Mark W. Horn

Lithography with 157 nm lasers

Projection photolithography at 157 nm was studied as a possible extension of current 248-nm and planned 193-nm technologies. At 157 nm, lasers are available with ∼8 W average power. Their line width is narrow enough as to enable the use of catadioptric, and maybe all-refractive optics similar to those used at 248 and 193 nm. The practicality of such designs is further enhanced by measurements of calcium fluoride, which show that its absorption is sufficiently small (∼0.004 cm−1) at 157 nm. Binary masks with chromium and chromeless phase shifting masks were fabricated on calcium fluoride as the transparent substrate. Robust photoresists at 157 nm still need to be developed, and they probably will be of the top surface imaging or bilayer type. Indeed, a silylation resist process was shown to have characteristics at 157 nm similar to those at 193 nm. The calcium fluoride based masks were integrated with the silylation process and a home-built, small-field, 0.5-numerical aperture stepper to provide projection...

A new method for measuring thermal conductivity of thin films

A new, relatively inexpensive, easy-to-use instrument has been developed for measuring the thermal conductivity of thin films based on a differential photoacoustic method. Measurements made on silicon dioxide and silicon nitride are consistent with those reported previously for a different technique. In addition, the room temperature thermal conductivity of conventional polymer thin films and plasma deposited thin films has been determined relative to thermally grown silicon dioxide. Knowledge of the thermal conductivity of thin films, which is critical for many applications, can now be obtained for any thin film which can be deposited on a high thermal conductivity substrate.

Plasma‐deposited organosilicon thin films as dry resists for deep ultraviolet lithography

Organosilicon thin films have been formed by plasma enhanced chemical vapor deposition and employed as deep‐UV photoresists. Films 20–200 nm thick were deposited from liquid organosilicon sources onto carbon‐based planarizing layers and patterned in projection with a 193 nm excimer laser. At fluences below ∼5 mJ/cm2/pulse, exposure to 193 nm radiation induced oxygen incorporation into the film. Following either a wet or dry development step, negative‐tone imaging was achieved, with the remaining photooxidized film being highly resistant to the O2 reactive ion etching employed in the subsequent pattern transfer step. At higher fluences, ∼15 mJ/cm2, positive‐tone imaging by self‐development was obtained for single‐pulse exposures.

Photolithography at 193 nm

Photolithography at 193 nm is a natural continuation of the progression from 436 to 365 to 248 nm in lithography, dictated by the requirement for continually higher resolution. It is anticipated that 193‐nm lithography will enable 0.25‐μm patterning in volume production with conventional masks, and 0.18‐μm resolution with phase‐shifting masks. The main issues related to lithography at this new wavelength are being addressed. It has been shown that highly transparent optical materials are available at 193 nm. Also, they are damaged by the laser radiation at a slow enough rate that high‐quality projection optics are expected to perform within specifications for ten years of full‐time operation. Consequently, a 193‐nm step‐and‐scan system is being constructed, and it has been designed to attain 0.25‐μm resolution over a 22 by 35 mm field. A range of 193‐nm photoresist schemes has been demonstrated. They include semitransparent single‐layer resists, positive‐tone surface imaging (silylation), and negative‐ton...

Aluminum oxides as imaging materials for 193‐nm excimer laser lithography

Aluminum oxide films deposited over amorphous carbon(a‐C:H) planarizing layers have been investigated as imaging layers for 193‐nm excimer laser lithography. The AlOx films were deposited by ion beam deposition, e‐beam evaporation, or sputter deposition. The films have been analyzed by Auger electron spectroscopy and x‐ray photoelectron spectroscopy. Depending on the deposition conditions, AlOx films with compositions varying from metallic to fully oxidized Al can be formed. The optical appearance of these films varies from highly reflective for metallic Al to highly transparent for fully oxidized layers. Using pulsed 193‐nm radiation from an ArF excimer laser, the single‐pulse self‐development threshold energy is similar for films with different compositions when the film reflectivity is taken into account. It was found that pure Al films and highly oxidized Al films do not provide good adhesion on a‐C:H layers and have a tendency to peel during laser exposure. In contrast, films with intermediate compos...

Optimization of a 193-nm silylation process for sub-0.25-um lithography

We have optimized a positive-tone silylation process using polyvinylphenol resist and dimethylsilyldimethylamine as the silylating agent. Imaging quality and process latitude have been evaluated at 193 nm using a 0.5-NA SVGL prototype exposure system. A low- temperature dry etch process was developed that produces vertical resist profiles resulting in large exposure and defocus latitudes, linearity of gratings down to 0.175 micrometers , and resolution of 0.15-micrometers gratings and isolated lines.

Polysilyne thin films as resists for deep ultraviolet lithography

A new class of organosilane polymers, the polysilynes, have been examined as resist materials for deep ultraviolet lithography. These polymers have the formula [SiR]n, and polymers with R=n‐propyl, n‐butyl, isobutyl, amyl, cyclohexyl, and phenyl have been examined. Photolysis of polysilyne films with 193, 215, or 254 nm light results in the formation of an organosiloxane. Latent images of this oxidized material can be selectively developed yielding a negative tone image either by wet development in toluene or dry development in a Cl2 or HBr plasma. The best sensitivities at 193 nm were 20 mJ/cm2 for wet developed poly(n‐butylsilyne) and 200 mJ/cm2 for dry developed poly(phenylsilyne). Positive tone wet development in polar solvents, such as acetone, is incomplete and yields a thin, insoluble residue layer.

193-nm lithography

The trend in microelectronics toward printing features 0.25 /spl mu/m and below has motivated the development of lithography at the 193-nm wavelength of argon fluoride excimer lasers. This technology is in its early stages, but a picture is emerging of its strengths and limitations. The change in wavelength from 248 to 193 nm will require parallel progress in projection systems, optical materials, and photo-resist chemistries and processes. This paper reviews the current status of these various topics as they have been engineered under a multiyear program at MIT Lincoln Laboratory. >

Profile control in dry development of high‐aspect‐ratio resist structures

Anisotropic etching for dry development of thick resist layers in oxygen‐based plasmas has been developed for two particular applications: trilayer resist for planarization of steep binary optics structures, and top surface imaged (silylated) resists for deep submicron lithography. To reduce linewidth degradation during etching of high‐aspect‐ratio resist structures, lateral etching and bow are minimized in two different reactors, a conventional parallel‐plate reactive ion etching (RIE) system and a low‐pressure, high‐ion‐density helicon etcher. In the conventional RIE system, C5H8, a polymer‐forming gas is added to the O2 feed gas to form a sidewall inhibition layer. In the helicon reactor, very low temperatures (−100 °C) are used to suppress sidewall etching. Different optimization conditions are found for trilayer resist etching and pattern transfer of silylated resist in the helicon reactor.

Wet‐developed bilayer resists for 193‐nm excimer laser lithography

A high‐contrast resist process using polysilynes has been developed for 193‐nm excimer laser lithography. Copolymerization allows for control of both polymer molecular weight and the net polymer solubility parameter. Optimal formulations yield sensitivities of 35–60 mJ/cm2 and line‐edge roughness of ≤20 nm. Addition of sensitizers into the resist further improves sensitivity and values from 5 to 30 mJ/cm2 have been demonstrated. Use of high‐density, low‐bias etching sources for the oxygen‐plasma pattern transfer improves process windows. For example, etch rate selectivities of 80:1 for the planarizing layer versus the polysilyne imaging layer have been observed even when the planarizing layer etch rate exceeds 1 μm/min. Under these conditions, the exposure latitude is 40% for k1=0.57 and the development latitude is 100% (20±10 s) for 10% linewidth control.