This invention relates to charge coupled memory devices.
The recent discovery of the charge coupled functional device concept that is described and claimed in copending U.S. Pat. application Ser. No. 11,541, filed Feb. 16, 1970, by W. S. Boyle and G. E. Smith, allows for the fabrication of a new form of memory device in which the capacity of each charge storage site is selectively fixed usually by adjusting the electrical capacity at each site of the MIS structure in accordance with input information. With the storage capacity of each site independently fixed, charge can be repeatedly accumulated in the same fixed pattern and as the charge is shifted out, the signal level will correspond to the input information.
The charge coupled storage concept is fully explained in the application referred to above. That device in its basic form is a dynamic memory store although with the incorporation of recirculation and regeneration, as taught therein, the storage duration can be extended to provide an essentially permanent memory feature.
According to the present invention a memory function is built into each storage site so that recirculation and regeneration is not necessary.
The memory devices of the invention are conveniently categorized into one of two forms. In one, the storage capacity of each site is permanently fixed, generally by the structural characteristics of the device, to give a "read-only" memory.
In the second category are permanent memories in which the storage capacity of the sites can be conveniently adjusted or reprogrammed. This adjustable feature suggests that these devices can, if desired, be operated in a nonpermanent mode. All of these devices share the common feature that the electrical capacitance of the MIS storage elements is selectively altered. In the usual case it will be preferable to employ parallel readout for these devices although with suitable adjustment of the accumulation and shift potentials, serial readout can be used.
Structures in which the capacitance is permanently fixed include those in which the physical thickness of the insulating layer is varied according to the information program. Alternatively, at least two (two for digital devices) different insulators are used, different in the sense that they exhibit significantly different dielectric properties. Included under this device concept are homogeneous insulators in which the dielectric strength is locally altered as by selective diffusion or implantation of more, or less, conductive ions.
The electrical capacitance of the individual MIS elements can also be fixed by selecting the properties of the metal. Using metal conductors having different work functions results in storage elements with different capacitances.
Preferred from the standpoint of versatility are those embodiments wherein the storage capacity of the individual sites can be conveniently adjusted with new information. In one such embodiment, floating or dielectrically isolated capacitors are provided at each memory site. Information is shifted into the charge coupled register by normal charge coupled action and is transferred to the isolated capacitors by a prescribed read-in process. The charge remains in the isolated capacitor as long as desired depending upon the effectiveness of the dielectric isolation. The amount of this charge, in an analogue sense, or the presence or absense of charge, in a digital sense, determines the charge capacity of the storage elements. The information in the memory elements can be erased conveniently for reprogramming.
These and other aspects of the invention may be more evident from the following detailed description. In the drawing:
FIG. 1 is a front section of a portion of a charge coupled memory device according to one embodiment of this invention;
FIGS. 2A and 2B are sectional views of an alternative charge coupled memory device employing parallel coupling for readout;
FIG. 3 is a perspective view of a further embodiment of the invention;
FIG. 4 is a front sectional view of a charge coupled memory device in which the memory is semipermanently fixed;
FIG. 5 is a current voltage plot describing a property of the insulating barrier between the storage medium and the memory controlling element in the device of FIG. 4; and
FIGS. 6A and 6B are plots of the band structure of the memory element of FIG. 5 with and without fixed charge in the memory controlling element.
In the device represented by FIG. 1, a series of conventional charge coupled elements are shown in combination with memory storage elements according to the invention. With a three wire drive scheme, as in this device, every third electrode is used for accumulation. The structure includes semiconductor 10, insulating layer 11, the metal drive electrodes 12a -12d, 13a -13d and 14a -14d and their associated conductors 12, 13 and 14. Of the drive electrodes, 12a -12d comprise accumulation stages and provide the memory function. As shown in the figure certain of these, i.e., 12b and 12d, have thick insulating layers 15 additionally provided. Thus the capacitance of these elements is less than that of the elements comprising electrodes 12a and 12c. With a bias on conductor 12, carriers are allowed to accumulate under conductors 12a -12d to their equilibrium value. The accumulation process can be facilitated by several methods. Illuminating the semiconductor 10 produces an excess of free carriers by photon absorption. Alternatively, carriers can be shifted in by charge coupled action and allowed to equilibrate to the capacity of each element. A similar result is conveniently obtained by driving all elements to avalanche so that carriers are injected at each site. Accumulation also occurs inherently due to thermal processes. This last mechanism is attractive in its simplicity and is adequate except where very short accumulation periods are required. The accumulated carriers are then shifted out by normal charge coupled action by sequentially biasing conductors 12, 13 and 14. The signal produced by carriers accumulated under electrodes 12a and 12c will be larger than the corresponding signal from elements associated with electrodes 12b and 12d.
Using this series readout mode the variation in capacitance of the accumulation elements may in some cases interfere wit the shift operation. However, this can be minimized if the drive or shift potentials are large (i.e., at least twice) the potential used for accumulation.
A preferred way of avoiding the problem just alluded to is to use parallel readout. An embodiment illustrating this expedient is shown in plan view in FIG. 2A. In this device the charge coupled array of elements 12a -12d, 13a -13d, and 14a -14d are similar to those of FIG. 1 except that elements 12a - 12d are simply conventional drive elements. The three wire drive conductors 12, 13 and 14 are biased in sequence to affect the shift mechanism. The accumulation or memory stage is parallel to the shift row and comprises a single conductor strip 17 with a bias connection 16. Memory elements are shown at 12a' and 12c' . These elements are simply thin portions in a relatively thick insulator 11'. The structure is evident from FIG. 2B which is a section through element 12c' . The high capacitance associated with these elements will allow accumulation of carriers for, for example, a digital "one," as compared with the absence of carriers accumulated adjacent electrodes 12b and 12d, which might represent digital "zeros." When the accumulation is complete, conductor 12 is biased and the sequential shift operation produces the digital signal at the output. For this segment of the device, that signal would be 0101.
Charge transfer between the accumulation or memory stage and the charge coupled line is to be avoided during the shift operation. Several simple techniques can be used to achieve this. For example, the accumulation process can be made long in comparison to the readout time so that interaction during readout will involve too few carriers to impair the signal. Alternatively, the bias on electrode 17 is removed during the shift operation so that no carriers will accumulate.
The accumulation elements 12a' -12d' can be physically isolated from the charge coupled line with a gate electrode. This will be evident from the discussion below in connection with FIG. 3.
There are various convenient ways of fabricating the structure of FIGS. 1 and 2A taking advantage of known semiconductor processing techniques. For example, the insulator can be deposited to the thickness desired at elements 12b and 12d (i.e., the combined thickness of layers 11 and 15 in FIG. 1 or the thickness of layer 11' in FIG. 2A) and then selectively etched to form the thinned regions. Alternatively a composite layer such as SiO2 and Si3 N4 can be deposited which is then selectively etched with a preferential etchant to give the desired structure. These techniques are well known and form no part of the invention.
Another means for obtaining a difference in storage capacity between selected elements is to use metal electrodes having significantly different work functions. The structure in this case would be essentially that shown in FIG. 1 except that the insulating layer would be uniform in thickness at each element associated with electrodes 12a, 12b, 12c and 12d. Electrodes 12a and 12c would be, e.g., platinum, with electrodes 12b and 12d tungsten. The difference in work function between these metals is approximately 1.0 volts, giving an easily detectable variation in charge storage with normal bias voltages.
The parallel readout arrangement of FIG. 2A can be easily adapted to this embodiment. This would require simply that electrode 17 be segmented so that region 12a' is covered with platinum and region 12c' with tungsten. A common conductor 16 is still appropriate as these sites are normally biased simultaneously.
A modification of the parallel-coupled read-only memory of FIG. 2A is shown in FIG. 3. For illustration, this device employs the two wire scheme described and claimed in application Ser. No. 11,448, filed Feb. 16, 1970 by D. Kahng and E. H. Nicollian. In this case every other element comprises a memory stage. The device includes the familiar semiconductor storage medium 30, insulating layer 31, and the sequence of drive electrodes 32a, 33a, 32b, 33b, 32c, 33c, 32d, 33d, 32e and 33e, all connected to conductors 32 and 33 as shown. The sequential drive operation is described completely in the application alluded to above and the function of the stepped insulator and the drive electrode configuration will not be repeated here. The front portion of the semiconductor 30 is not covered with insulator and contains a continuous longitudinally extending diffused region 34 which forms with the substrate a p-n junction. An electrode 35 is provided to externally short the junction. This p-n junction functions similarly to the source of an IGFET and provides a continuous supply of charge carriers in close proximity to the electrode sequence 32a to 33e, but as yet uncoupled to it. The coupling is made selectively by gate electrodes 36, 37, and 40. Gates at 38 and 39 are intentionally omitted as called for by the memory code. When the gate electrodes are biased via conductor 41, charge flows from the source junction 34 to the region under the gates. Conductor 32 is biased coincidentally with conductor 41 and charge flows through the biased gates into the charge coupled line. The charge accumulated under the associated drive electrodes 32a, 32b, 32c, 32d and 32e is shifted out in the normal way by sequentially biasing conductors 32 and 33. The binary signal derived from this sequence would be 10011.
These memory stages resemble hybrid IGFETS with a p-n junction source and an MIS drain. Dimensions and other specifications for the device are straightforward extensions of prior art teachings.
At this point it is evident that the device of FIG. 3 can alternatively be made with individual p-n junctions made at selected sites and with a continuous gate electrode. This embodiment is such an obvious alternative that it need not be depicted.
It is also worth noting that these devices are digital and in the form shown do not have analog capability. They are also characteristically distinct from the other devices described herein in that those devices involve programming the inherent storage capacity of selected storage sites while these embodiments function by charging only selected elements of a charge coupled line (all elements having essentially the same charge capacity) and wherein the charging is accomplished by coupling to an instantaneous supply of carriers. Note that in the device of FIG. 2A the charging of the charge coupled line is selective but the supply of carriers requires a finite accumulation period. Consequently it is evident that devices of the type suggested by FIG. 3 are inherently faster and appear at this point to be preferred charge coupled memory devices.
An embodiment of a charge coupled memory device in which the memory can be adjusted is shown in FIG. 4. The substrate 50 is a semiconductor such as silicon, and the three electrodes 51, 52 and 53 comprise the three wire drive system with associated conductors 54, 55 and 56. This drive scheme is exemplary only, and for a more complete description of the operation of this and alternative systems, reference is made to the aforementioned copending applications. The intermediate layer, which is normally a homogeneous insulator of an MIS structure, in this case contains the memory element. A thin insulating layer 57 covers the substrate 50 and separates it from the floating capacitor plate 58. The capacitor plate may be metal or semiconductor and serves simply to store charge. A second insulating layer 59 isolates the capacitor from the drive electrodes 51, 52 and 53. The insulating layer 57 is partially conducting so as to allow charge to be transferred between the capacitor plate 58 and the substrate 50. The insulating layer 59 should be sufficiently thick to prevent significant amounts of charge from leaking from the drive electrodes 51, 52 and 53 under normal bias conditions.
The partially conducting insulator 57 should exhibit the non-ohmic behavior described by FIG. 5. Charge stored at the semiconductor-insulator interface should not leak to the capacitive memory plate except during the read operation. If the insulator is non-ohmic, a threshold field, Et, allows fields above this value to be used for reading into and erasing the memory, while the use of a field below E1 permits normal storage and drive functions without affecting the memory.
The read-in operation through which the floating capacitor plate 58 is charged is described by FIGS. 6A and 6B. FIG. 6A is a band structure diagram of the device of FIG. 4 without charge present at the semiconductor-insulator interface (n-semiconductor), while FIG. 6B is a corresponding band structure diagram with charge stored at the interface. The presence or absence of charge (or the amount of charge) is the information being read into the memory and can be placed locally under the memory plate by normal charge coupled action. With the charge pattern in place a relatively large voltage V1 is imposed across the composite structure. The magnitude of V1 is such that with no charge at the interface (FIG. 6A) the electric field at the barrier between the floating capacitor 58 and the semiconductor 50 is low enough to prevent conduction, i.e., below the threshold field Et. However, with positive charge at the interface, there is a larger voltage drop across the insulator as compared to the semiconductor, resulting in a field across the thin insulator greater than Et. This larger field allows conduction of electrons through the thin insulator 57 and leaves a net positive charge in the floating capacitor. This charge is effectively isolated and will decay with a time characteristic of the leakage current for fields below Et. The operating voltages are chosen smaller in magnitude than V1 so that the leakage current is kept acceptably small. With proper choice of the insulator 57 and the drive voltage, the charge decay time can be made essentially infinite. A positive voltage equal to or exceeding V1 on electrode 52 will extract majority carriers from the semiconductor and erase the charge.
The amount of charge read into the capacitor 58 will determine the storage capacity for holes of the semiconductor-insulator interface. Thus the memory can be made analog as well as digital.
The read operation requires simply the accumulation of carriers in each of the memory stages (in this case those associated with conductor 55) to the extent of its capacity. With an n-type semiconductor a negative voltage is impressed on the conductor 55 associated with the memory element comprising electrode 52, and holes are allowed to accumulate to their thermal equilibrium value. The charge is then shifted out by charge coupled action. The accumulation of carriers can be accelerated by photo-induced holes or by shifting in charge by charge coupled action at a large negative voltage and then making the voltage less negative so that each site becomes saturated.
The accumulation of carriers can be selectively controlled by focusing a light image on the substrate and comparing the spatial intensity of the image with the memory. In this way the device can be made to function as an image comparator or for pattern recognition.
The capacitive memory plates are similar in function and structure to the floating gate field effect transistor described in Bell System Technical Journal, July-Aug. 1967, pp. 1,288-1300.
The following is a specific example of this embodiment of the invention and prescribes appropriate specifications from which all the devices described herein can be conveniently derived.
The device of FIG. 4 is made with 10 Ωcm. silicon as the substrate material 50. The thin insulating layer 57 is SiO2 grown or deposited to a thickness of 10 to 1,000 A. The capacitor memory plates 58 are platinum or silicon with a thickness of 100 to 1,000 A. The plate thickness is not functionally important and the range given is simply convenient. A thick plate 58 risks dielectric discontinuities in the insulator 59. The voltage required for conduction between plate 58 and substrate 50 is of the order of 50 millivolts per angstrom of insulator. Therefore the voltage range corresponding approximately to the threshold field Et of FIG. 5 would range from 0.5 volts to 50 volts for the recommended range of insulator thickness. The insulator 59 can also be SiO2 with a thickness of the order of 200 to 10,000 A. and, for reasons evident from the foregoing description, at least twice the thickness of the insulating layer 57. If the insulating layers are composed of different materials, e.g., a combination of SiO2 and Si3 N4, then the thickness and dielectric strengths of the materials should be chosen so that the insulator 57 has at least twice the conduction of the layer 59 for a given write voltage. The drive electrodes 51, 52, and 53 can be of any conducting material such as gold, platinum or polycrystalline silicon. The memory plates 58 can also be formed of semiconductor material such as silicon. Advantageously the memory plates and the storage medium 10 are of different conductivity type.
The memory device of the invention has been described with a conventional semiconductor substrate and the depletion mode of operation according to the teachings of application Ser. No. 11,541 filed Feb. 16, 1970 by W. S. Boyle and G. E. Smith. The memory mechanism will function in an equivalent manner with the enhancement type charge coupled device using insulating semiconductors as described and claimed in an application filed by D. Kahng (D. Kahng Case 23 ), Ser. No. 47,205 on June 19, 1970. For the purposes of this invention the term insulating semiconductor means those materials defined in that application.
Alternative structures can be used for the floating plate capacitor shown in FIG. 4. For example if two different insulating materials are deposited in a dual layer, charge is typically trapped at the interface. This charge resides in deep traps and these traps can be depleted and replenished via the mechanism described above. The interfacial traps can be thought of as directly analogous to the memory plates 58 of FIG. 4. A useful combination of insulators for this embodiment are SiO2 -Si3 N4 and SiO2 -Al2 O3. The latter combination is convenient from the point of view of fabrication. A dual layer of silicon and aluminum can be deposited and then anodized, e.g., by plasma anodization. The technique provides good control over the interface properties. A similar result would be expected for Si3 N4 and AlN. An explanation of these interfacial states and the mechanism for filling and emptying them is described in RCA Review, Vol. 30, June 1969, pp. 335-382.
Another related charge storage mechanism relies on deep traps in the bulk of the insulator. Taking advantage of these, a very simple memory device can be constructed. It requires simply a homogenous insulating layer between the drive electrodes and the storage mechanism. Thus it is structurally indistinct from the basic charge coupled device except for the voltages used. The basic charge coupled device can use drive voltages of different values but all these would be below the threshold for conduction across the insulator. In the normal operating mode, carrier injection to or from the storage medium is undesirable. Thus the provision of a bias means for impressing a voltage large enough to fill or empty traps in the insulator with an operating sequence appropriate to the memory mode differentiates this device from the conventional charge coupled device.
Yet another embodiment in which the storage site capacitance can be semipermanently adjusted uses a structure with a stepped insulator similar to those of FIGS. 1 and 2A except that the thickness of the insulator can be adjusted by using, as the insulating material, a thermoplastic material of the type described in the Journal of Applied Physics, Dec. 1959, pp. 1,870- 1,873, and RCA Review, Vol. XXIII, Sept. 1962, p. 413. These materials are typically polymers with a low-temperature glass transition. When they are heated to their plastic transformation point in the presence of an electric field they constrict in the direction of the field until the electrostatic forces balance the surface tension forces. Lowering the temperature will freeze the material in the deformed state, and raising the temperature again in the absence of a field will allow the surface tension forces to restore the material to its original state. Thermoplastics are not very compressible so that constriction will displace the plastic from the area under the electrode.
The device just described functions basically in the same manner as those described in connection with FIG. 1 except for the memory read-in. To accomplish the latter, charge representing the desired memory code is shifted into electrodes 12a, 12b, 12c and 12d. Following the previous example this will leave charge at sites accompanying electrodes 12a and 12c with no charge below electrodes 12b and 12d. The thermoplastic layer is then heated causing constriction of the plastic at sites 12a and 12c and no constriction at sites 12b and 12d. Cooling the thermoplastic leaves the desired stepped insulator. Extension of this scheme to analogue memory devices is straightforward.
Various additional modifications and extensions of this invention will become apparent to those skilled in the art. All such variations and deviations which basically rely on the teachings through which this invention has advanced the art are properly considered within the spirit and scope of this invention.