Letters from Readers

03/01/2002

"Quick-and-dirty" metrology earns kudos
I was happy to see your Jan. 2002 editorial ("Time for 'quick-and-dirty' metrology?" p. 12) regarding the need for in situ metrology, and the likely cost implications. There's one point I felt you didn't quite drive home. The key to effective in situ metrology is to find NEW ways of monitoring the tool's performance that are LOW cost. Often the tool architecture must be changed, too. When done this way, the benefits to the toolmaker far exceed the cost for such solutions. I continue to be amazed that the whole IC production line hasn't gone to in situ controls. You are right when you say they need to do so.

Getting in situ sensing and feedback incorporated into a process tool is really the toolmakers' responsibility. The technology can't be economically bought and patched on to a tool. The off-line technologies are too expensive, as you say, and are NOT a good solution for the real-time control needs of in situ sensing and feedback.

I'll offer a crude example. For many years, focus in steppers was monitored using focus-exposure wafers and measuring these off line. The built-in "auto-focus," which is really a monitor of lens to wafer gap, is low cost, relies on a glancing light beam, and bears no resemblance to the off-line method. The architecture change was to add a programmable and servo-controlled z motion to drive to best focus. Some put this into the stage, others into the column structure. No one bought a CD-measuring video system and tried to patch it onto a stepper for real-time control of camera focus.

Later at MRS Technology, we built a simple detector and knife-edge (slit) sensor into our stages. This unit scans for best contrast in the image, working with a knife-edge image from the mask that projects through the main lens. 4000 x, y, and z locations were measured in under 45 min, and a complex best-fit surface was calculated to find the lens best focus. The cost of the parts was minimal. But the stage and its travel have to be designed from the start to include the sensor unit and enough travel to reach the four corners of the lens field. This simple in situ system became the main method for checking lens quality. We gained savings in system integration and installation speed, which were unexpected benefits.

If you write another piece on in situ metrology and feedback, please prod the lingering toolmakers more. It really is their job. They'll see many benefits.

Griffith L. Resor III, president, Resor Associates

Early work on atomic layer deposition cited
We would like to comment on the article "Assault on ITRS roadblocks led by atomic layer deposition," Jan. 2001, pp. 70-72. Some sources mentioned atomic layer deposition (ALD) or epitaxy (ALE), which we contend is based on the principles of molecular layer (ML) technology. Actually, ALD is a ML method with a different name. The statement "...Tuomo Suntola invented atomic layer epitaxy some 20 years ago" in the section "Emergence of atomic layer processes," by Ivo Raaijmakers, CTO, ASM International, p. 70, is in error.

The first implementors of the new method, then called "molecular layering," were V.B. Aleskovskii and S.I. Koltsov, as described in an article published in 1974 (V.B. Aleskovskii, "Chemistry and technology of solids," Zhurnal Prikladnoi Khimii, Vol. 47, No. 10, pp. 2145-2157, Oct. 1974. Original article submitted April 8, 1974.). The first publication on this new technology appeared in the USSR (in Russia) in 1965 (see Ref. 48 in the literature cited).

The patent of the Finnish scientists appeared in 1978. By 1978, however, in Russia, there were many publications about the new molecular layering technology (some of them are referenced in the literature cited above, including Refs. 48, 49, 51, 52, 53, 55, 61, 71, 78, etc.). Detailed information and a chronology of publications about the molecular layering method were also included in the monograph of V.B. Aleskovskii, A Stoichiometry and Synthesis of Solid Linkings, (Leningrad, Publishing house "Science" 1976); in the textbook of V.B. Aleskovskii, Chemistry of Solids (Moscow, Publishing house "Higher education" 1978); in an article by A.A. Malygin, A.A. Malkov, S.D. Dubrovenskii, "The chemical basis of surface modification technology of silica and alumina by molecular layering method," in the book Adsorption on New and Modified Inorganic Sorbents, ed. A. Dabrowski, V.A. Tertykh; in the series Studies in Surface Science and Catalysis, Amsterdam, The Netherlands, Elsevier, Vol. 99, pp. 213-237, 1996); etc.

Please note that the Finnish authors (Tuomo Suntola, etc.) and other scientists have information about prior Russian publications.

A.A. Malygin, professor and head, Chemical Technology of Materials for Electronic Instruments Department, and

V.M. Smirnov, professor, Chemistry of Solids Department, St. Petersburg State Technological Institute, St. Petersburg, Russia

ASM International chooses not to respond formally at this time, but maintains the correctness of the statements made in the article.