simulation of a structure containing thick and thin the response of the C-V curve. If we call Vmin(limit) oxides, and no bird’s beak, i.e. L,, = 0). the voltage for which the simulated and experimental Figure 6 shows the simulated curves obtained for curves coincide [arrow on Fig. 5(b)], we can infer, interface state density assumed to be. uniformly dis- from the equation below, that the distance lo in the tributed with respect to energy, from: 0 to lEl3 cm-* bird’s beak from the gate oxide edge in which we can eV_‘, with the experimental curve of Fig. 5(a) for detect defects is: comparison. It can be seen that the method is sensi1 = V,. (limit) - Vti, (tox) * Lbb= 1800A tive to concentrations of states at least: lEl1 cn-* 0 V~/,i,(fOX) - Vmin(tOX) eV-‘, and that the best fit to measurements is where obtained with N, = lE12 cmm2 eV-I. The comparison of simulation and measurement V,,(limit) = 1.5 V are shown in Fig. S(b) for the gate voltages ranging V,,(tox) = -0.29 V from - 5 to 15 V on an enlarged y-axis, and for the best fit in N, of the previous region. Vmin(fox)= 11.9v It can be seen that the simulation and experiment fit well the high positive gate voltage region ( VBfrom and 11 to 15 V). This indicates that there is agreement in L,, = 1.2 I*m. the field gate oxide region, (i.e. where the thick oxide passes from depletion into inversion) within the It is found that the limit 1, from the gate oxide edge in which the surface states in the bird’s beak region incertainty of the measurement. This incertainty arises due to the inaccuracy in the estimation of V,, play a role is 1800A. There are two possibilities as to why the part of for the thick oxide. In the “plateau” region between 0 and 11 V, the the bird’s beak beyond the limit 1, = 1800 A has an apparent zero density of states compared to that part comparison of simulation and experiment show that close to the thin oxide. The first is that it is the latter there is a greater and greater divergence as more and region that corresponds to the region under the more interface states are added to the bird’s beak
753
Interface states under locos bird’s beak region
itself. In our case, it is difficult to reconcile the effect of sweep rate mentioned above with such a model. CONCLUSIONS
0
Gate voltage (V 1 Fig. 7. Normalized experimental C-V curves for a 400 A thin oxide with two sweep rates: -, lOmV/s; ---, 100 mV/s.
Quasi-static measurements have been carried out on a special MOS capacitor test structure which enhances the ratio of the bird’s beak area and the total capacitor area. A simulation program has been developed for this special structure, and in order to match simulation to experiment it is found necessary to add a continuum of surface states to the bird’s beak region with concentrations of the order of lE12 cmW2 eV_‘. For the best fit these states need to be situated to within 1800 A of the thin oxide edge. For large distances it has not been possible to draw definitive conclusions. In addition a peak in the C-V curves has been found at voltages near the field oxide threshold voltage, which has not yet been explained, but which appears not to be due to interface states. Acknowledgements-The
authors would like to thank P.
nitride mask, which is more susceptible to the cre- Chantraine and his team for preparation of devices. This ation of defects during oxidation because of the work is supported in part by the E.E.C. “SPECTRE” project. nitride mask itself [8]. The second possibility is the sensitivity of the measurement in this zone. The REFERENCES current measured coming from each capacitor slice is proportional to the capacitance which is, in turn, 1. R. H. Dennard, F. H. Gaensslen, H.-N. Yu, V. L. inversely proportional to the oxide thickness. The Rideout, E. Bassous and A. R. LeBlanc, IEEE J. Solid-St. Circuits SC-9, 256 (1974). capacitance of each “slice” decreases with distance 2. P. P. Wang, IEEE Trans. Electron Dev. ED-U, 779 from the gate oxide edge [see eqn (7)], and so the (1978). current resulting from charge exchange of these states 3. S. A. Abbas and R. C. Dockerty, Appl. Phys. Lett. 27, near the field oxide edge becomes a negligible part 147 (1975). 4. T. H. Ning, C. M. Osburn and H. N. Yu, J. Electron. of the overall current, and is thus not seen on the Muter. 6, 65 (1977). experimental curve. For the moment, it has not been 5. P. E. Cotrell, R. R. Troutman and T. H. Ning, IEEE possible to distinguish between the two effects. Any Trans. Electron Dev. ED-24, 520 (1979). distinction would require a more sensitive measure6. R. G. Penning, Insulating Films on Semiconductors ment system than is possible with this instrument. (Edited by J. F. Verweij and D. R. Wolters), p. 171. North Holland, Amsterdam (1983). Returning to the anomalous peak seen in Fig. 5(b) 7. X. Q. Zheng, K. Hofmann, M. Schulz and L. Risch, just before field oxide inversion (at approx. lOV), J. uppl. Phys. 53, 9146 (1982). Fig. 7 shows two measurements made on the mosaic 8. J. Hui, P. V. Voorde and J. Moll, ZEDM 85 14.7, 392 structure at different sweep rates. The first is that of (1985). Fig. S(b), with a ramp sweep rate of 10 mV/s, whereas 9. M. Kuhn, Solid-St. Electron. 13, 873 (1970). 10. C. N. Berglund, IEEE Trans. Electron Dev. ED-13, 701 the other is made under the same conditions, but with (1966). a ramp speed of lOOmV/s. It can be seen that there 11. S. M. Sze, Physics of Semiconductor Devices, 2nd edn, is an increase in the size of the peak for the faster rate D. 367. Wiley-Interscience, New York (1981). of sweep, indicating that this might be due to the 12. E. H. Nicolhan and J. R. Brews, MOS (Metal Oxide Semi-conductor) Physics and Technology, p. 323. measurement system. It should be noted that this type Wiley-Interscience, New York (1982). -. _ of peak in the quasi-static response has also been seen 13. S. M. Sze, Physics of Semiconductor Deuices, 2nd edn, after electron injection in S-rich SO2 capacitors [ 141. pp. 375, 378. Wiley-Interscience, New York (1981). The explanation given here is a current due to the 14 C. Falcony and F. H. Salas, J. uppl. Phys. 59, 3787 neutralization of the positive charge in the oxide (1986).