NIR-enhanced image sensor using multiple epitaxial layers, presented Electronic Imaging 21 Jan 2004, San Jose CA; SPIE proceedings vol.
NIR-enhanced image sensor using multiple epitaxial layers
Bart Dierickx*, Jan Bogaerts
FillFactory NV, Schaliënhoevedreef 20 b, 2800 Mechelen, Belgium
ABSTRACT
We present the performance characteristics of a CMOS image sensor, manufactured on wafers with specially designed
multiple epitaxial layers. At the homo-junction between two consecutive epitaxial layers a small potential drop or
electric field represents a barrier for electrons diffusing backwards into the wafer. Such structure results in a net drive
or confinement of photo-charges towards the surface. As a result there is and enhanced, anisotropical, diffusion of
charge that are generated deep in the Silicon, e.g. by near infrared (NIR) or X-ray radiation. The spectral response is
an order of magnitude higher than for the same image sensor on "regular" wafers. The anisotropical diffusion results in
a limited MTF degradation compared to wafers with a single thick epitaxial layer.
Keywords: image sensor, multiple epitaxial layers, graded epitaxial layers, MTF, NIR, X-ray
1. INTRODUCTION
The STAR250-sensor is an integrating 3-transistor CMOS Active Pixel Sensor with 512 by 512 pixels on a 25 µm
pitch. Each pixel has 4 diodes for improved MTF and PRNU. The sensor has on-chip double sampling circuit to cancel
Fixed Pattern Noise. Its key feature is a radiation tolerance of more than 10 Mrad(Si) [1]-[3].
2. THICK MULTIPLE OR GRADED EPITAXIAL LAYERS
The device has been manufactured in the mixed 0.5µm CMOS process at the AMIS foundry, Oudenaarde, Belgium.
The 150 mm wafers have a custom made 100 µm thick epitaxial layer, composed of five 20 µm thick sub layers. Figure
1 shows a schematic cross section of the devices with the doping concentration of each layer.
light
n-type photodiodes
Epitaxial layers
of p-type Si
with increasing
concentration
p 1E14/cm3
3E14/cm3
1E15/cm3
3E15/cm3
1E16/cm3
Photoelectrons
1E18/cm3
Bulk wafer p-type Si
Figure 1. Schematic cross section of the multiple epi layer STAR250 devices.
photodiodes per pixel (see Figure 6).
*
bart.dierickx@fillfactory.com; phone +32 15 446340; www.fillfactory.com
This peculiar pixel has 4
This paper is a report of the most important electro-optical results, comparing them with the reference device, which is
the same STAR250 image sensor, processed in "standard" wafers. We try to match the observed results to 2D device
simulations.
The properties of the structure crystallize in two aspects:
• An enhanced spectral response in the near infrared. This is not only due to the thick epitaxial layer, but mainly
due to the faster collection time, which reduces the probability of recombination of a photo carrier on its way
to the collection junction.
• A moderate degradation of the MTF, at all wavelengths and mostly in the near infrared. The MTF degradation
- or loss of sharpness - however is significantly less important than would be seen on wafers with a single thick
epitaxial layer.
3. KEY MEASUREMENT RESULTS
3.1. Quantum efficiency
Figure 2 shows the measured and simulated quantum efficiency, including the fill factor and reflectivity, at different
wavelengths. The difference in QE at short wavelengths may be explained by the different cover glasses – it may also
be a side effect of the lower doped top layer in the multiple-epi device. In the range 800-1100 nm is the response is
considerably higher and only limited by the reflectivity and fill factor. The dotted lines are simulations of the
sensitivity that take the absorption of silicon into account but not the interferences in the dielectric layers.
0.6
0.5
QE x FF
0.4
Tmax = 100 µm
0.3
Tmax = 70 µm
0.2
pixel standard
0.1
0
400
pixel thick epi
500
600
700
800
900
1000
Wavelength [nm]
Figure 2. Spectral response of the STAR250 pixel with standard epi and the "thick" 5-layer multiple epi.
The dotted lines in the figure 2 are obtained with the following model.
In this model the effect of light absorption in silicon is mathematically described by:
I ( x) = I 0 exp(− λxa )
(1)
where x is the penetration depth of the photons in the silicon, λa the absorption length, I(x) the light intensity at position
x and I0 the incident light intensity (at x = 0). The absorption length is wavelength dependent as shown in Figure 3(a).
The model assumes that all photons absorbed at position tmin < x < tmax create electrons that diffuse, or drift under the
influence of the potential profile, to the photodiodes. Charges generated beyond position x > tmax are not collected and
only a fraction f of the electrons created at position x < tmin contribute to the quantum efficiency. The quantum
efficiency at wavelength λ is then modeled as:
t min
QE (λ ) = exp(− tλmina ) − exp(− tmax
λa ) + f [1 − exp( − λa )]
(2)
Figure 3(b) shows a schematic representation of this model.
I/I0
tmin
(a)
λ
tmax
tepi
x
(b)
Figure 3. Absorption length λa as function of the wavelength (a) and schematic representation of the used
model (b).
In our model, we may discriminate between tmax and the epi thickness tepi. The tmax would equal tepi in case the diffusion
length (or recombination length) of the charges is considerably larger than tepi. However, the electrons have a finite
lifetime τn. This lifetime is the result of the Shockley-Read-Hall recombination mechanism (in heavily doped silicon,
the lifetime is limited by Auger recombination) and is determined by the properties of the trap levels with respect to the
minority carriers (trap density, capture cross section and location in the bandgap). The parameter tmax is thus a very
rough way to account for the recombination limited diffusion length. We see that the measurements fit well with both
a tmax of 70 µm or 100 µm. Here and in the sequel, recombination will not be accounted for in the charge diffusion
models. This means that the simulated collected charges are overestimated, certainly for the cases that the diffusion
times are long.
The diffusion length Ln is related to the recombination life time τn as
Ln
=
Dn τ n
with
Dn
= µn
kT
q
(3)
The next paragraph will show that the diffusion length is anisotropical and maximal in the direction towards the
collecting surface due to the concentration gradient in the epi layers.
3.2. Effective pixel shape and MTF
The effective pixel shape (EPS) is obtained with the "knife-edge" method (Figure 4). Pixels size and pitch are 25 um,
with 4 diodes per pixel (Figure 6). A thick metal line at that position causes the "dip" in the right part of the pixel.
Compared to the reference pixel on "standard" material, we see moderate lateral charge diffusion.
100
600 nm
90
750 nm
Effective pixel shape [%]
80
800 nm
900 nm
70
950 nm
60
500 nm standard
50
40
30
20
10
0
-75
-62.5
-50
-37.5
-25
-12.5
0
12.5
25
37.5
50
62.5
75
x [um ]
Figure 4. Effective pixel shape (in X-direction) obtained by the knife-edge method with 1 µm step. The dip in the
profile is caused by a thick metal line running over the pixel at that position.
0.4
0.35
STAR250 standard device
0.3
0.25
0.15
0.1
0.05
0
500
MTFat Nyquist
0.2
STAR250 M-epi
Wavelength [nm]
550
600
650
700
750
800
850
900
950
Figure 5. calculated MTF at Nyquist of the effective pixels shapes of Figure 4
4. SIMULATION OF THE CHARGE DIFFUSION
It is not straightforward to simulate the MTF behavior, or effective pixel shape behaviour, of such a structure. We
simulated the minority charge diffusion as a finite element transient simulation on a grid of 100 x 400 points, where the
grid is on a 1 µm spacing.
As an initial condition, the grid is filled with a minority charge distribution, as would be originating from a light flash of
a certain wavelength (a certain absorption length) at a certain x-position. It is thus possible to introduce a needle-like
sheet of charge into the grid, which emulates an ideal point-like illumination. We let the simulator will then run for an
arbitrary amount of time; the charges will diffuse to one of the front side collecting photodiodes. At the end of the
simulation, the effective sensitivity for that illuminated position is then recorded as the number of electrons collected in
the "intended" photodiode, as referred to the charges that diffused towards the other photodiodes.
4.1 The diffusion equation
The diffusion current density for minority carriers Jn and majority carriers Jp is as follows [5].
dn
Jn = q.n.µn.E + q.Dn.dx
(4)
dp
Jp = q.p.µp.E – q.Dp. dx = 0
(5)
Here Jp is put to zero as we assume that the majority carriers are in thermal equilibrium. Even if Jp compensates Jn, it
will be negligible compared to the two other terms of equation (5). The electric field E is then be determined from (5)
as (6), and a simple closed-form for Jn is obtained as:
dp
Dp.
dx kT
dp
E = p.µ = q . p.dx
p
Jn = q. n . Dn .
dp
dn
+ q . Dn .
p.dx
dx
(6)
(7)
dJn
dp
dn
n = q. Dn . ( p.dx + n.dx )
(8)
dJn
dNA
dn
n = q. Dn . (NA.dx + n.dx )
(9)
dp
dNA
We approximated p.dx by N .dx : this is valid as long as the electric field layers are thin compared to the overall
A
structure. The electron diffusion current (9) thus equals zero where the relative gradients in electrons n and majority
dopants NA are equal but opposite.
Equation (9) is the base of the simulator's finite element algorithm. Per time step the diffusion from each grid point a
to each of it's neighboring grid points j is calculated as:
∆Qa = Σj(C.nj-na)×D/16
(10)
where na is the electrons concentration in grid point a, and nj are the electron concentrations in it's 4 neighboring grid
points, D is the diffusion constant normalized to the grid geometry and simulation time step, and C is a correction
coefficient, which is normally equal to 1, except at a homo junction, taking into account the equation (9).
In numbers: We assumed a bulk mobility µn = 1500 cm2/V.s at T=300K, hence kT/q = 25mV and Dn is 3.75 µm2/ns.
For a diffusion distance of 1 µm the diffusion time is 0.267 ns. The simulator time step is 16th part of the diffusion time
constant, hence 16.7 ps.
4.2 Simulated effective pixel shape
The simulator starts from the true Silicon structure of the pixel, but does not account for the opaque metal layers.
Figure 6 shows the pixel geometry with indication of the metal lines running over the array and the position of the 4
photodiodes in the pixel. The photodiodes are on a pitch of 12.5 µm. In each pixel, 4 diodes are interconnected to
achieve a relatively small photodiode capacitance, and thus a high conversion gain, while maintaining a good collection
efficiency and MTF.
Figure 6. STAR250 pixel geometry indicating the metal lines (dark grey structures) and 4 photodiodes (small
squares) at 12.5 µm pitch. In each pixel (large square), 4 diodes are interconnected.
effective pixel shape [a.u.]
100
10
simulation
1
measurement (knife edge)
x [um]
0.1
-75
-50
-25
0
25
Figure 7. Simulated and measured pixel profile at 900 nm (logarithmic y-axis).
50
75
effective pixel shape [%]
60
5_layers
1_layer
50
continuous
40
thin_epi
30
20
10
0
-100
-75
-50
-25
0
25
50
75
100
x [um]
Figure 8. Simulation of the effective pixel shape at 900 nm.
In figure 8 we compare by simulation the two realized devices: "5_layer" (with 5-layer 100 um thick epi layer) and
"thin_epi" (the standard device on a 5 µm thick single epi layer ), and two hypothetical devices: "1_layer" (a single 100
µm thick epi layer), and "continuous" (an infinitesimal number of epi layers, thus an continuous, exponential, gradient
of substrate concentration, with approximately the same overall concentration trend as in the 5-layer device). The
simulations are normalized for collected charge. Hence the "thin_epi" device has an overall lower response.
The above simulations prove that the 5-layer epitaxial device yields a significant improvement in sharpness over the
single thick epitaxial layer device. But as could be expected, a continuous graded concentration device would even
yield better results.
4.3 Further simulations and prediction
As we have the simulator engine available, we were interested to check a few other hypothetical devices.
An interesting experiment is to check what is the effect of the homo junction barrier height. This barrier height is a
direct function of the concentration ratio of both sides. We have run the simulation with the step of the concentration as
parameter. The real STAR250 devices consist of 5 layers of different concentration increasing with a factor of about 3.
The simulation in figure 9 proves that the multiple layer approach dramatically reduces the charge collection time. This
will have a side effect on the spectral response, as the probability for recombination drops too. As we have no reference
devices, not a usable model for the recombination in this material, we did not enter in hypothetical simulations of the
recombination effects.
2000
1800
1600
single layer
90% collection time [ns]
1400
1200
1000
800
600
the present 5-layer mulitple epi
400
200
concentration step factor
0
0
2
4
6
8
10
Figure 9. 90 % charge collection time as function of the concentration step factor. The thick epi devices
described in this paper have an epi layer consisting of 5 layers with step factor of about 3.
The above figure is quite interesting, as it shows that the time to collect (90% of) the charge is almost ten times longer
for a single thick epitaxial layer compared to 5 layers that take the same overall thickness, and have a factor 3
concentration ratio. Consider that the recombination lifetime in good material is in the order of a µs. It must be clear
that the multiple epitaxial structure helps to avoid recombination during the collection time.
5. CONCLUSIONS
The result obtained with this type of epitaxial wafers, i.c. a spectacular increase of the NIR response, is explained by the
model. The degradation of the MTF is, in view of the geometrical constraints, relatively small and tolerable.
The increase in spectral response is supposed to come from a thicker layer, but also to a faster collection time reducing
the probability of recombination.
The technique is promising to enhance sensitivity for NIR applications, as spectroscopy, automotive, space, security
cameras etc. Drawbacks are obviously the decrease in MTF, and the logistics and costs to manufacture these wafers.
ACKNOWLEDGMENTS
We acknowledge the contributions of the complete FillFactory staff, especially Danny Scheffer and Geert Van Cuyck.
The key results, the growth of the multiple epitaxial layers, of this study have been made possible by the staff of AMI
Semiconductor, Oudenaarde (Belgium), in particular Ziad Hocin and Paul Colson.
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2.
3.
4.
5.
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