GB2132818A

GB2132818A - Imaging array

Info

Publication number: GB2132818A
Application number: GB08334173A
Authority: GB
Inventors: Eugene Diedrich Savoye; Walter Frank Kosonocky; Lloyd Franklin Wallace
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1983-01-03
Filing date: 1983-12-22
Publication date: 1984-07-11
Also published as: FR2538952B1; JPS59135763A; GB8334173D0; JPH0512866B2; GB2132818B; FR2538952A1; DE3347449C2; DE3347449A1

Abstract

An imaging array (10) of the charge-transfer type having improved sensitivity includes a semiconductor body (12) having a plurality of substantially parallel charge-transfer channels (20) with channel stops (22) therebetween which extend a distance into the semiconductor body (12). At least some of the channel stops (22) have blooming drains (26) therein for the removal of excess photogenerated charge. Buried barrier regions (28) stop the floor of electrical charge, generated by absorption of light in the body (12), into the channels (20) while preventing the loss of such charge by direct flow into the blooming drains (26). The buried barrier regions (28) extend a further distance into the body (12) from those channel stops (22) having blooming drains (26) therein and containing a greater concentration of conductivity modifiers than that contained in the channel stops. <IMAGE>

Description

SPECIFICATION Imaging array having higher sensitivity and a method of making same This invention pertains to an imaging array of the charge-transfer type exhibiting increased sensitivity to light, and a method of making the array.

Imaging arrays of the charge-transfer type, such as the charge-coupled device disclosed in U.S. Patent No. 4,362,575 issued to L.F. Wallace on 7 December 1982 and assigned to RCA Corporation, store lightgenerated electrical charge and transfer such charge to a charge detector for detection and display. The array generally includes a semiconductor body of a first conductivity type having a plurality of substantially parallel charge-transfer channels of opposite conductivity type extending a distance into the body from a first major surface thereof. The chargetransfer channels are isolated from one another by channel stops which provide potential barriers to the flow of stored charge between adjacent channels.

Each channel stop is generally composed of a region which extends a distance into the body from the first major surface between a pair of charge-transfer channels, and which has the same type conductivity as the body but has a higher concentration of conductivity modifiers. A dielectric insulator overlies the first major surface, and a plurality of substantially parallel electrodes overlie the dielectric layer and extend in a direction transverse to the direction of the charge-transfer channels.

Application of a voltage of the proper polarity to a particular electrode will cause light-generated electrical charge to accummulate in potential wells in the charge-transfer channels under the electrode. The sequential application of voltages to successive electrodes over a channel will cause the charge to be transferred to the charge detector.

If the amount of charge generated in a particular portion of the array is in excess of the amount which can be stored in the potential well formed under a particular electrode, the excess charge will spread into adjacent potential wells along the chargetransfer channel. This spreading of the charge causes an increase in the size of the brighest portions of an image and is known as blooming. A solution to the blooming problem is to reduce the concentration of conductivity modifiers in the channel stops and to form blooming drains in the channel stops which have the opposite type conductivity to that of the semiconductor body.The lighter doping of the channel stops reduces the height of the potential barrier between the potential wells in the channels and the blooming drains, so that the excess charge preferentially flows over the barrier and into the blooming drains rather than over the higher barriers along the charge-transfer channels. The drains signficantly reduce the blooming but at the price of also reducing the light sensitivity of the array, since a portion of the charge generated in the semiconductor body, rather than flowing to the potential wells in the channels, flows directly into the drains and is lost. Thus, it would be desirable to have an imaging array having blooming control but without the corresponding loss of sensitivity.

The invention is an imaging array of the chargetransfer type having a high sensitivity to incident light, where the improvement comprises buried barrier regions for preventing the direct flow of photogenerated charge into a blooming drain. The buried barrier regions are of the same type conductivity as the semiconductor body, but contain a greater concentration of conductivity modifiers, and extend a further distance into the body from those channel stops having a blooming drain therein. The concentration of conductivity modifiers is sufficient to form a potential barrier to the direct flow of charge, generated in the bulk of the semiconductor body, into the blooming drains, thereby increasing the quantum efficiency of the array.

The invention also includes a method of forming an array having a higher quantum efficiency wherein the improvement comprises forming buried barrier regions containing a greater concentration of the same type conductivity modifiers as that contained in the body and extending a further distance into the semiconductor body under the blooming drains.

In the drawing: Figure 1 is a perspective cutaway view of a portion of an imaging array incorporating the present invention.

Figure 2 and 3 are cross-sectional views of portions of two different embodiments of the imaging array of the present invention.

Figure 1 shows a portion of an imaging array 10 of the charge-coupled type. The array 10 includes a semiconductor body 12 having first and second major surfaces 14 and 16, respectively. The semiconductor body 12 is composed of a semiconductor material, such as silicon, having a first type conductivity. A plurality of substantially parallel chargetransfer channels 20 of the opposite type conductivity extend a distance into the body 12 from the first major surface 14. Channel stops 22 extend a distance into the body 12 from the surface 14 between the charge-transfer channels 20. The channel stops 22 comprise channel barrier regions 24, each having a blooming drain 26 extending a distance from the major surface 14.The channel barrier regions 24 contain a greater concentration of first type conductivity modifiers than the body 12 and have a slight excess net concentration of either first or second type conductivity modifiers. The blooming drains 26 contain a considerably greater concentration of second type conductivity modifiers than do the channels 20. Buried barrier regions 28 extend a further distance into the body 12 below the blooming drains 26 and contain a greater concentration of first type conductivity modifiers than do either the body 12 or the channel barrier regions 24. A backside-surface barrier region 30 extends a distance into the body 12 from the second major surface 1.6, and contains a much greater concentration of first type conductivity modifiers than does the body 12 and is typically less than 100 nanometers (nm) thick.

An electrically insulating layer 32, typically a thermally grown silicon dioxide (channel oxide), overlies the first major surface 14 of the body 12. A plurality of channel electrodes 34, which are sub stantially perpendicular to the charge-transfer channels 20, overlie the electrically insulating layer 32.

The channel electrodes 34 are spaced apart from one another by interelectrode insulators 36.

Figure 1 shows a three-phase arrangement for the charge transfer in which every third channel electrode 34 is connected to the same voltage signal. It is to be understood that the invention is also applicable to any other arrangement for the transfer of photogenerated charge, such as a two-phase transfer system, which includes a blooming drain to control excess charge.

It was discovered that the flow of photogenerated electrical charge directly to the blooming drains 28 causes a significant reduction in the amount of charge flowing into the potential wells in the chargetransfer channels 20, thereby reducing the quantum efficiency of the light-sensing array 10. The buried barrier regions 28 form a potential barrier to the flow of photogenerated charge directly into the blooming drains, thereby, constraining such charge to flow into the channels 20 from whence it can be detected.

The presence of the buried barrier regions 28 results in about a twofold improvement in the quantum efficiency. The presence of the buried barrier regions 28 does not prevent the flow of excess charge from a channel 20 into a blooming drain 26 and, therefore, blooming control is preserved.

In the remaining Figures, the identification of elements common to those Figures and to Figure 1 is the same.

The light-sensing array 50 of Figure 2 differs from the light-sensing array 10 to Figure 1 in that the buried barrier regions 52 extend a further distance into the body from the channel stops 22 over the full width of the channel stops 22.

In a light-sensing array without buried barrier regions 28, it is desirable that blooming drains 26 be positioned in the channel stops 22 on both sides of a charge-transfer channel 20so that the loss of charge by direct flow to the blooming drains 26 is symmetrical. Since the presence of a buried barrier region 28 adjacent to a blooming drain 26 prevents this loss of charge, the need for symmetrical blooming drains 26 about a channel 20 is eliminated. Thus, adjacent channels 20 can share a common blooming drain 26, with the blooming drains 26 for alternate channels 20 being eliminated. This should result in an increase in the manufacturing yield of useable devices since roughly one-ha!f of the blooming drains 26 are eliminated.This principle is illustrated in Figure 3 where the light sensing array 60 differs from the light-sensing array 50 of Figure 2 in that alternate channel stops 22 are replaced by channel stops 62 which consist of only the channel barrier regions 24 of the channel stops 22. The channel stops 62 are regions extending a distance into the body 12 from the first major surface 14 between charge-transfer channels 20 and which contain a greater concentration of first type conductivity modifiers than the body 12, and contain a slight excess concentration of either type conductivity modifiers. Only those channel stops 22 having blooming drains 26 therein require the presence of a buried barrier region 52.

The substrate 12 typically has P- type conductivity and contains a concentration of uncompensated P type conductivity modifiers of between about 1.0 and 15 x 1054/cm3 corresponding, respectively, to a bulk resistivity between about 150 and 10 Q-cm. The channels 20 have N type conductivity and contain a concentration of uncompensated N type conductivity modifiers between about 1 and 10 1016/cm3. The blooming drains 26 have N+ type conductivity and typically contain a concentration of uncompensated N+ type conductivity modifiers between about 1018 and 1025/cm3. The channel barrier regions 24 typically contain between about 1 and 5 x 015/cm3 P type conductivity modifiers.The channel barrier regions 24 also typically contain N type conductivity modifiers in about the same concentration as the channels 20, since th N type implant or diffusion used to form the channels 20 is done uniformly over the area of the channels 20 and channel stops 22. This results in a lightly N- type conducting portion of the channel barrier regions 24 adjacent to the major surface 14 of the body 12. The concentration of conductivity modifiers in the channel barrier regions 24 should be such that the magnitude of the potential barrier between the channel 20 and the blooming drain 26 causes excess charge preferentially to flow into the blooming drain 26 rather than along the channel 20.

The buried barrier regions 28 typically contain between about five and five hundred times greater, and preferably about one hundred times greater, concentration of P type conductivity modifiers than the body 12. Preferably, the concentration of P type conductivity modifiers is greater than 2.5 x 10'7/cm3.

The concentration of conductivity modifiers in the buried barrier regions 28 must be sufficient to provide a barrier to charge flow directly from the body 12 to the blooming drains 26. The concentration must also be great enough to prevent the depletion region generated by application of a bias voltage to the blooming drains 26 from reaching through the buried barrier region 28 to the body 12.

The imaging arrays 10 of the invention may be fabricated using the self-aligned techniques disclosed in U.S. Patent No. 4,362,575 issued to L.F.

Wallace on 7 December 1982 and assigned to RCA Corporation, referred to above. A thermal oxide about 500 nanometers (nm) thick is grown on a Ptype silicon substrate 12 having a resistivity between about 10 and 150 Q-cm. The thermal oxide is covered with a layer of photoresist and the blooming drain pattern is defined therein. The photoresist and the thermal oxide are then removed in the defined areas, and the silicon is doped through the openings in the oxide by ion implantation of arsenic to a dose of 3.6 x 1014/cm2 at a beam energy of 200 keV to form the blooming drains 26. The silicon body 12 is then subjected to a "deep" boron implant to a dose of 7 x 1012/cm2 at 400 keV to form the buried barrier regions 28. The openings in the oxide are then enlarged by etching back the oxide under the photoresist by about 3 micrometers (plum) to each side of the original opening. The photoresist is then removed and a "shallow" boron implant to a dose of 7 x 10"/cm2 at 100 keV is carried out. At this point in the process, the P- type semiconductor body 12 contains the heavily doped N + type blooming drains 26 extending a distance into the body 12 of about 150 nm and surrounded by a moderately doped P type region 24 extending about 330 nm into the body from the surface 14 and about 3 Fm laterally along the surface 14 from the N+ type blooming drains 26.

The buried barrier regions 28, formed by a "deep" boron implant, extend a further distance of about 1000 nm into the body 12 from the moderately doped P type channel barrier regions 24 and contain about a factor of ten greater concentration of acceptors than does the moderately doped channel barrier regions 24.

After the arsenic and boron implants are completed, the thermal oxide is stripped from the surface 14, a capping oxide is formed over the entire surface 14, and phosphorus in implanted to a dose of 2.0 x 1012/cm2 at 175 keV in the area if the charge-transfer channels 20 and channel stops 22 to form the channels 20. This implant, since the phosphorus is also implanted into the barrier regions 24 of the channel stops 22, compensates this region 24 leaving it lightly P- or N- type conducting.

After this step is completed, additional processing steps well known in the art are carried out to fabricate the remainder of the array 10 including the electrically insulating layer 32 and the channel electrodes 34, The array 50 of Figure 2 is fabricated by performing the "deep" boron implant after the openings in the oxide layer are enlarged and either before, after or simultaneous with the "shallow" boron implant.

The array 60 of Figure 3 may be fabricated by forming the channel stops 62 after the arsenic and "deep" boron implants have been completed.

While the fabrication process has been described in terms of particular type conductivity modifers, it is to be understood that other elements which have the same effect may be used. It is also to be understood that the conductivity types of the different regions may be reversed so long as the relationship in the conductivity types of the body and the different regions is maintained.

Claims

1. In an imaging array of the charge-transfer type comprising a semiconductor body of a first type conductivity having first and second major surfaces and including a plurality of substantially parallel charge-transfer channels comprising regions of a second type conductivity extending a distance into the body from the first major surface thereof and channel stops comprising channel barrier regions extending a distance into said body from the first major surface between the charge-transfer channels, at least some of said channel stops having blooming drains therein comprising regions of said second type conductivity extending a distance from the first major surface into said channel barrier regions, the improvement comprising buried barrier regions extending a further distance into the body from the blooming drains for preventing the direct flow of photogenerated charge into said blooming drains, said buried barrier regions containing a greater concentration of first type conductivity modifiers than the channel stops.

2. The array of Claim 1 wherein each channel stop has a blooming drain therein.

3. The array of Claim 1 wherein adjacent chargetransfer channels share a common channel stop with a blooming drain therein.

4. The array of Claim 1 wherein the semiconductor body has P type conductivity, the charge-transfer channels and the blooming drains have N type conductivity, and the channel stops and the buried barrier regions have P type conductivity.

5. The array of Claim 4 wherein those channel stops having blooming drains, have an N-type conducting portion therein disposed adjacent to the first major surface.

6. The array of Claim 1 wherein the buried barrier regions extend a distance into the body from that portion of the channel stops beneath the blooming drains.

7. The array of Claim 1 wherein the buried barrier regions extend a further distance into the body from the channel stops over the full width of the channel stops.

8. The array of Claim 1 wherein the buried barrier regions contain an excess concentration of P type conductivity modifiers which is between about five and fifty times greater than the excess concentration of P type conductivity modifiers in the channel stops.

9. The array of Claim 1 wherein the buried barrier regions contain an excess concentration of P type conductivity modifiers which is between about five and five hundred times greater than the exess concentration of P type conductivity modifiers in the body.

10. in a method of fabricating an imaging array in a semiconductor body of a first type conductivity which has a major surface, including the steps of forming spaced-apart blooming drains extending a distance into said body from the major surface and comprising regions of a second type conductivity, forming channel stops around the blooming drains comprising channel barrier regions extending a distance into said body from the major surface, and forming charge-transfer channels extending a distance into the body from the major surface comprising regions of said second type conductivity whereby there is at least one channel stop with a blooming drain therein adjacent to each channel, the improvement comprising, after the step of forming the blooming drains, forming buried barrier regions extending a further distance into the body from the channel stops containing the blooming drains.

11. The method of Claim 10 wherein the step of forming the buried barrier regions is performed after the step of forming the channel stops.

12. The method of Claim 10 wherein the step of forming the buried barrier regions is performed simultaneously with the step of forming the channel stops.

13. The method of Claim 10 wherein the steps of forming the channel stops and the buried barrier regions are performed by ion implantation of the same type of conductivity modifier.

14. The method of Claim 13 wherein the number of first type conductivity modifiers implanted to form the buried barrier regions is between about five and five hundred times greater than the number of first type conductivity modifiers in the body.

15. A method of fabricating an imaging array of the charge-transfer type substantially as described hereinbefore.

16. An imaging array of the charge-transfer type substantially as described hereinbefore with referpence to Figures 1 through 3 of the accompanying drawing.