Chunwei Chen

Large area interposer lithography

Large area silicon or glass interposers may exceed the maximum imaging field of step and repeat lithography tools. This paper discusses the lithographic process used to create a large area interposer on a stepper by the combination of multiple subfield exposures. Overlay metrology structures are used to confirm the relative placement of the subfields to construct the interposer. Routing lines from 1.5 to 4.0 μm in width are evaluated to measure critical dimension (CD) control where the lines cross the subfield boundaries. CD metrology at the bottom and top of the photoresist is performed using a top down CD-SEM. Finally large area test interposers are patterned using two subfields on a 1X stepper and processed through a Cu electroplating module for detailed characterization. The CD control of routing lines as they cross the subfield boundary can be optimized by using a shaped or tapered line end design. Lithography simulation using Prolith modeling software by KLA-Tencor is matched to experimental results and then used to evaluate performance of various line end designs. Larger latitude for overlap error was observed for the tapered line end compared to the standard square line end. The experimental and modeled results in this study show the capability of using stepper lithography to produce large area interposers with 1.5 μm I/O routing line dimensions.

Development of thick negative photoresists for electroplating applications

We report about the development of a thick negative photoresist series, AZ(R) EXP 125nXT, and their use in electroplating levels up to 160 μm thickness. The new photoresist series enables coatings of 5-120 μm with acceptable uniformity and edge bead in a single coat step. 200 μm photoresist coating was achieved by a double coating processes. The lithographic performance of the photoresists was evaluated using broad band aligners and steppers. Optimized lithographic parameters to achieve straight and nearly vertical side wall profiles are reported. The photoresists show not only excellent adhesion to copper with no surface treatment and electroplating tolerance in a variety of metal plating solutions, but is also compatible with silicon and gold substrates. The photoresists have been found to be easily stripped with no residues in solvent based stripper solutions.

Chemically amplified thick film i-line positive resist for electroplating and redistribution applications

Adapting chemically amplified (CA) resist technology to thick film applications is demonstrated in this paper over a wide range of thicknesses and types of substrates. Substantial performance differences were observed over copper (Cu) substrates compared to silicon (Si). These differences are attributed to different photo acid generator (PAG) distribution in the resist depth influenced by its structure and the nature of the substrate. Optimized resist formulations were developed to provide acceptable performance on Cu wafers. A family of new chemically amplified thick film resist products is being introduced to the market. This technology offers significant advantages in throughput and performance over conventional novolak / diazonaphthoquinone (DNQ) products at a competitive cost.

A study on post-exposure delay of negative tone resist and its chemistry

Exceptional post exposure delay (PED), CD stability, up to 72 hours was reported. This study was conducted using two negative resist formulations identical in their composition except for their PAG type. A mechanism by which the photoacid is protected from relatively moderate levels of airborne amines is proposed. Evidence of room temperature interaction between the resist components and the acid during post exposure delay was also suggested. Therefore, the PED outcome could be the result of two opposing mechanisms.

Performance comparison of negative resists for copper rerouting and other electroplating applications

Two types of chemically amplified (CA) negative resists were compared lithographically. An acid catalyzed resist and a photopolymerizable type resist. The optimum lithographic performance of the acid catalyzed resist on Cu is in the thickness range below 15μm, with vertical profiles. This resist exhibits inverted profiles on Cu above 15μm of thickness. The Photopolymer type resist performs best above 25μm thickness, and can be used for 120μm thick applications with single coat. Top line rounding is more observed with this resist as its applied thickness is reduced below 20μm. This effect is believed to be related to oxygen uptake in the resist surface. Thus it has a more pronounced effect at relatively thinner films. Both resists are compatible with the electroplating process.

Development of nanocomposite resists with high plasma etch resistance

We report about the development of novel nanocomposite resists that incorporate colloidal silica nanoparticles into conventional resist materials to yield thick coatings with both excellent lithographic properties and significantly increased plasma etch resistance. 10-50 wt% silica nanoparticles of 10-15 nm in size were dispersed homogeneously in a variety of standard resist resins by a simple process. The nanocomposite resists have similar lithographic performances to conventional resists without silica nanoparticles. The nanocomposite resists also show excellent process window capability and stability. Oxygen plasma etch and deep reactive ion etching (DRIE) processes were used to evaluate the etch resistance of the nanocomposite resists. Compared with standard photoresists, the oxygen plasma etch rate is reduced by 38-80% when the silica content increases from 20 to 50 wt%. The etch selectivity of nanocomposite resists with 40 wt% silica is increased by 70% in DRIE test.

PAG distribution and acid thermal diffusion study in ultra-thick chemically amplified resist films

The introduction of chemically amplified (CA) resist technology to thick films, 10 to 100 um in thickness introduced a number of behavior differences not experienced in thinner films to the same magnitudes. Resist image profile deformation, insensitivity to standing waves and the reduction in polymer deblocking temperatures are significantly affected in thick films to a larger extend than in thinner films. The major contributing factors to these differences are discussed in this paper: 1) the influence of photo-acid generator (PAG) structure on its distribution in resist depth on Cu substrates and 2) thermal acid diffusion, influenced by greater amounts of retained solvents in thick films than in thinner films.