Flash memory cell and fabrication method

Memory cells, formed as trench transistors, having a respective floating gate electrode and a control gate electrode at a trench wall above a channel region between doped regions for source and drain are provided with a gate electrode arranged in a further trench, via which gate electrode the channel region present in a semiconductor ridge between the trenches can additionally be driven.

FIELD OF THE INVENTION

The present invention relates to a flash memory cell and a matrix arrangement of such memory cells as a semiconductor memory, and to an associated fabrication method.

BACKGROUND OF THE INVENTION

A flash memory cell has a transistor structure containing a floating gate electrode and a control gate electrode, which are isolated from the semiconductor material and from one another in each case by thin layers of a dielectric. When a suitable voltage is applied to the control gate electrode, charge carriers tunnel from the channel region of the transistor through the thin dielectric onto the floating gate electrode, as a result of which the memory cell is programmed. Since the threshold voltage of the transistor changes by virtue of the charge carriers on the floating gate electrode, the programmed state can be distinguished from the original state, i.e. the cell can be read. During the erasure process, the charge is removed from the floating gate electrode by oppositely applied potentials, so that the original (uncharged) state of the memory transistor is attained again, at least approximately. In the case of previous flash memory cells there are problems with the miniaturization of the transistors since the thickness of the tunnel oxide between the semiconductor material and the floating gate electrode cannot be reduced below 8 nm for reasons of adequate data retention. A scaled miniaturization of this transistor with the thickness of the gate oxide remaining the same is not possible.

SUMMARY OF THE INVENTION

It is an object of the present invention to specify a flash memory cell which, despite smaller dimensions, has a performance comparable to conventional flash memory cells. In addition, the intention is to specify an associated fabrication method.

The flash memory cell has a double gate transistor with a very thin semiconductor ridge, on one side of which a floating gate electrode and also a control gate electrode are arranged and on the other side of which a further gate electrode is arranged. In the case of this arrangement, the transistor property is determined by both gates. The semiconductor ridge present between the electrodes is fully depleted of charge carriers. If the gates are driven separately, the channel potential is influenced differently from both sides of the semiconductor ridge. On one side of the semiconductor ridge there is a structure of a customary field effect transistor, and on the other side a floating gate transistor that is driven via a control gate. The customary transistor structure is provided for reading from the memory cell; its threshold voltage can be controlled by means of the electrical potential on the gate of the floating gate transistor.

A description is given below of a preferred exemplary embodiment of the flash memory cell on the basis of a preferred fabrication method.FIG. 1shows a cross section through an intermediate product after the first steps of the method. An SOI substrate comprising a bulk silicon layer1, a thin insulation layer2made of silicon dioxide and a thin body silicon layer3is preferably taken as a basis. Preferably, in this case, firstly an auxiliary layer4is applied to the body silicon layer3, which auxiliary layer may be e.g. silicon nitride. The auxiliary layer4is patterned by means of a suitable mask technique, e.g. a photo mask or a resist mask, such that the remaining portions can be used as a mask for etching trenches5oriented parallel to one another. The trenches5are fabricated with a depth such that the body silicon layer3is completely severed by each trench. The trenches are preferably fabricated right into the insulation layer2as shown inFIG. 1in order to obtain a stronger coupling between the floating gate electrode and the control gate electrode. A photoresist that is possibly used is then removed. A plurality of trenches5arranged parallel to one another is provided for fabricating not only one flash memory cell but a matrix-like arrangement of a memory cell array.

The first dielectric layer6provided as a gate dielectric is subsequently fabricated, preferably an oxide, in particular silicon dioxide, for which purpose the semiconductor material may be oxidized superficially with a small thickness. In this case, it is unimportant whether the first dielectric layer6is also applied on the top side of the auxiliary layer4. The floating gate electrodes7are fabricated adjoining the first dielectric layer6by a procedure in which firstly the material provided for the floating gate electrodes7, preferably doped polysilicon, is deposited into the trenches5. The deposited material is etched back in a manner known per se to form spacer-like portions at the sidewalls of the trenches5. A further mask provided with openings running transversely with respect to the trenches is used to etch away sections between portions of the order of magnitude of an individual transistor, so that the remaining spacer-like portions provided for the floating gate electrodes are interrupted in sections in the longitudinal direction of the trenches5, i.e. perpendicularly to the plane of the drawing ofFIG. 1.

The next method steps lead to the arrangement in accordance withFIG. 2. Firstly, a second dielectric layer8is applied and patterned, which is provided as a dielectric between the floating gate electrode and the control gate electrode and is preferably applied as an ONO layer (oxide-nitride-oxide layer) comprising successively SiO2, Si3N4and SiO2. The material of the control gate electrodes9is then deposited, which is preferably once again doped polysilicon. This material is likewise etched back, but not interrupted in the longitudinal direction of the trenches. The control gate electrodes of the individual memory cells thus remain electrically conductively interconnected along the trenches, with the result that respective word lines of the memory cell array are formed.

The trenches are then filled with a trench filling10made of an electrically insulating material, e.g. with silicon dioxide. Before the trench filling is introduced, the polysilicon may be at least partly removed at the ends of the trenches by means of a further mask technique, in order to isolate the individual word lines from one another. The trench filling is planarized on the top side e.g. by means of CMP (chemical mechanical polishing). By means of a further etching step, the auxiliary layer4is removed to an extent such that a residual portion of the auxiliary layer in each case remains as a spacer only at the sidewalls of the trench filling10.

In accordance with the cross section illustrated inFIG. 3, using the residual portions of the auxiliary layer4as a mask, further trenches15are etched into the body silicon layer3, the trenches running parallel between the trenches5that are already present and filled. The depth of the further trenches15reaches only as far as the top side of the insulation layer (2). A third dielectric layer11, e.g. made of silicon dioxide, is then in each case fabricated at the sidewalls of the further trenches15. Gate electrodes12are fabricated in a manner adjoining the dielectric layer, preferably by doped polysilicon once again being deposited and patterned to form spacer-like portions.

The semiconductor ridges13that have remained between the trenches are dimensioned such that it is possible to control the charge carriers in the semiconductor material of the semiconductor ridges13from both sides by means of electrical potentials on the control gate electrodes9and the gate electrodes12. The material provided for the gate electrodes12is etched away at the ends of the further trenches15, so that, here as well, the gate electrodes12which succeed one another at a respective sidewall of a further trench15are electrically conductively interconnected and form mutually isolated further word lines provided for reading the memory cells. The further trenches are also filled with a trench filling10that is subsequently planarized.

FIG. 4shows a cross section through the arrangement in the region between the floating gate electrodes where, in this exemplary embodiment, the second dielectric layer8is situated directly on the first dielectric layer6. The intermediate sections19of the word lines that connect the control gate electrodes9to one another are arranged near the sidewalls of the trenches. In the regions between the floating gate electrodes7, the portions of the auxiliary layer are removed, and implantations of dopant are introduced into the semiconductor ridges13through the resulting openings in order to form doped regions14for source and drain. These doped regions14are contact-connected through the openings by a procedure in which preferably firstly a diffusion barrier16made of titanium and/or titanium nitride is applied and then contact hole fillings17e.g. made of tungsten are introduced in a manner known per se. The source and drain regions present in each case on a line running transversely with respect to the longitudinal direction of the trenches are electrically conductively connected to one another by means of a bit line18patterned in strip-type fashion on the top side.

FIG. 5shows the arrangement of the flash memory cells in a memory cell matrix in plan view. The sectional positions of the cross sections ofFIGS. 3 and 4are specified inFIG. 5. The reference symbols correspond to the reference symbols of the previous figures. It is discernable here that the floating gate electrodes7are arranged in each case at the walls of those portions of the semiconductor ridges13which lie between two successive doped regions14, above the channel regions provided there. The control gate electrodes9are electrically conductively connected to one another by the intermediate sections19to form word lines. An associated gate electrode12for driving the channel region on both sides is arranged on the side of a semiconductor ridge13that is in each case opposite to the floating gate electrode7. The bit lines, which are not depicted in this plan view, run on the top side in parallel strips transversely with respect to the longitudinal direction of the trenches, that is to say in each case in perpendicular strips inFIG. 5.

FIG. 6illustrates the circuit diagram for this memory cell matrix. Each memory cell is formed by a double gate transistor. The source and drain regions are connected to one another in columns by means of the bit lines running perpendicular inFIG. 6. Instead of a word line, two drive lines are in each case present here in rows, to be precise in each case a line CG-Gate for driving the control gate electrodes and in each case a drive line Tr-Gate for driving the gate electrodes of the customary transistor structures that are opposite to the floating gate electrodes.

A table with suitable and typical voltage values for programming (Prog), erasing (Erase) and reading (Read) the memory cells is specified above the circuit diagram in FIG.6. The voltage values entered in the table are present respectively at the drain region, at the control gate electrode, at the gate electrode of the customary transistor structure and at the source region. The memory cell matrix represents a “virtual ground” architecture. Dividing the flash memory cells into a read transistor, for which the gate electrodes12are provided, and into a programming/erasing transistor, for which the control gate electrodes9are provided, has the advantage that the read transistor can be realized with a particularly thin gate oxide (third dielectric layer11). The read current when reading from the memory cells via the Tr-Gate line is significantly greater than the read current when reading from conventional flash memory cells, thereby enabling a distinct miniaturization of the memory cells.