OUTPUT CIRCUIT FOR IMAGE SENSOR

Abstract: An output circuit for sequentially receiving and converting charge collected in the photoelements of an image sensor and converting such charge into an output voltage. The output circuit includes a buried-channel LDD transistor having gate, source and drain electrodes. The source electrode provides a floating diffusion which collects charge. An output source-follower amplifier also employing buried-channel LDD transistors is connected to the floating diffusion and produces the output voltage.

Description: OUTPUT CIRCUIT FOR IMAGE SENSOR Technical Field The present invention relates to output circuits for sequentially receiving and then converting charge collected in the photoelements of an image sensor into an output voltage. Background Art The resolution of image sensors is increased by increasing the number of photoelements of such sensors. As the number of photoelements increases, their size necessarily decreases for a given sensor image area. Charge is collected by these photoelements and transferred to an output circuit which includes a floating diffusion (FD). This output circuit is adapted to convert charge into an output voltage. As the size of sensor elements decreases, the number of signal charge they collect also decreases. Unfortunately, the portion of the output signal caused by noise actually increases. Noise therefore becomes a more significant component of any output signal. Consider that we have a given photoelement signal charge as determined by its responsivity and signal exposure level. Such charge is independent of the sense node's capacitance (C FD ). Also consider that the source-follower output circuit as described herein has a given 2 input—referred noise voltage ( i SF ) primarily due to thermal noise of the first-stage drive transistor. Thermal noise is also independent of C FD for a given transistor width-to-length ratio (W/L) as determined by drive requirements. 2 (Vi SF ) can be converted to an input—referred noise in terms of charge by the expression, ^SF 2 - C FD 2vi SF 2 Another noise term associated with the described output circuit is termed reset or kTC noise, and is given in terms of charge by the expression, Q kTC ~ kTC FD, where k is Boltzman's constant and T is the temperature. Therefore the signal—to—noise ratio (S/N) is given by, CLsig S/N = ? - j -kTC FD + C FD i SF + other terms , where the other terms are also independent of C FD - From this expression, it can be seen that the signal—to—noise ratio is improved by decreasing C ™ . It it therefore an object of the present invention to provide a highly—sensitive floating—diffusion electrometer design which minimizes input capacitance and thereby reduces noise in the total output signal voltage. Disclosure of the Invention This object is achieved by an output circuit for sequentially receiving charge and converting such charge collected in the photoelements of an image sensor into output voltages, comprising: a. a first buried—channel LDD transistor having gate, source and drain electrodes, the source electrode provides a floating diffusion; means for selectively applying pulses to the gate electrode of said first transistor to effect the operating characteristics of said first transistor to create a potential well in such floating diffusion preparing it to receive charge; b. an output source—follower amplifier having a second buried-channel LDD transistor electrically connected to said floating diffusion, said source—follower amplifier producing an output voltage in response to charge collected in such floating di usion. Brief Description of the Drawings Fig. 1 is a block diagram of the basic elements of an image sensor in accordance with the present invention; Fig. 2 shows in schematic form the output circuit 18 shown in Fig. 1; Fig. 3 is a top view showing the layout of " portions of the output circuit 18; Fig. 4 is a sectional view taken along the lines A-A of Fig. 3; Fig. 5 is a sectional view taken along the lines B-B of Fig. 3; and Fig. 6 is a sectional view taken along the lines C-C of Fig. 3. For convenience of illustration, none of the drawings are to scale. Modes of Carrying Out the Invention Turning first to Fig. 1, there is shown in block diagrammatic form a full-frame image sensor 10. The image sensor 10 includes photoelements 12 which collect charge as a linear function of the intensity of incident light and integration time, and as a non-linear function of incident light wavelength. Each photoelement represents one pixel of an image scene. These photoelements can for example be photocapacitors which accumulate electrons in an n region of a buried channel. During readout, charge is transferred vertically from photocapacitor to photocapacitor in each column to a buried—channel horizontal charge—coupled device (CCD) 14. A portion of the charge—coupled device is shown in more detail in Fig. 4. Each packet of electrons from each photoelement is sequentially delivered to a horizontal CCD element preceding an output gate 16 and then from this element through the output gate to an output circuit 18. The output circuit 18 is integrated on the same chip as the sensor 10. The output circuit 18 provides an output voltage V . proportional to each packet of electrons (charge) it receives. The output circuit 18 includes a floating diffusion 33 (FD) which, as will be seen, has its effective capacitance minimized to reduce noise in the output signal. Turning now to Fig. 2, the circuit 18 is shown in schematic form. In response to the removal of the pulse ΦR applied to the gate electrode 20 of a reset transistor Q R , the transistor Q_ is turned off and shortly thereafter, charge is transferred from under the output gate 16 to a floating diffusion FD, 33. As shown in Fig..4, the floating diffusion is actually the source electrode 33 of the transistor Q R . When the pulse ΦR is applied, the transistor Q_ is turned on and the R potential across the floating diffusion FD is returned to a reference level set by V , the reset drain potential. When transistor Q is off, a R potential well is created in the floating diffusion. Electrons are once again transferred to this potential well from the output gate 16. The floating diffusion 33 is electrically connected to the gate electrode of a transistor QD_ of the first stage of the source—follower output amplifier 18. In this first stage, there are two transistors QD.. and QL- . Both of these transistors continuously operate in a saturated mode. At the electrical junction of the transistors QD, and QL, , a voltage is produced which follows the voltage level across the floating diffusion FD. This voltage is applied as an input to the gate electrode of transistor QD,,. The drain of QD_ is connected to the same potential source V, , which is coupled to dα the drain of transistor QD . All of the transistors, Q_, QD. , QL. and QD are NMOS K 1 _. lightly-diffused drain (LDD) buried-channel transistors. Transistor QD, is shown in section in Fig. 5. The output voltage V __ is taken from the out source electrode of transistor QD-. V __ is 2 out applied as an input to conventional off-chip signal processing circuitry. Fig. 3 shows a top layout view of transistor Q R and transistor QD, of the first stage of the source-follower output amplifier 18. Fig. 3 should be consulted during the description of Figs. 4 and 5. Turning now to Fig. 4 where there is shown in cross—section the reset transistor Q„ and the output gate 16 and two gates of the horizontal CCD 14. The CCD 14 is shown as a two—phase device. There are two levels of polysilicon, poly-1 and poly—2 which respectively provide shift register gate electrodes 22 and 24. A substrate 26 is of a p— ype conductivity and an n—type layer 28, which can be provided by implanting arsenic into the substrate 26, provides a buried—channel structure. Directly over the n-type layer 28 is a layer of thermally grown silicon dioxide 29. Directly over the p—type substrate 26 is a p field threshold adjust implant 46 in the non—active regions of the device. Over the P field threshold adjusted regions 46 is a thick field silicon dioxide layer 31 provided by a conventional LOCOS (Local Oxidation of Silicon) process. CCD shift register electrodes 22 and 24 are formed on the thin gate oxide 29. Separating each of the electrodes is an insulating layer 30 of silicon dioxide which is provided by a conventional LTO (Low Temperature Oxide). The output gate 16 has a positive potential V_ continuously applied to the electrode. If we assume that electrons are held under the last gate 24 of the horizontal shift CCD 14 and that at this time the gate potential Φ 2 is reduced while the gate potential φ is raised, the electrons will flow down a "potential hill" under the output gate 16 to the floating diffusion 33. At this time, the transistor Q R is off; that is, signal electrons collect on the source electrode 33. The transistor Q R is an NMOS LDD buried-channel transistor. The source electrode 33 provides the function of a floating diffusion FD. The electrode 33 is a floating diffusion because the potential developed across it is allowed to float when the transistor Q__, is off. The floating diffusion is + provided at the PN junction between the n diffused electrode and p (substrate) regions. Then when the pulse ΦR is applied to the gate electrode 20 of transistor Q R , the transistor Q R turns on and the potential across the floating diffusion 33 is reset by the electrons draining off onto the drain of transistor Q R which is at a potential V _. During these times, a voltage change is produced across the floating diffusion which is electrically connected to the gate electrode 40 of transistor QD, shown in Figs. 2 and 5. As shown in Fig. 5, transistor QD is also an NMOS LDD buried—channel transistor. The transistor Q , QD- QL and QD 2 are constructed by defining the active and field regions through a conventional LOCOS process. All these transistors are formed by implanting arsenic to form the buried—channel layer 28. By using buried—channel transistors, as opposed to surface—channel transistors, the flicker noise component of the transistors is reduced since the carriers do not interact as strongly with silicon-silicon dioxide interface states. This buried—channel implant layer 28 also serves to replace the n source and drain implanted regions typically used to form LDD NMOSFETs thereby simplifying the process. In accordance with the invention, the use of an LDD structure which has the n layer 28 connected to n source and n drain electrodes as shown, causes the gate—to—source and drain overlap capacitances to be greatly reduced. This capacitance makes up part of the total capacitance at the sensing node FD 33. In addition, the n buried—channel layer 28 is separated from the p field implanted regions 46 at the source node 33 of QR which forms the floating diffusion as shown in Fig. 6. This serves to reduce the floating diffusion's sidewall junction capacitance, and hence the total capacitance of the sensing node FD, since the depletion spreading is increased. A boron FET threshold voltage adjust implant 34 is implanted through the gate electrodes for the following elements: QO 1 (see Fig. 5), QL., QD_ and OG. A number of conventional processes can be used to provide these implants 34. A particularly suitable process is described in commonly assigned EP-A-0207328, entitled "Method of Making Edge-Aligned Implants and Electrodes Therefor" in the names of Losee and Lavine. The disclosure of this application is incorporated by reference herein. The n source and n drain electrodes of the transistors have been constructed by implanting arsenic though source and drain contact holes etched through the overlaying LTO layer 30 into layer 28. This saves a masking step hence simplifying the construction of these + electrodes, and results in the n contacts being self—aligned. This also stops the n source and drain regions from spreading beneath the polysilicon gates (see Figs. 4 and 5), by lateral diffusion thereby greatly reducing the overlap capacitance. Aluminum contacts 45 are sputtered into the etched holes to make electrical connection with the electrodes. Since the n source/drain regions are also separated from the p field or channel stop regions 46 (see Figs. 5 and 6), the capacitance of the reverse biased junctions is also reduced due to an increased depletion region width. An added advantage of separating the n regions off of the p regions is an increase of the diode's reverse—bias breakdown voltage. Fig. 5 will now be described. The NMOS LDD buried—channel transistor QD, includes a drain electrode 42 and a source electrode 44. A p region 46 provides a field threshold adjust function. It will be understood that a lightly—doped p region 34 may be provided just under the layer 28 to adjust the threshold voltage of the transistor as • discussed above in connection with QD, . This p region 34 can be provided by conventional boron ion implantation. It has also been found quite advantageous to have transistors QL, and QD 2 as NMOS LDD buried—channel transistors, which increase speed by reducing the capacitive load seen at the output of the first—stage source—follower. The effective floating diffusion capacitance is reduced for three reasons. The first is by reducing the gate—to—drain and source overlap capacitance since the n regions are offset far enough from the gate so that it will not diffuse laterally underneath the gate. This reduction is effected through the use of the LDD structure. The second reduction in capacitance is effected by the separation of the n floating diffusion region from the p field threshold adjust regions. The third reduction in effective floating diffusion capacitance results from separating the CCD buried-channel n region 28 at the floating diffusion from the p field threshold adjust regions 46 as shown in Fig. 6. This causes a larger depletion region of this reverse biased junction which results in a lower junction capacitance. In accordance with this invention, an output circuit was constructed in accordance with the present invention using four buried—channel NMOS, LDD transistors. Each of these transistors employed self—aligned n contacts. By means of this arrangement, an effective floating diffusion capacitance of approximately 10 fF was obtained. The circuit provided low—flicker noise with a relatively high output voltage. Flicker noise was reduced by using buried—channel transistors. Industrial Applicability and Advantages The output circuit of the present is particularly useful in image sensors. Among the features of this integrated—output circuit are that it has low-power dissipation, requires very little surface area and is quite versatile. This circuit reduces noise in the output voltage through high input—referred sensitivity and also increases reliability and simplicity of overall system design. NMOS transistors are particularly suitable for use in the present invention.

Claims: Claims : 1. An output circuit for sequentially receiving charge and converting such charge collected in photoelements of an image sensor into an output voltage, said circuit being characterized by: a. a first buried—channel LDD transistor having gate, source and drain electrodes, said source electrode providing a floating diffusion; b. means for selectively applying pulses to the gate electrode of said first transistor to reset the potential across said floating diffusion and prepare it to receive charge produced in a photoelement; and c. an output source—follower amplifier having a second buried—channel LDD transistor, the gate of which is electrically connected to said floating diffusion, whereby said amplifier produces a change in output voltage in response to charge collected in said floating diffusion. 2. The output circuit as set forth in claim 1, characterized in that 'said output source—follower amplifier is a two—stage configuration including said second transistor in said first stage and at least one buried—channel LDD output transistor in said second stage, with said second stage being electrically connected to said first stage and adapted to follow the voltage produced as charge are collected in said floating diffusion. 3. The output circuit of claim 2 ' , characterized in that all of said buried—channel transistors are NMOS and include a p substrate, an n buried—channel layer formed in the substrate, a thermally grown oxide layer on the n channel layer, a gate electrode on said thermally grown oxide layer, an insulating layer formed on the thermally grown layer and on the gate, and n source and n drain electrodes implanted in the n layer through holes etched in said insulating layer.

Inventors: STEVENS ERIC G, STEVENS, ERIC, G.
Applicants: EASTMAN KODAK CO, EASTMAN KODAK COMPANY
Application year: 1987
Main classification: H01L29
IPC codes: H01L21, H01L27, H01L29, H04N5