Next week, Intel and Numonyx will present a paper on 3D cell-stacking developments for PCM (phase-change memory) at the IEDM conference in Baltimore, Maryland. The two companies previewed this paper in an announcement a few weeks ago (discussed in this blog here). Just before Thanksgiving, Numonyx presented a Webinar on PCM that contained some excellent background information on PCM. Four decades after its invention—when it appeared on the cover of Electronics magazine—PCM may be about to become a serious challenger to NAND Flash, the current king of nonvolatile memory technologies and the current low-cost leader among all semiconductor memories. These next few blog entries leading up to the Intel/Numonyx paper presentation will elaborate on some of the ideas presented in the pre-Thanksgiving Numonyx Webinar.

PCM manufacture involves introducing “foreign” elements (not silicon) from the periodic table into the IC-manufacturing process. Normally, this is something IC manufacturers avoid at all costs, but the material being introduced is glass—albeit something called chalcogenide (pronounces “kal-KAW-gen-ide”) glass—composed of germanium, antimony, and tellurium. The glass is pretty inert, so it apparently doesn’t present too many contamination problems that would absolutely preclude the material’s use in IC manufacturing.

If you’re an electrical engineer, it’s likely you’ve never heard of chalcogenide glass, but it’s one of the most closely studied materials with one of the highest manufacturing volumes in high tech—just not in electronics. A chalcogenide glass layer is the active component in recordable CDs and DVDs. In its crystalline form, the glass is highly reflective. In its amorphous form, the glass is not so reflective, resulting in a nice, binary, optical-storage mechanism. In an optical disk burner, laser-induced thermal heating switches the glass from one state to the other. A fast, strong laser pulse disrupts a spot of sputtered crystalline material and causes it to become amorphous, reducing its reflectivity. You can see the difference if you look closely at a written disk.

These optical differences between the crystalline and amorphous states are essential to recordable, optical-disk operation but they’re not at all relevant to PCM data storage. However, the chalcogenide glass also has measurably different resistance between the crystalline and amorphous states. The crystalline form of the glass has relatively low resistivity and the amorphous form has higher resistivity, until the glass melts. Now you’re talking memory.

You can see the differential resistivity between the crystalline and amorphous states at low “read” voltages in the figure below. At higher voltages, the glass heats and starts to melt. At that point, the crystalline and amorphous V/I curves merge as the glass starts to soften and melt.

PCM cells exploit this V/I curve, which is conceptually similar to the hysteresis curve for magnetic memories. A fast, high-voltage pulse melts a spot of glass in the PCM cell through Joule heating. Joule heating causes a small amount of chalcogenide material to switch from amorphous to crystalline or back again depending on the size and shape of the write pulse (as shown in the following diagram). Once the pulse is removed, quick cooling allows the glass to solidify in amorphous form. A longer, less intense voltage pulse anneals the glass and puts in the crystalline state.

A PCM cell is pretty simple as shown below. The memory cell resides between a bit electrode and a word electrode. The cell itself consists of a current-limiting/heating resistor and a dot of polycrystalline chalcogenide glass. Current flow through this structure and the amount of that current depends on the voltage impressed on the word and bit lines. When the current is high enough, a region of glass next to the resistor (the dark gray mushroom cap atop the resistor in the figure) starts to melt. The PCM chip’s read/write control circuitry controls the size, shape, and timing of the write pulse.

It’s the simplicity of this mechanism and of the PCM cell design that excites chip makers like Intel, Numonyx, and the other vendors chasing after the 4-decade dream of a new form of semiconductor memory.

However, don’t get the impression that this is a trouble-free memory poised to wipe out all existing semiconductor memories overnight. Won’t happen. If this was simple stuff, PCM would have won the semiconductor memory wars long ago. That obviously didn’t happen. Why? Tune in for the next installment.