Image sensor forming method capable of defining connection...

Details Image sensor forming method capable of defining connection... Image sensor forming method capable of defining connection...

2001-08-28

2002-10-29

Chaudhari, Chandra (Department: 2813)

Semiconductor device manufacturing: process

Making device or circuit responsive to nonelectrical signal

Responsive to electromagnetic radiation

C438S200000, C438S444000

Reexamination Certificate

active

06472245

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates generally to an image sensor forming method and, more particularly, to an image sensor forming method capable of defining a connection window structure by etching a field oxide.
DESCRIPTION OF THE PRIOR ART
An image sensor is an apparatus for converting optical information of one or two dimensions, to an electrical signal. Image sensors may be provided with a pick up tube and a solid state imaging device, for example. The pick up tube has been developed and applied in areas of measurement, control and recognition that employ image processing techniques and can be similar to what is used in the television industry. There are two kinds of devices commercially available as the solid state imaging device, MOS-based (Metal-Oxide-Semiconductor) detectors and CCDs (charge coupled devices).
The CMOS image sensor is a device for converting an optical image to an electrical signal, by using a CMOS manufacturing technique. CMOS image sensors employ switching schemes for detecting sequential outputs by using numerous MOS transistors per unit pixel. The CMOS image sensor is simpler to drive and more versatile in scanning than the widely used conventional CCD image sensor. Furthermore, the CMOS image sensor can integrate a signal processing circuit on a single chip, resulting in a smaller product of lower cost and reduced power consumption.
FIG. 1
is a circuit diagram showing schematically a unit pixel detection structure of a conventional CMOS image sensor, in which the unit pixel comprises a photo diode PD for photo sensing and 4 NMOS transistors. A transfer transistor Tx transports photo charge to a floating diffusion region; a reset transistor Rx draws out the charge at the floating diffusion region for signal detection; a drive transistor Dx acts as a source follower; and a select transistor Sx is for switching and addressing. In
FIG. 1
, Cf represents the capacitance of the floating diffusion region and Cp represents the capacitance of the photo diode.
The operation of the image sensor unit pixel above will now be described. At first, during unit pixel reset, the reset transistor Rx, the transfer transistor Tx and the select transistor Sx are turned on. At this time, the photo diode PD starts to be depleted so that carrier charging occurs at the capacitance Cp and carrier charging builds up to a voltage VDD at the capacitance Cf in the floating diffusion region. After turning off the transfer transistor Tx, the reset transistor Rx is turned off, while the select transistor Sx is left on. During this operation after an output voltage V
1
from the unit pixel output Out is read and buffered, the carriers of the capacitance Cp, are transported to the capacitance Cf by turning on the transfer transistor Tx. After another output voltage V
2
from the output Out is read, an analog data value corresponding to V
1
-V
2
is converted to a digital data value.
FIGS. 2A
to
2
D show processing sectional diagrams for a conventional method of forming CMOS image sensors. Hereinafter, problems associated with the conventional CMOS image sensor manufacturing method will be described, with reference to these figures.
As shown in
FIG. 2A
, a field oxide
21
is formed on a p-type silicon substrate
20
and a gate insulation film
22
and a gate electrode
23
of the transfer transistor are additionally formed, as shown.
Next, as shown in
FIG. 2B
, a first photo resist pattern PR
1
is formed for forming an n-type impurity ion injection region of the photo diode PD into the p-type silicon substrate
20
. The incident n-type dopant, or impurity, is shown by arrows.
Next, as shown in
FIG. 2C
, after oxide film spacers
25
are formed on side walls of the gate electrode
23
, such as by eliminating the first photo resist pattern PR
1
and etching an oxide film layer placed on the p-type silicon substrate
20
, a second photo resist pattern PR
2
for exposing the p-type silicon substrate
20
and the previously formed n-type silicon substrate
24
is formed. A p-type impurity is then injected.
From this p-type impurity ion injection processing, as shown in
FIG. 2D
, a first p-type impurity ion injection region
26
is formed with a top side exposed on the upper surface of the p-type silicon substrate
20
and a bottom side in contact with the n-type impurity ion injection region
24
is formed. This p-type region
26
constructs the photo diode along with the n-type impurity ion injection region
24
. A second p-type impurity ion injection region
27
electrically connecting the p-type impurity ion injection region
26
and the p-type silicon substrate
20
is formed by ion-injecting p-type impurity into the p-type silicon substrate
20
between the leftmost field oxide
21
and the n-type impurity ion injection region
24
.
The second p-type impurity ion injection region
27
serves as a connection window for matching the potentials of the first p-type impurity ion injection region
26
and the p-type silicon substrate
20
.
In order to form the second p-type impurity ion injection region
27
, the n-type impurity ion injection region
24
and the leftmost field oxide
21
of the photo diode should be spaced by a width equal to that of the second impurity ion injection region
27
. This detrimentally results in a smaller n-type impurity ion injection region
24
, which leads to reduced charge capacity of the photo diode and a reduced saturation level of the CMOS image sensor.
Additionally, during formation of the n-type impurity ion injection region
24
, if the first photo resist pattern PR
1
is mis-aligned, then the width of the second p-type impurity ion injection region
27
is affected. In fact the second p-type impurity ion injection region
27
may not be formed at all depending on the mis-alignment, in which case the first p-type impurity ion injection region
26
and the n-type impurity ion injection region
24
would not operate as photo diode, but only as a simple p-n junction.
SUMMARY OF THE INVENTION
In accordance with an aspect of the present disclosure, there is provided a method for manufacturing an image sensor including a photo diode having: (a) a first region formed with a first dopant type, the first region being formed within a semiconductor substrate having a second dopant type, and (b) a second region formed with the second dopant type, whereby a top side of the second region is exposed on the surface of the semiconductor substrate, the method comprising the steps of; forming a field oxide on the semiconductor substrate; etching selectively the field oxide in contact with the photo diode to create an exposed portion between the etched field oxide and the photo diode region; and forming a connection window region for connecting the second region of the photo diode to the semiconductor substrate by ion-injecting the second dopant into the semiconductor substrate defined by the exposed portion.
In accordance with another aspect of the present disclosure, there is provided a method for manufacturing an image sensor including a photo diode for photo sensing and a transfer transistor for transferring photo charge from the photo diode to a signal processing region, the method comprising the steps of: (a) forming a field oxide on a semiconductor substrate having a first dopant; (b) forming a first region of the photo diode by ion-injecting a second dopant selectively into the semiconductor substrate between the field oxide and the transfer transistor; (c) etching selectively the field oxide in contact with the photo diode to expose a portion of the semiconductor substrate between the etched field oxide and the photo diode region; (d) forming the second region of the photo diode by ion-injecting the first dopant into the first region of the photo diode, such that an upper surface of a top side of the second region is exposed on the semiconductor substrate and a bottom side of the second region is in contact with the first region of the photo diode; and (e) forming a connection window region connecting the second region of the photo

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