EPROMs and Flash E.sup.2 PROMs (hereafter collectively, PROMs) have several structures which allow them to hold a charge without refresh for extended periods of time (see FIG. 1). The charge itself is stored on a "floating gate" 10 also referred to as Poly 1 or P1, which is a structure of polycrystalline silicon (hereafter, poly) surrounded on all sides by a layer of oxide 12. Located superjacent and parallel to this P1 structure is another poly structure, the word line or "control gate" 14 or P2. P1 10 and P2 14 act as the two plates of a capacitor. Below the P1 layer are two N+ junctions, one which acts as the transistor source 16 and the other as the drain 18, which are doped into a p-type substrate 20. The portion of the substrate 20 between the source 16 and the drain 18 is the channel 22. The cell of FIG. 1 functions like an enhancement-type n-channel metal oxide semiconductor field effect transistor (MOSFET) with two gates of poly.
There are many ways to program a flash E.sup.2 PROM. For example, a high voltage such as 12 V, is applied to the control gate. Then a voltage pulse such as 8 V is applied between source and drain. The large positive voltage on the control gate establishes an electric field in the insulating oxide. This electric field attracts the electrons generated from the so-called avalanche breakdown of the transistor due to the high drain and control gate voltages, and accelerates them toward the floating gate, which they enter through the oxide. In this way the floating gate is charged, and the charge that accumulates on it becomes trapped.
To return the floating gate from a charged state to a state with no charge, the charge is returned to the substrate or other electrodes. In an EPROM, this is accomplished with ultraviolet light which excites the electrons past a certain energy state, thereby allowing them to pass through the oxide. In an E.sup.2 PROM, this is accomplished with an electrical field.
The voltage which must be applied on the control gate to turn on the transistor is much higher in a device storing a charge than in a device which does not have a voltage potential stored on P1. To read the content of the floating gate, a voltage somewhere between the low and high voltage values (i.e. the threshold voltage V.sub.t) is applied to the control gate. A cell that trips at V.sub.t has no charge stored on P1, while a cell which does not trip is determined to be storing a charge.
There are structures which make up a PROM array which are common to several transistors in the array. FIG. 2 shows a top view of an array showing transistor sources 16, drains 18, digit lines 24, floating gates 10, and control lines 26 which form control gates 14 as they pass over the floating gates 10. Also shown as a dotted line is the "active area" 28 interspersed with areas of field oxide 30. As shown in FIG. 2, a single control line 26 is common to all transistors in a single column, and when selected it activates all transistors in the column. The source regions 16, which run parallel with the control lines 26, are common to all transistors in two adjacent columns. Individual transistor drains 18 are common to two transistors in adjacent columns. The digit (or bit) lines 24 are common with the drains 18 of all transistors in a single row.
To read the datum on a floating gate 10, the control line 26 of the cell to be read is activated which causes all transistors in the selected column to become active and to output the cell information on their respective digit lines 24. The information on the digit line 24 which corresponds to the cell to be read is obtained with a sense amplifier (not shown), with one sense amp for each digit line.
The active area 28 is defined during the manufacturing process after the thin pad oxide is initially formed on the surface of the wafer. After the pad oxide is formed on the surface of the wafer a layer nitride is formed and a patterned layer of photoresist is formed over the nitride. The exposed nitride is removed, and the exposed oxide is further oxidized. The nitride prevents the growth of the oxide underneath it, while allowing the exposed oxide to grow. The area under the nitride which remains thin is referred to as the gate oxide and is also called the active area. The exposed oxide which thickens becomes the field oxide. During a spacer etch, the gate oxide will erode away to bare silicon, as it is thinner than the field oxide. The bare areas can then be doped, with the field oxide protecting the other silicon areas from being doped.
There are many methods used to manufacture Flash E.sup.2 PROM and EPROM memory, one of which is shown in FIGS. 3-7. To obtain the structure of FIG. 1, a first blanket layer of oxide, the pad oxide, is formed on top of the silicon substrate. Nitride is patterned in a criss-cross pattern on the oxide to define the active area 28 as shown in FIG. 3, and the field oxide 30 is formed from the exposed pad oxide while the unexposed pad oxide becomes the gate oxide 28. As shown in FIGS. 4A and 4B, a first blanket layer of poly 40 which will make up P1 is formed on the appropriate areas of the gate oxide 28, and a second layer of oxide 42 and a layer of nitride 44 (or a nitride-oxide sandwich), which separate P1 from a second poly layer is formed on the first poly layer 40. The P1 layer is patterned with a layer of resist 46 in rows, perpendicular to the source lines of the active area. As shown in FIG. 5, after a P1 sidewall oxidation (not shown) a second blanket poly layer 50, P2, is formed on the nitride layer 44 (or nitride-oxide), and an optional oxide layer 52 is formed on the Poly 2 50 layer, and photoresist 54 is patterned on the surface in columns perpendicular to the rows of P1 40. An etch forms the floating gates 10 and control gates 14. Referring to FIG. 6, a third blanket layer of oxide 60 is formed and etched, which forms the spacers 70 as shown in FIG. 7. During the spacer etch, exposed gate oxide 28 is also removed, thereby exposing the areas of silicon which will later become the transistor diffusion areas.
After the structure of FIG. 7 is formed, the substrate 20 is doped to form the N+ areas of the transistor sources 16 and drains 18, and conductive areas which couple the sources between the rows. An implant source-drain can also be formed before the spacer formation for additional overlap of the impurity regions under the spacers.
A second method of forming the Flash E.sup.2 PROM structure is described in the article "A 5-Volt Contactless Array 256KBIT Flash EEPROM Technology," M. Gill et al, IEDM, 1988, pp 428. This structure, which is formed with a buried N+ line process, has higher digit line capacitance, more process complexity, and also has a high degree of lateral diffusion in the buried digit line and is not easily scalable.
With the advent of laptop computers, reducing the power consumption of devices has become a major design focus for engineers. The high voltage required to program and operate Flash EPROMS can decrease the usable battery life of portable and notebook computers.
A Flash EPROM design is described in "Buried Source-Side Injection (BSSI) for Flash EPROM Programming," Cetin Kaya et al, IEEE, 1992, pp. 465-467, which is incorporated herein by reference. Kaya, et al. claims the cell, which has a small cell area, can be programmed in a voltage range of 3-4 V. The device structure comprises the use of a buried source-side injector which requires the formation of a source junction 0.1-0.2 microns (.mu.) under the silicon substrate surface, and a conventionally designed drain junction. To program the cell the source is grounded while 13 V and 3.5 V are applied to the gate and to the drain respectively. To manufacture the buried source injector, a self-aligned high-energy n-type dopant implant is performed at the source side, which is followed by a heavy dose of As at the drain side. Subsequently, after the sidewall oxide is formed, additional impurities are deposited in the source to reduce the resistance of the diffusion and to form a low-resistance contact region.
In effect, the structure that allows the low-voltage program in the cell of Kaya, et al. is the "sharp corner" of impurities at the source close to the floating gate which forms the source injector. Maximizing the sharpness of the implanted region is a goal of the high-energy implant, but the implant is difficult to control and the shape of the injector will vary widely depending on the subsequent process heat treatment. A cell which has the advantages of the Kaya et al. cell but which allows the sharpness of the source injector to be maximized would be a desirable structure.