Capacitor Aspect Ratio Scaling

The storage capacitor’s charge is proportional to the surface area of the inside and outside surfaces of the deep hole. The ratio of the hole’s height to its diameter is the aspect ratio. 2D scaling of the memory cells causes the diameter to shrink and, to maintain adequate surface area and charge, the aspect ratio must increase. To continue 2D scaling, chipmakers keep squeezing the capacitor diameter, which pushes the aspect ratio to the extreme.

While a practical idea, the approach is hitting the physical limits of conventional patterning techniques. Specifically, the capacitor holes are defined with sacrificial hard masks made of amorphous polysilicon which are etched through, creating a stencil for a subsequent etch into thick molds that create the increasingly narrow capacitor holes. 

As high-energy ions etch the capacitor holes, they also eat away at the hard mask. The higher the aspect ratio, the greater the risk that the hard mask is eroded before the capacitor hole is fully formed, which ruins the chip.

One approach to mitigating this risk has been increasing hard mask thickness, but this also creates issues. The aspect ratio of the taller hard mask plus the narrower capacitor hole combined makes it increasingly difficult to complete the etch and remove all of the etch byproducts. These byproducts can cause twisting, bending, under-etching and other physical defects (see Figure 2).

What DRAM makers really need in order to continue scaling is a denser, harder mask material that erodes at a substantially slower rate than the underlying capacitor mold. This thinner hard mask would allow deeper etches along with byproduct removal for perfectly cylindrical and straight capacitor holes with evenly matched depths.

Periphery Circuitry Scaling

A second opportunity to continue DRAM shrinking is to reduce the size of the peripheral circuitry area which consists of the logic transistors and interconnect wiring that surrounds the DRAM cell array (see Figure 3). If the memory cells scale but the periphery circuitry doesn’t, then the periphery takes up an increasing percentage of the die size. The logic is important: it helps determine the performance and power consumption of today’s high-speed DDR4 and emerging DDR5 DRAMs. Each of the metal lines that connect the logic transistors to the cell areas needs to be surrounded by an insulating dielectric material to prevent interference between electrical signals, and the thickness of this dielectric is another key factor impacting scaling. For the past 25 years, DRAM makers have used one of two silicon oxides—silane and tetraethoxysilane (TEOS)—as the dielectric material.

Today, DRAM makers are facing the same dielectric scaling challenges faced by leading-edge logic makers around 20 years ago. Beyond a certain point, thinning the dielectrics brings the metal lines so close together that capacitive coupling increases. As a result, the device suffers from increased power consumption, reduced performance, excess heat and potential reliability issues. In logic scaling, the solution was new copper low-k interconnects. In DRAM, the time for a new material that allows metal lines to be placed closer together without causing signal interference is now.

New Materials Research

These DRAM scaling materials engineering challenges—involving patterning hard masks and insulating dielectrics—have been the subject of intense research. In my next blog, I’ll discuss new materials that Applied Materials has been developing to help DRAM makers extend their product roadmaps to meet the world’s growing need for affordable, high-performance DRAMs.

Tags: DRAM, scaling, PPACt, interconnect, hard mask, etch, new ways to shrink