Ed’s Threads 070713
Musings by Ed Korczynski on July 13, 2007
High-k, low-k, special-k, super-k
SEMATECH has announced that the R&D; organization
has developed a “super High-k” dielectric for ICs. How “super” can it be at 30-40 k (double the 15-20 k of hafnium oxide)? How easy might it be to integrate? We can’t guess since the material and its properties beyond the dielectric constant remain secret. All we know is that some people want us to call it “super-k” or “SHK”, and I’m against this as title inflation.
As the semiconductor manufacturing industry pushes the limits of CMOS architectures to ever smaller physical dimensions—45nm node production now ramping—materials properties must improve to ensure proper IC function. New materials are used throughout the chip, yet some of the basic terms used to describe these new materials were never standardized. In particular, the dielectric constant (k)—the measure of a material’s polarizability by a passing electromagnetic wave—was formerly kept in a tight range by using only silicon oxide (k~4) and silicon nitride (k~7) films. With 4-7 established as the “medium” range of k by default, anything <4>7 counts as “High-k” (HK). Note that industry convention capitalizes “High” while not capitalizing “low” in these terminologies. Also note that "
k" is properly itallicized but does not always appears as such.
Now 45nm node chips will employ materials with k values ranging from 2.5 to 20, and even lower and higher k materials are under development. Relatively higher k is desired in transistor gates to ensure minimal current leakage when biasing the gate to open the channel, while relatively lower k is desired in intermetal dielectrics (IMD) to ensure minimal coupling and delay to propagating signal pulses.
As the industry has developed low-k dielectrics for IMD, and High-k dielectrics for gates (as well as for memory storage), terminology has been confusing.
Looking first at low-k, the industry first used fluorinated silicon-oxide glass (FSG) with k~3.5, then silicon oxycarbide (SiOC) and silicon-carbon oxyhydride (SiCOH, often pronounced “psycho”) films with k~3.0 for IMD. Since air or vacuum has k of 1, adding pores or gaps to SiCOH as a fraction of the volume proportionally decreases k for the final film. Porous low-k (PKL) films may also be termed ultra low-k (ULK) or extreme low-k (ELK), regardless of where they fall in the 2.0-2.7 range.
Polyimide, benzo-cyclo-butene (BCB), and parylene are all 2.5-3.0 k range films used in passivation and packaging, though they are not commonly termed ULK or ELK. So, for a given chip, it’s possible that a porous SiCOH film of k=2.6 would be termed ULK, while the k=2.6 BCB film used on the same chip is merely “low-k”.
Terminology moving in the other direction was formerly simpler. Starting with k ~7 for silicon nitride as the top end of the “medium” k range, the industry currently uses aluminum oxide and hafnium oxide as HK films in the 8-10 and 15-20 ranges, respectively. Less publicized in recent years but used in volume production nonetheless, ferroelectric RAM (FRAM) fabs use lead-zirconium-titanate (PZT) and barium-strontium-titanate (BST) materials with k values in the 100-300 range. For years, any dielectric with k>7 was simply termed “High.”
Now that SEMATECH wants to call 30-40 the “super” dielectric constant range, what are we to call k>50? Shall we follow the hard-disk drive (HDD) industry terminology for magneto-resistive heads and call PZT films “giant-high-k” and BST films “colossal-high-k” starting now? What about the poor FRAM marketeers who suffered without having these terms to describe their products for so many years—who could they sue for lost brand-value? Why not retroactively inflate terminology for other materials and call graded-SiON and ONO-stacks “special-k?”
In all seriousness, we should employ moderation in terminology, and just call this new material another high-k (HK). In the name of simplification, that to me would indeed be "super."
—E.K.
Labels: dielectric, high-k, IC, interconnect, low-k, metal gate, terms
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070713: High-k, low-k, special-k, super-k
