Susan C. Palmateer

Low‐threshold InGaAs strained‐layer quantum‐well lasers (λ=0.98 μm) with GaInP cladding layers and mass‐transported buried heterostructure

Buried‐heterostructure quantum‐well lasers fabricated by mass transport are reported for In0.18Ga0.82As/GaAs/Ga0.5In0.5P strained‐layer structures grown by atmospheric pressure organometallic vapor‐phase epitaxy. Threshold current densities as low as 85 A/cm2 are measured for broad‐stripe lasers, and buried‐stripe devices show threshold currents as low as 3 mA and differential quantum efficiencies as high as 34% per facet without coatings.

Line-edge roughness in sub-0.18-μm resist patterns

We have characterized line-edge roughness in single-layer, top-surface imaging, bilayer and trilayer resist schemes. The results indicate that in dry developed resists there is inherent line-edge roughness which results from the etch mask, resist (planarizing layer) erosion, and their dependence on plasma etch conditions. In top surface imaging the abruptness of the etch mask, i.e., the silylation contrast, and the silicon content in the silylated areas are the most significant contributors to line-edge roughness. Nevertheless, even in the case of a trilayer, where the SiO2 layer represents the near ideal mask, there is still resist sidewall roughness of the planarizing layer observed which is plasma induced and polymer dependent. The mechanism and magnitude of line-edge roughness are different for different resist schemes, and require specific optimization. Plasma etching of silicon, like O2 dry development, contributes to the final line-edge roughness of patterned features.

Limits to etch resistance for 193-nm single-layer resists

An important aspect of single-layer resist use at 193-nm is the inherently poor etch resistance of the polymers currently under evaluation for use. In order to provide the information necessary for resist process selection at 193 nm, we have projected the ultimate etch resistance possible in 193-nm transparent polymers based on a model we have developed. First, a data base of etch rates was assembled for various alicyclic methacrylates. This data base has been used to develop an empirical structure-property relationship for predicting polymer etch rates relative to novolac-based resist. This relationship takes the functional form normalized rate equals -3.80r3 plus 6.71r2 minus 4.42r plus 2.10, where r is the mass fraction of polymer existing as cyclic carbon. From this analysis, it appears as though methacrylate resists equal in etch resistance to deep UV resists will be possible. Early generations of methacrylate-based 193-nm resists were also evaluated in actual IC process steps, and those results are presented with a brief discussion of how new plasma etch chemistries may be able to further enhance resist etch selectivity.

GaInP mass transport and GaInP/GaAs buried‐heterostructure lasers

Mass transport of a semiconductor alloy has been demonstrated using Ga0.51In0.49P which is lattice matched to GaAs. Buried‐heterostructure diode lasers with Ga0.51In0.49P as cladding and GaAs as the active layer have been made using this fabrication technique. Initial attempts produce devices with room‐temperature lasing thresholds of ∼33 mA and 15% differential power efficiency per facet.

Lithography at a wavelength of 193 nm

The trend in microelectronics toward printing features 0.25 μm 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 requires parallel progress in projection systems, optical materials, and photoresist 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.

Metrology methods for the quantification of edge roughness

As the critical dimension (CD) for semiconductor devices continues to shrink, new thin-layer imaging processes such as bi-layer, Top-Surface Imaging (TSI), Plasma Polymerized Methylsilane (PPMS), and CARL may be required. However, features patterned with these non-traditional processes have inherent high-frequency edge-roughness. If this edge-roughness can not be reduced, it will limit the use of these processes below 0.15 micrometer by reducing process latitude, since the edge-roughness contributes to CD variation and possibly affects device reliability. In order to measure the edge- roughness, a quantitative metrology method needs to be developed. This paper covers the use of a Digital Instruments AFM, a Veeco AFM, and old FE SEM, and a new high resolution SEM for the measurement of the edge-roughness of these patterned features. Quantitative measurements, both in magnitude and spatial frequency are described for each metrology tool. Discussions are made of the parameters that limit the edge-roughness measurement and compared to the parameters that are known to affect CD measurement. Examples of measured edge-roughness are given for a variety of dry developed samples including features processed with an oxide hard mask and TSI. Edge-roughness of chrome features on the reticle, patterned TSI features, and patterned single-layer features are compared to confirm that the higher frequency roughness observed in TSI is not transferred from the reticle.

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.

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. >

Monolithic optoelectronic transistor: A new smart‐pixel device

A new optical switching and logic device, the monolithic optoelectronic transistor (MOET), is demonstrated. The MOET is an integrated circuit consisting of a p‐i‐n photodiode, a resonant tunneling diode, a multiple‐quantum‐well modulator, and a field‐effect transistor. The device can function as an optical inverter or NOR gate. Present devices switch at an input optical power of 12.5 μW and have a large‐signal optical gain exceeding ten. The advantages of the MOET include abrupt switching thresholds, saturated ‘‘on’’ and ‘‘off’’ states, and nonlatching operation.

All-dry resist processes for 193-nm lithography

We report on two different all-dry resist schemes for 193-nm lithography, one negative tone and one positive tone. Our negative tone resist is an extension of our initial work on all-dry photoresists. This scheme employs a bilayer in which the imaging layer is formed by plasma enhanced chemical vapor deposition (PECVD) from tetramethylsilane (TMS) and deposited onto PECVD carbon-based planarizing layers. Figure 1 shows SEMs of dark field and light field octagons patterned in projection on Lincoln Laboratorys 0.5-NA 193-nm Micrascan system. These 0.225-micrometers and 0.200-micrometers line and space features were obtained at a dose of approximately 58 mJ/cm2. Dry development of the exposed resist was accomplished using Cl2 chemistry in a helicon high-ion-density etching tool. Pattern transfer was performed in the helicon tool with oxygen-based chemistries. Recently, we have also developed an all-dry positive-tone silylation photoresist. This photoresist is a PECVD carbon-based polymer which is crosslinked by 193-nm exposure, enabling selective silylation similar to that initially reported by Hartney et al., with spin-applied polymers. In those polymers, for example polyvinylphenol, the silylation site concentration is fixed by the hydroxyl groups on the polymer precursors, thus limiting the silicon uptake per unit volume. With PECVD polymers, the total concentration of silylation sites and their depth can be tailored by varying plasma species as a function of time during the deposition. This affords the possibility of greater silicon uptake per unit volume and better depth control of the silylation profile. Figure 2 shows a SEM of 0.5-micrometers features patterned in plasma deposited silylation resist.