Making a good impression: Nanoimprint lithography tests at NIST

Apr 29, 2008
Making a good impression: Nanoimprint lithography tests at NIST
Electron micrograph shows a cross-section of a typical SOG microcircuit feature. Nanoporous regions in the interior are lighter. The process forms a dense, stronger skin about 2 nanometers thick on the outside. (Color added for clarity.) Credit: NIST

In what should be good news for integrated circuit manufacturers, recent studies by the National Institute of Standards and Technology have helped resolve two important questions about an emerging microcircuit manufacturing technology called nanoimprint lithography—yes, it can accurately stamp delicate insulating structures on advanced microchips, and, no, it doesn’t damage them, in fact it makes them better.

An emerging manufacturing technique, nanoimprint lithography (NIL) is basically an embossing process. A stamp with a nanoscale pattern in its surface is pressed into a soft film on the surface of a semiconductor wafer. The film is hardened, usually by heating or exposure to ultraviolet light, and the film retains the impressed pattern from the stamp. The process is astonishingly accurate. NIL has been used to create features as small as ten nanometers across with relatively complex shapes.

NIL is being eyed in particular for building the complexly patterned insulating layers sandwiched between layers of logic devices in future generations of integrated circuits. State-of-the-art semiconductors contain over a billion transistors, packed together into a footprint of silicon that is no bigger than a few square centimeters. Several miles of nanoscale copper wiring are required to connect the devices, and these wires must be separated by a highly efficient insulator.

One candidate is a porous glassy material called SOG* that can be applied as a thin fluid film. When heated, SOG turns into a thin glass film laced with nanometer pores that enhance the electrical insulation. But SOG is relatively delicate, and the conventional photoresist etching process used to cut trenches for the wiring can compromise it. NIL, on the other hand, might be able to pattern SOG layers with wiring trenches and eliminate several time-consuming and expensive photolithography steps if it could pattern the film accurately and do so without destroying the delicate nanopore lacework.

In a paper published last fall,** NIST materials scientists addressed the first question. Using sensitive X-ray measurements they demonstrated that NIL could be used on a functional SOG material to transfer patterns with details finer than 100 nanometers with minimal distortion due to the processing. In a new paper this month,*** they extend this work to study the effect of the embossing process on the nanopore structure in the glass. Using a combination of techniques to measure the distribution of nanopores in the insulator material, they found that the NIL embossing process actually has a beneficial effect—it increases the population of small pores, which improve performance, reduces the population of larger pores that can cause problems and creates a thin, dense protective skin across the surface of the material. All of these effects are highly attractive for minimizing short circuits in semiconductor devices.

Taken together, the two papers suggest that nanoimprint lithography can produce superior nanoporous insulator layers in advanced semiconductor devices with significantly fewer—and easier—processing steps than conventional lithography.

* “Spin-on organosilicate glass”

** H.W. Ro, R.L. Jones, H. Peng, D.R. Hines, H-J. Lee, E.K. Lin, A. Karim, D.Y. Yoon, D.W. Gidley and C.L. Soles. The direct patterning of nanoporous interlayer dielectric insulator films by nanoimprint lithography. Advanced Materials. 2007, 19, 2919–2924.

*** H.W. Ro, H. Peng, K.-i. Niihara, H.-J. Lee, E.K. Lin, A. Karim, D.W. Gidley, H. Jinnai, D.Y. Yoon and C.L. Soles. Self-sealing of nanoporous low dielectric constant patterns fabricated by nanoimprint lithography. Advanced Materials 2008, Early View: April 15, 2008.

Source: National Institute of Standards and Technology

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guiding_light
5 / 5 (2) May 02, 2008
The test doesn't address if you needed to imprint a layer on top of another layer of already patterned porous material. If you imprint the top layer, it would dent the underlying layer (smush its pores basically). HSQ is a porous low-k dielectric, also used as a nanoimprint resist.
plasma_guy
5 / 5 (2) May 02, 2008
Sponge on top of sponge effect?
guiding_light
5 / 5 (2) May 02, 2008
Sponge on top of sponge effect?


Yes, exactly. Imprint assumes a stiff support under the imprinted layer. If the support is spongy, there is less resistance to the imprint force.
gongii
5 / 5 (1) May 02, 2008
It makes sense, even chemical-mechanical polishing (CMP) has a concern when applied to ultra-low-k (ULK) materials, for pretty much the same reasons.

Slurry Development for Cu/Ultra Low-k CMP

Author(s):
Hugh Li, Matt VanHaneham, and John Quanci
Slurry R&D , Rodel
United States

Conventional CMP for Cu/Ultra-low k (k
gongii
5 / 5 (1) May 02, 2008
Conventional CMP for Cu/Ultra-low k integration faces significant technical challenges. The majority of ULK materials are made porous to reduce the dielectric constant, while trading off on the mechanical strength. With diminished hardness, elasticity and adhesion, the CMP process has to be %u201Ckinder and gentler%u201D: lower down force, lower relative velocity, softer pad, and slurry with lower abrasive content. In a word, the mechanical portion of the planarization process would be greatly reduced. To maintain the same performance, one has to rely on the chemical reactions to make the Cu/ULK CMP a viable process. In addition to the fragile nature of these ULK materials, topography correction becomes very important as more metal layers are built to achieve the performance requirement. Without topography correction, residual metal and depth of focus at upper layers become formidable threats to final yield. Therefore, it is desirable to design adjustable removal rates into the Cu/ULK CMP process to reduce dishing and recess. Because of the variety of ULK materials, the Cu/ULK integration schemes also differ, creating the need for tunable selectivities in the CMP slurry design. In developing barrier slurries for Cu/ULK CMP, we considered both mechanical and chemical aspects: gentle polish and tunable selectivities. Since film delamination is essentially an adhesion breakdown, the shear stress was estimated by measuring the in-situ frictional force during polishing, which in turn guides us in slurry design. To compensate for diminished mechanical force and to obtain tunable selectivities, chemically active ingredients are added in the slurry to independently enhance and control material removal rates. After development of surface response models, it is possible to quickly find a slurry formulation for a specific integration scheme.
plasma_guy
not rated yet May 08, 2008
I believe nanoimprint does not go all the way through the layer but leaves a residual layer, which must then be etched. Plasma damage for porous nanoimprintable materials is pretty bad.