Charge coupled device with buried zones in a semiconductor substrate for use especially as a light sensor

A CCD is disclosed having a semiconductor substrate of a first conductivity type with a plurality of electrodes serially located above one planar surface thereof, a plurality of buried doped zones of a conductivity type opposite to that of the substrate and located in a plane spaced below a surface of the substrate. The rear edge of each electrode is in line with the front edge of a buried doped zone. The front edge of the same electrode overlaps the rear end of the next succeeding buried doped zone. The upper front corner of each electrode is bevelled.

BACKGROUND OF THE INVENTION 
1. Field of the Invention: 
The invention relates to a charge-coupled device, and more particularly to 
a CCD which is especially suited for use as a light sensor; and to a 
method of making the same. 
2. Description of the Prior Art: 
Charge coupled elements are known components in semiconductor technology 
(see, for example, Siemens Forschungs- and Entwick-lungsberichte, Vol. 4, 
1975, pages 226 ff; German OS No. 2,201,150). It is further known that 
charge coupled elements can be used as light sensors (see, for example, 
"IBM Tech. Disc. Bull.", Vol. 16, No. 1, June 1973, pages 173--174 and 
"Bell System Technical J1.", Oct. 1972, pages 1923-1926). CCD circuits of 
this type fundamentally consist of a series of MIS capacitors, each of 
which MIS-capacitors are constructed in such manner that on the surface of 
a semiconductor substrate there is applied an electrical insulating layer 
which bears an electrode. CCD-components of this type can be used, for 
example, as a store or as radiation sensors for light. At least in each 
MIS-capacitor, a radiation-transmissive point must be provided in the 
electrode through which radiation can penetrate into the substrate. The 
radiation produces change carries in the substrate. When an appropriate 
voltage is connected between the substrate terminal of the element and the 
relevant electrode of a MIS-capacitor, these charge carriers can be 
accumulated in this capacitor in the substrate beneath the capacitor 
electrode. When sensors of this kind are used to record very weak 
radiation intensities, it proves disadvantageous that the radiation which, 
in the sensor, produces the pairs of charge carriers required for 
recording, must pass through electrodes of such a MIS-capacitor and 
through the insulating layer into the semiconductor, as both in the 
electrode and the insulating layer a part of the radiation is already 
absorbed and thus is no longer available for the production of charge 
carriers in the semiconductor substrate. 
The aim of the invention is to provide a CCD which can be used as a charge 
store or as a radiation sensor (e.g., for light or ionizing radiation) and 
which can be used even with weak radiation intensities. As a result of its 
design, it obviates the need for radiation which forms the pairs of charge 
carriers to pass through one of the electrodes of this CCD. 
BRIEF SUMMARY OF THE INVENTION 
The present invention is based on the following considerations: the buried, 
doped zones which lie below the gaps of the electrode and which possess a 
doping which is opposite to that of the substrate can serve to store a 
charge if, in the case of a p-substrate and n-doped buried zones, the 
electrodes are connected to a negative potential relative to the doped 
zones so that the potential barrier which is thus formed prevents an 
exchange of charge between the individual buried, doped zones. This 
negative potential can be achieved either by means of an external voltage 
source or by means of contact potential differences in the case of 
Schottky electrodes. The one-sided overlap of the electrodes with the 
interlying, buried doped zones produces an asymmetry of the arrangement 
which, for example when a positive voltage is connected to one of the 
electrodes, means that the charge sored in a buried, doped zone of this 
type can only flow in one direction into the adjacent, buried, doped zone. 
As a result, this component can be operated as a CCD. 
A preferred use of this CCD in accordance with the invention consists in 
use as light sensor or sensor for other radiations, as the active storage 
zones which are formed by the buried, doped zones are not covered by an 
absorbant electrode. Furthermore, the insulating layer can be removed in 
the intermediate zone between the electrodes; in the case of Schottky 
electrodes it can also be entirely absent. A further advantage of this CCD 
component in accordance with the invention consists in that its production 
requires only one single mask step, and that therefore the mask adjustment 
processes which normally require a high outlay can be omitted from the 
production. 
In the following, the invention will be described and explained in detail 
making reference to preferred embodiments which are illustrated in the 
Figures.

DESCRIPTION OF PREFERRED EMBODIMENTS 
The production of a CCD in accordance with the invention in a MOS-technique 
will serve as an example. On a semiconductor substrate 1, for example a 
silicon substrate which is p-doped, with a carrier concentration of 
10.sup.14 cm.sup.-3, there is deposited a silicon dioxide layer 2 having a 
thickness of approximately 120 nm. An electrode layer 13, for example an 
aluminum layer, is deposited onto this silicon dioxide layer 2. Using a 
photolithographic technique, the electrodes 3 of the CCD are then etched 
out of this aluminum layer. The electrodes 3 have a spacing between one 
another of as little as 1 to 2 .mu.m. Then the doped, buried zones 4 are 
produced. These zones 4 are arranged at a depth of between approximately 
50 nm and 1 .mu.m beneath the substrate surface. This is effected by 
employing ion implantation, e.g., phosphorus ions 5 which are injected at 
an acceleration voltage of between 30 and 800 keV and in an irradiation 
dose of more than approximately 10.sup.12 cm.sup.-2 into the substrate. 
The doping of these buried zones 4 is "self-adjusting" as the electrodes 3 
of the CCD which remain upon the insulating layer serve as an implantation 
mask. As the implantation profile in the semiconductor substrate is not 
exactly delimited by the shadow zone of the electrodes 3 arranged on the 
insulating layer, but also, as a result of the deceleration occurring in 
the substrate of the implanted ions, also has a lateral extent and 
therefore extends into the shadow zones of the masks, relative to the 
surface normal of the substrate 1, an overlap occurs between the buried, 
doped zones 4 and the electrodes 3. This can also occur as a result of 
diffusion phenomena during the heating process which is necessary in order 
to activate the implanted particles serving to drive the doping profile 
even further into the shadow zone of the mask 3. The extent of the overlap 
zone is preferably in the order of the distance of the buried zones 4 from 
the substrate surface; in the quoted example it amounts to between 
approximately 50 nm and 1 .mu.m. In the next stage of the production 
process, an ion etching is carried out on the electrodes 3 arranged upon 
the insulating layer. This ion etching is carried out with ions 15 
delivered obliquely, so that the rear, edge zone 33 of the electrodes 
illustrated in broken lines in FIG. 2 is removed. The front edge zone 34 
of these electrodes is bevelled in wedge shape (FIG. 2). As a result of 
this ion etching process, the component acquires the structure shown in 
FIG. 3 in which the electrodes 3 and the buried, doped zones 4 overlap 
only on one side. 
FIG. 4 illustrates an alternative embodiment of the CCD in accordance with 
the invention, wherein Schottky electrodes are arranged on the substrate 
surface. In this case no insulating layer is present on the substrate 
surface 11 so that the light can enter the semiconductor substrate 
completely unobstructed. 
Making reference to FIGS. 5 and 5A, the fundamental mode of operation of 
the CCD in accordance with the invention will now be explained. FIG. 5A 
schematically illustrates the structure of the CCD element in accordance 
with the invention, with the electrodes 300, 301 and 302 and with the 
buried, doped zones 400 and 401 arranged in the substrate. FIG. 5A 
schematically illustrates the course of the potential U for electrons 
present in the substrate, in two storage states. The line 308 represents a 
potential course for a state in which each of the electrodes 300, 301 and 
302 is connected to a potential which is negative relative to the buried, 
doped zones 400 and 401. Radiation or incident light between the 
electrodes 300 and 301 serves to produce pairs of charge carriers, the 
electrons 500 of these pairs of charge carriers gathering in the n-doped 
zone 400, whereas the positive holes flow away to the substrate 100. The 
line 309 in FIG. 5 illustrates a potential course for a charge shift 
process. For this purpose the negative potential connected to the 
electrodes 300 and 302 is maintained and the electrode 301 is connected 
not to a negative potential but to a potential which is positive in such a 
manner that the potential barrier beneath the electrode 301 is broken 
down. As a result of the overlap zone between the electrode 301 and the 
buried doped zone 401, an additional potential well 501 is formed. This 
additional potential well serves to accumulate the charge carriers. If, in 
the next step of the operating process, the electrode 301 is reconnected 
to the negative potential, the electrons present in the potential well 501 
can no longer flow back into the potential well between the electrodes 300 
and 301. 
During the operation of the CCD in accordance with the invention, those 
elements of the CCD which enable the read-out and forwarding of the 
information governed by the value of the shifted charge, can be protected 
from the radiation which is to be identified by means of screening. This 
ensures that during the further transportation of the charge produced by 
the radiation out of a specific zone of the sensor, this quantity of 
charge is not changed by further formation of charge carrier pairs 
produced in the read-out section as a result of radiation. 
It will be apparent to those skilled in the art that many modifications and 
variations may be effected without departing from the spirit and scope of 
the novel concepts of the present invention.