A DRAM cell typically comprises a MOS transistor and a capacitor. An example of such a DRAM cell is shown in FIG. 1. The DRAM cell 10 of FIG. 1 comprises the MOSFET 12 and the capacitor 14. A word line is connected to the gate of the MOSFET 14. A bit line is connected to the source of the MOSFET 12. The capacitor 14 is connected to the drain of the MOSFET 12. The state of the DRAM cell 10 is determined by whether or not the capacitor 14 is holding a charge.
The DRAM cell is read by using the bit line to determine whether or not a charge is stored in the capacitor. The DRAM cell is written by using the bit line to add or remove charge from the capacitor. However, the cell can only be read or written when the cell is addressed (i.e. activated) by the word line.
With the continuous advancement of DRAM technology, the chip area used for one DRAM cell is getting smaller. There are two methods used to increase the capacitance of the capacitor in the DRAM cell. One method is to decrease the effective dielectric thickness and the other method is to increase the effective capacitor surface area.
It is expected that future scaled DRAM cells will rely heavily on the quality and storage capacity of ultra thin dielectric materials that are sandwiched between two heavily doped polysilicon and/or silicon electrodes. However, higher capacitance values cannot be obtained without seriously degrading the device retention time. Specifically, films thinner than 50 angstroms present excessive leakage current due to direct carrier tunneling. For a given capacitor dielectric film, the larger the surface area, the larger the capacitance.
FIGS. 2, 3, 4, 5 and 6 illustrate prior art Dram cells. The DRAM cell 20 of FIG. 2 comprises a MOSFET 22 and a capacitor 24 which are both formed on a silicon substrate 25. The MOSFET 22 comprises the source and drain regions 26 and 28, which regions are separated by the channel 29. It should be noted that the source and drain regions 26, 28 have a conductivity type opposite to that of the substrate. A metal contact 27 directly contacts the source region 26. A gate 30 is formed from polysilicon material (Poly-1) and is separated from the surface of the substrate by an oxide layer 31. An additional oxide region 32 is on top of the gate 30.
The capacitor 24 sits on top of the drain region 28. The capacitor 24 has a first electrode 40 formed by a conducting polysilicon material (poly-2), a thin dielectric layer 42 which may be NO or ONO, and a second electrode 44 which is formed from a conducting polysilicon material (poly-3). The capacitor contacts the drain region 28 in a space between the oxide region 32 and the oxide region 52.
FIG. 3 shows a DRAM cell 20' which is similar to the DRAM cell 20 of FIG. 2. The main difference is that the shape of the poly-2 electrode layer 40, dielectric layer 42 and poly-3 electrode layer 44 have been altered to increase the surface area of the capacitor 24.
FIG. 4 shows a DRAM cell 20" in which the poly-2 electrode 40 has been made rugged to increase the surface area of the capacitor.
FIG. 5 shows a DRAM cell 20'" in which the polysilicon electrode 40 is formed from a plurality of horizontal layers 41 to increase the surface area of the capacitor.
Another capacitor structure which can be used to increase the capacitance of a DRAM cell is known as MOST (modulated stacked) capacitor. (See e.g. Y. K. Jun et al, "The Fabrication and Electrical Properties of Modulated Stacked Capacitor for Advance DRAM Applications" IEEE Electron Device Letters, Vol. 13, No. 8, August 1992, the contexts of which are incorporated herein by reference) .
A DRAM cell 20"" having a MOST capacitor structure is illustrated in FIG. 6. In the capacitor 24 of FIG. 6, the poly-2 electrode 40 comprises a plurality of spaced apart vertical pillars 43. The capacitor dielectric 42 and poly-3 electrode 44 are then deposited over the spaced apart vertical pillars 43. In this manner a capacitor with a large effective surface area is formed in a DRAM cell.
It is an object of the present invention to provide a technique for manufacture of a MOST capacitor for use in a DRAM cell.