Active solid-state devices (e.g. – transistors – solid-state diode – Responsive to non-electrical signal – Electromagnetic or particle radiation
Reexamination Certificate
1999-11-23
2003-04-15
Chaudhuri, Olik (Department: 2814)
Active solid-state devices (e.g., transistors, solid-state diode
Responsive to non-electrical signal
Electromagnetic or particle radiation
Reexamination Certificate
active
06548878
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed toward an improved manufacturing process for a producing a high speed PN photodiode having a distributed photodiode structure and the resulting photodiode.
2. Description of the Related Art
Photodiodes are diodes in which charge carriers are generated responsive to light incident upon the photodiode. Any PN junction diode which admits light can function as a photodiode. A photodiode outputs voltage or current when absorbing light. In a photodiode which is intended for high speed communication systems, it is important to optimize the performance for light conversion efficiency, speed (minimal transit time delay), minimum RC time constant, ability to operate at low reverse bias voltage, and cost in the application in which the photodiode will be employed.
The structure of a conventional discrete PIN photodiode
24
is illustrated in
FIG. 1B. A
wafer
50
is lightly doped with N dopant in order to produce an intrinsic region
56
. A P+ region
52
is formed on one surface of the wafer and an N+ region
58
is formed on the opposing surface of wafer
50
with intrinsic region
56
interposed P+ region
52
and N+ region
58
. A reflective layer
60
, typically gold, is disposed on the surface containing P+ region
58
with reflective layer
60
also serving as the electrical contact to N+ region
58
. A metal contact
54
is disposed on the surface containing P+ region
52
to provide the electrical connection to the P+ region.
Typically, one power supply potential is applied to the reflective layer
60
and another power supply voltage is applied to contact
54
to reverse bias the PN junction formed by P+ region
52
and N+ region
18
. This forms a large depletion region within the intrinsic region
56
wherein electron and hole charge carrier pairs generated by light photons incident upon the intrinsic region
56
are rapidly accelerated toward the P+ and N+ regions respectively by the electric field of the reverse bias voltage. Charge carrier pairs are also typically generated outside the depletion region within intrinsic region
56
which diffuse, due to random thermal motion of the carriers, at a much slower velocity until they reach either the depletion region or the junction formed by P+ region
52
and intrinsic region
56
of photodiode
24
.
A conventional photodiode that is designed for high quantum, i.e. light conversion, efficiency requires that the light path within the photo current collection zone, i.e. the depletion and non-depletion zones within intrinsic region
56
, be sufficient in length so that most of the light photons of the incident light signal area are absorbed and converted into electron-hole pairs that are collectable at the P+ and N+ regions. Usually, this requires that the width of the intrinsic region
56
, which is the primary light collection region, be several times the length required for light absorption. If diode
10
has an efficient back-side reflector, such as reflective layer
60
, which effectively doubles the light path within diode
24
, then the intrinsic region
56
of the photodiode can be made narrower. For a typical near infrared silicon photodiode, the nominal absorption path length is about 15-25 microns. The path length should be at least two to three times the nominal absorption path length to obtain good light conversion efficiency.
Wafer
50
can be lapped to as thin as 100 microns in order to obtain a thinner intrinsic region
56
and better performance for the resulting PIN photodiode. However, it is generally not practical to thin wafers beyond this limit without an excessive level of wafer breakage along with severe wafer handling and processing problems. As noted above, however, the width of intrinsic region
56
that is optimal for the performance of the photodiode can be as little as 30 microns.
A photodiode designed for high frequency response requires that the photo current pairs generated by the light signal be collected rapidly and that the diode RC time constant is fast. Rapid photo current pair collection usually requires that most of the photo current pairs generated by the light signal be generated with the depletion region formed by the reverse bias voltage because the pairs will have a high drift velocity. Otherwise, the photo generated charge carrier pairs produced in the non-depletion regions within intrinsic region
56
and within diffusion distance of the collection electrodes
52
and
58
will have a diffusion velocity that is several hundred times slower than the velocity of the pairs generated within the depletion zone. The photo generated charge carrier pairs in the non-depletion zones will slowly migrate for collection at P+ region
52
and N+ region
58
resulting in a tall on the trailing edge of the electrical signal corresponding to the light signal. The diffusion distance of the charge carriers is determined by the carrier mean free path before re-combination and may exceed 150 microns.
A fast RC time constant for photodiode
24
requires minimal capacitance and low series resistance between the electrical contacts
54
and
60
and the photo current pair collection sites at the margin between P+ region
52
and the depletion zone and the margin between N+ region
58
and the depletion zone. The greater the width of the intrinsic region
56
, the greater the width of the depletion zone and the lower the capacitance per unit area of photodiode
24
. Since the width of the depletion zone increases with the magnitude of the reverse bias voltage, it is typical for high speed photodiodes to have a relatively high reverse voltage applied to them.
The inclusion of lightly doped intrinsic region
56
between the P+ and N+ regions
52
and
58
results in a PIN photodiode with a wider depletion region, depending on the magnitude of the reverse bias voltage, which improves the light collection efficiency, increases speed, and reduces capacitance over that of a simple PN diode structure.
The PIN photodiode is typically produced by diffusing the N+ region
58
on the back side of the lightly doped (N) wafer
50
, diffusing the P+ region
52
on the topside of the wafer
50
, and then adding metal contacts to each side of the wafer. Typically, the backside contact area connected to N+ region
58
is reflective layer
60
and is made of gold. The reflective layer is then typically connected to the ground voltage terminal.
Although a PIN photodiode outperforms a standard PN diode, the PIN photodiode structure cannot be easily manufactured by standard semiconductor processes wherein fabrication is typically performed on only one side of the semiconductor wafer
50
. In typical high volume applications, it is now standard practice to fabricate the receiver circuitry and transmitter driver in a single integrated circuit (IC) to produce a transceiver IC. As described above, it is difficult to integrate an efficient photodiode on the same semiconductor substrate as the transceiver circuit. As a result, a discrete infrared photodiode is typically assembled with the transceiver circuit and an LED, along with lenses for the photodiode and LED, into a plastic molded package to form a transceiver module. The transceiver module is designed to be small in size and allow placement in the incorporating electronic device so as to have a wide angle of view, typically through an infrared window on the transceiver casing. The transceiver IC is designed to digitally interface to some type of serial data communications device such as an Infrared Communication Controller (ICC), UART, USART, or a microprocessor performing the same function.
As noted above, any PN junction diode which admits light can function as a photodiode. A PN diode junction can also be fabricated using standard IC processes. However, the photo-current collection region within an electric field, the drift region, in a PN photodiode is limited to the
Holcombe Wayne T.
Irissou Pierre
Nauleau Jean-Luc
Francissen Vernon W.
Gardner Carton & Douglas
Integration Associates Inc.
Wille Douglas A.
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