Michael J. Lercel

Self‐assembled monolayer electron‐beam resists on GaAs and SiO2

It was demonstrated that self‐assembled monolayers of n‐octadecanethiol [ODT; CH3(CH2)17SH] on GaAs and n‐octadecyltrichlorosilane [OTS; CH3(CH2)17SiCl3] on SiO2 act as self‐developing positive electron beam resists with electron‐beam sensitivities of ∼100–200 μC/cm2. For the OTS monolayer on a silicon native oxide, atomic force microscopy (AFM) images of the exposed layer before etching demonstrate the removal of all or part of the layer upon electron‐beam exposure. Features as small as 25 nm were resolvable in a 50 nm period grating. A resist contrast curve for OTS was obtained from AFM depth measurements as a function of dose. An ammonium hydroxide water etch was used to transfer patterns into the GaAs to a depth of at least 30 nm and buffered HF was used for pattern transfer into the SiO2 to a depth of at least 50 nm.

Electron beam lithography with monolayers of alkylthiols and alkylsiloxanes

Self‐assembled monolayers have been modified with focused electron beams of energy 1–50 keV and scanning tunneling microscopy (STM) based lithography with energies of ∼10 eV. Modifications ∼15 nm in size have been formed by STM and ∼25 nm in size by 50 keV beams. The fact that these materials work as self‐developing electron beam resists is demonstrated by both atomic force microscopy imaging and pattern transfer using conventional wet etchants. Patterns have been transferred to silicon substrates to a depth of ≳120 nm with a multistep wet etching process. The mechanism of electron beam modification has also been explored to better design future monolayer processes.

High‐resolution silicon patterning with self‐assembled monolayer resists

Single crystal and polycrystalline silicon films have been patterned and etched with a novel high‐selectivity process using self‐assembled monolayer resists of octadecylsiloxanes (ODS). The highest resolution patterning of sub‐10 nm features has been demonstrated by scanning force microscopy imaging of ODS layers patterned with a focused electron beam. An all‐dry UV/ozone developer has been used to remove residual carbon from the electron beam exposed regions to improve etch selectivity. The positive tone pattern transfer process consisted of a short buffered hydrofluoric acid wet etch to remove the silicon native oxide followed by a high‐selectivity, low ion energy etch using Cl2 and BCl3 in an electron cyclotron resonance reactive ion etch. Features have been etched up to 90 nm deep into Si(100) wafers and minimum feature sizes obtained are ∼25 nm. Poly‐Si films on SiO2 insulator layers have been similarly patterned and have been used in a combined process with photolithographic definition of microbridg...

Plasma etching with self‐assembled monolayer masks for nanostructure fabrication

Self‐assembled monolayers of octadecylthiol on GaAs and octadecylsiloxanes on titanium, aluminum, and silicon have been used as electron beam resists for plasma etching into the substrates. An electron cyclotron resonance source was used to excite a low‐pressure, high‐density plasma with low ion energy to achieve high selectivity with the thin masking layers. Patterning of monolayers on GaAs usually produced a negative tone, but etching of the metal surfaces resulted in positive tone patterning. The maximum etch depth into the GaAs was ∼100 nm using the negative tone process. Lines have been etched into Ti with linewidths down to ∼20 nm. The negative tone process can be explained by the cross linking of the monolayer under high‐dose electron beam exposure; however, the positive tone process must rely on contrast either from different etching characteristics of the oxides or different structural arrangements of the different SAMs.

Etching processes and characteristics for the fabrication of refractory x-ray masks

Refractory x-ray masks for a wide variety of pattern types were fabricated using tantalum silicon as the absorber material. Both positive (Shipley UVIII®) and negative (Shipley SNR200®) chemically amplified electron beam resists were exposed and the patterns transferred into a silicon oxynitride hardmask. The amorphous TaSi absorber was then etched using a Cl2/O2 reactive ion etch (RIE). From a mask manufacturing standpoint, the challenge is etching the wide variety of feature types that commonly occur in device processing. The overall etch process was characterized for the formation of both freestanding lines (using negative electron beam resist) and narrow trenches (using positive resist). RIE lag, feature shape dependence, and cross-mask uniformity in the etch bias were characterized for feature sizes down to 125 nm. The etch process has been implemented in a pilot line environment and is being used to produce product masks.

Simulating the effects of pattern density gradients on electron-beam projection lithography pattern transfer distortions

The development of a low-distortion mask is critical to the success of the sub-0.1 μm lithography technologies. Electron-beam projection lithography (EPL) is one of the potential candidates for next-generation lithography. In order to minimize mask image placement (IP) errors, it is important to understand the factors that induce pattern distortions during mask fabrication and pattern transfer. The fabrication process flows for two EPL mask formats were numerically simulated and experimentally assessed for IP. This study included continuous membranes and stencil membranes for 1 mm ×1 mm and 1 mm ×12 mm window sizes on a 4 in. wafer. Both intramembrane (i.e., within a single window) and intermembrane (i.e., cross-mask) results are reported with excellent correlation between the finite element (FE) data and the experimental measurements. In this article details of the FE simulations are presented; an article by (M. Lercel et al., J. Vac. Sci. Technol. B, these proceedings) describes the corresponding experi...

Patterning-induced image placement distortions on electron beam projection lithography membrane masks

Membrane masks are needed for charged particle lithography and can include both stencil masks and masks with thin continuous membranes. Producing accurate image placement on membrane masks requires careful control of mask shape, pattern writing, and stress control of the mask materials. Pattern density and pattern density gradients also affect image placement (IP) control. This article discusses IP distortions on electron projection lithography masks caused by patterning the imaging layers with low and high density patterns and patterns with large gradients in the density. The process-induced distortion has been found to be largest with the largest vector distortion at the boundary when high pattern density gradients are present. The anisotropic stiffness of the unit cell also affects the process-induced distortion. Qualitatively, the results between continuous membrane and stencil masks show similar characters. The results provide distortion information that could be used to determine the maximum allowab...

Imaging capabilities of proximity X-ray lithography at 70 nm ground rules

This paper discusses the resolution capabilities of proximity x-ray lithography (PXRL) system. Exposure characteristics of features designed at 150 nm pitch size: 75 nm dense lines with 1:1 duty ratio, 2D features at 1:1 and 1:2 duty ratios and isolated lines have been studied. Aerial image simulations were compared to the experimental data. Verification of the aerial image model has been accomplished by measurements of exposure windows of 100 nm and 125 nm nested lines. The PXRL aerial image parameter, equivalent penumbra blur, has been determined from the experimental data. Contributions from the synchrotron radiation x-ray source, stepper and the chemically amplified resist to the degradation of the aerial image have been evaluated. Patterning capability of PXRL at 75 nm feature size is compared to projection optics using the optical k1 factor as a common figure of merit. To facilitate the comparison, optical imagin was at pattern sizes currently manufacturable by the mainstream optical tools while the PXRL imaging was at 75 nm pattern size. Requirements for a PXRL system of manufacturing VLSI at 70 nm minimum feature sizes with the critical dimension control better than 10 percent are also discussed.

Image size control in next generation lithography masks

Mask image size variation is a major contributor to the total image size budget. To understand the source and contribution of various errors, we have characterized the image size variations on next generation lithography masks. CD control experiments initiated on x-ray masks are now being extended to other NGL technologies through the application of similar patterns, measurement strategy, and error budget partitioning. A systematic measurement methodology has been used to partition the variations into known components. Long-range variations have been found to be the dominant error, and in x- ray masks, are typically membrane edge effects and cross-mask bow. The membrane effects have been shown to be primarily driven by temperature differences during the post-expose bake (PEB) of the chemically amplified resist. To further understand the source of these temperature variations, the x- ray and SCALPEL mask PEB have been modeled through a finite- difference model. Key contributors to controlling bake temperature uniformity have been identified.

Next-generation lithography mask development at the NGL Mask Center of Competency

Mask fabrication is one of the difficult challenges with all Next Generation Lithography (NGL) technologies. X-ray, e-beam projection, and ion-beam projection lithography all use some form of membrane mask, and extreme ultraviolet (EUV) lithography uses a reflective mask. Despite some differences, the various mask technologies share some common features and present similar fabrication difficulties. Over the past several years, the IBM Advanced Mask Facility (AMF) has focused on the fabrication of x-ray masks. Several key accomplishments have been demonstrated including fabricating masks with critical dimensions (CD) as small as 75 nm, producing line monitor masks in a pilot line mode to evaluate mask yields, and fabricating masks to make working microprocessors with the gate level defined by x-ray lithography. The experience on fabricating 1X x-ray masks is now being applied to the other NGL mask technologies. Progress on membrane and absorber materials can be applied to all the technologies, and patterning with advanced e-beam writing with chemically amplified resists utilizes learning from writing and baking on x-ray membrane masks.