At the early stage of a new technology development, Ground Rule (GR) calculations are performed assuming design targets are met, and all the process variations are within certain process assumptions. However, as technology matures, it is expensive, and time consuming to verify these assumptions for all the designs allowed by the rules. Thus, there’s a loop hole in the GRs if these assumptions are not met on the design. This issue becomes even worse for 2D dense design such as SRAM, where the design target and wafer image are so different due to all the corners, jogs, line ends etc, and the process variations are much larger for the 2D designs. As a result, SRAM designs almost never follow a standard GR approach but its own unique rules. In fact, for SRAM designs of slightly different style, the rules will be significantly different. On the other hand, Process Variation (PV) contours offer much more details of process variations even though their usage is very limited. One reason for that is that off-nominal conditions having larger risks from rule point of view but have lower probability at the same time, making it a dilemma for us. In this talk we propose a method to incorporate PV contours in GR calculation, and each PV contour are used in Monte-Carlo calculation in accordance with its own probability. We apply this method in SRAM layout optimization as an example. This work was performed at the IBM Semiconductor Research Center, Albany NY 12203
This paper presents a design and technology co-optimization (DTCO) study of metal cut formation in the sub-20-nmregime. We propose to form the cuts by applying grapho-epitaxial directed self-assembly. The construction of a DTCO flow is explained and results of a process variation analysis are presented. We examined two different DSA models and evaluated their performance and speed tradeoff. The applicability of each model type in DTCO is discussed and categorized.
The progress of using DSA for metal cut to achieve sub-20nm tip-to-tip (t2t) critical dimension (CD) is reported. Small and uniform t2t CD is very challenging due to lithographic limitation but holds the key to backend-of-the-line (BEOL) scaling. An integration scheme is demonstrated that allows the combination of design flexibility and fine, rectified local CD uniformity (LCDU). The combined effect of LCDU and centroid jittering will be discussed and compared to a hole shrink process using atomic layer deposition and spacer formation. The learning from this case study can provide perspectives that may not have been investigated thoroughly in the past. By including more important elements during DSA process development, such as metal cut, the DSA maturit y can be further advanced and move DSA closer to HVM adoption.
The chemical waste generated in today's microelectronic fabrication processes has driven the need to develop a more
environmentally benign process. Supercritical CO<sub>2</sub> (scCO<sub>2</sub>) has been evaluated as an environmentally friendly solvent for photoresist development. It is nontoxic, nonflammable, and inert under most conditions. It also possesses advantages
such as liquid-like densities, gas-like diffusivity, and zero surface tension. Although scCO<sub>2</sub> is a poor solvent for most
polymers, certain fluorine-and silicon-containing polymers have shown solubility in scCO<sub>2</sub>. Previously, negative-tone
patterns of 100nm have also been developed in scCO<sub>2</sub> using conventional photoresists such as ESCAP and PBOCST
with the aid of fluorinated quaternary ammonium salts (QAS). However, the incorporation of fluorine degrades plasma
etch resistance, and because of their persistence in nature, fluorinated compounds are coming under increased scrutiny.
In order to make the process more environmentally benign, the elimination of fluorine is desirable. Some molecular glass
photoresists without the incorporation of fluorine and silicon have thus been designed and synthesized to be processed in
scCO<sub>2</sub>. In addition to scCO<sub>2</sub>, another environmentally friendly, low VOC solvent, decamethyltetrasiloxane has also been investigated to develop conventional photoresists. In this paper, we demonstrate the patterning of photoresists in both
scCO<sub>2</sub> and decamethyltetrasiloxane.
Chemically amplified photoresists require a post exposure bake (PEB), typically on a hot plate at 90-150°C for 30-120
seconds, to catalytically deprotect the polymer backbone. During PEB, excessive diffusion of the photo-generated acid
results in loss of line edge definition, blurring of latent images and changes in the line edge roughness. Both acid
diffusion and deprotection are thermally activated processes, with the relative rates affected by the time/temperature
profile of the PEB. In this work, we introduce an alternate PEB method involving 500 μs time scale heating over a
temperature range of 130°C to 450°C using a continuous wave CO<sub>2</sub> laser. A methodology is developed for characterizing
this laser PEB and comparing the behavior with conventional hot plate PEB. The thermal stability of several polymer
and photoacid generator (PAG) resist systems were studied and shown to be stable at these high temperatures due to the
short heating duration. Sensitivity of resists under hot plate and laser PEB were measured. Under moderate temperatures,
the laser PEB sensitivity can exceed that of hot plate PEB by an order of magnitude. Quantitative determination of the
acid diffusion was obtained using resist bilayers (PAG loaded / PAG free). Despite the five orders of magnitude
difference in PEB time, systems with l-PEB and hot-plate PEB exhibit comparable imaging quality under deep ultraviolet exposure.
The semiconductor industry is pushing the limits of resolution to sub-30nm through the extension of 193nm lithography
as well as next generation techniques such as EUV lithography. Molecular glass photoresists may provide enhanced
resolution and performance advantages compared to traditional polymeric resists. These organic compounds have a low
molecular weight but still display high glass transition temperatures (Tgs). Enhanced design aspects are employed to
give beneficial resist properties such as transparency, high Tg and etch resistance. Asymmetrical, rigid structures are
used to create amorphous structures with high Tg molecular glasses, such as branched structures and carborane inclusion
complexes. Alicyclic cyclodextrin ring compounds have also been employed for 193nm lithography. Unconventional
atoms such as boron have been incorporated to increase etch resistance while supercritical CO<sub>2</sub> was employed as an
environmentally friendly solvent free developer. Exploring structural aspects and their effect on resist performance is
important in the design of new molecules for next generation lithography and will be discussed.
In order to meet the growing demand for smaller and higher-performance microelectronic devices, attention has been
focused on developing molecular glass photoresists which can be employed under next-generation 193-nm immersion
lithography conditions. These amorphous organic compounds produce high-resolution patterns due to their smaller pixel
size and lack of chain entanglement compared with polymer photoresists. Specially designed molecular resists have
substantial solubilities in supercritical carbon dioxide (scCO<sub>2</sub>) which can be altered through acid-catalyzed deprotection
reactions. While molecular resists based on phenols have been demonstrated for high-resolution patternability, scCO<sub>2</sub>-
developable molecular materials have not yet been reported for 193-nm lithography. In this paper, we introduce alicyclic
materials based on naturally occurring backbones as chemically amplified molecular resists developable in scCO<sub>2</sub>.
Methylated β-cyclodextrin and cholic acid derivatives with acid-labile protecting groups form good amorphous thin
films with high glass transition temperatures (>100 °C). These molecules show the capability of being patterned and
developed in scCO<sub>2</sub> with resolution below 200 nm.
The idea of using small molecules instead of polymers for next generation lithography may enable
improved resolution and line edge roughness (LER). Rather than using polymeric materials we are
focusing on a new class of materials known as molecular glasses (MGs). These are low molecular
weight organic materials that demonstrate high glass transition temperatures despite their modest size.
Unlike polymeric resists, these molecules have the added advantages of distinct size and uniformity.
We have synthesized a series of molecular resists containing rigid aromatic backbones and phenolic
moieties suited for electron beam and Extreme Ultraviolet (EUV) lithography as both positive tone
and negative tone photoresists. An increase in glass transition temperature is observed with increasing
size and rigidity. Glass transition temperatures (Tgs) between 80-130°C have been observed for t-
BOC protected positive tone resists with molecular weights within the range of 800-1200g/mol. MGs
with branched, dense structures are explored to design high Tg resist systems with improved
sensitivity and contrast. The effects of protection ratio, high and low activation protecting groups,
post exposure bake conditions, etch resistance and outgassing have been tested using selected phenolic
MG resists. These new resist architectures are synthesized and evaluated to attain sub 30nm feature
sizes required of candidates for next generation lithography.
Dissolution rates of molecular glass photoresists in supercritical CO2 have been measured with the assistance of an interferometric dissolution rate monitor coupled to the supercritical CO2 vessel. Data show that fully protected polyphenolic molecular glass cores can show dissolution rates >500 nm/min depending on processing conditions. This extends to large branched structures and ring-type molecules approaching molecular weight 2000 g/mol. Molecular glass resists of this type that possess glass transition temperatures above 100oC can be patterned and developed in scCO2 with resolution <65 nm. Using these concepts, positive-tone photoresists based on acid-catalyzed decrosslinking reactions have also been developed. This represents the first report of intrinsically positive tone photoresists developable in pure scCO2.