Extending Optical Lithography with the CARL Process

 RuthDeJule
 Associate Editor

A two-layer photoresist technology, chemical amplification of resist lines (CARL, Fig.1), is a process developed and trademarked by Siemens AG (Erlangen, Germany) that extendsthe capability of i-line lithography tools to critical dimension's below 0.20 µm. Yieldsand throughput comparable to standard resist technology have been demonstrated in 4Mb DRAMproduction at Siemens. With an i-line, 0.6 NA stepper, contact holes of 0.2 µm (Fig. 2)and l/s of 0.22 µm have been demonstrated using the CARL technique. Smaller features arepossible if phase shift masks or other optical enhancements are used, noted Dr. MichaelMeier, technology manager at Clariant AG (Wiesbaden, Germany). The CARL process isdesigned to implement the best of two well-known techniques: top surface imaging (TSI) andbi-layer resist techniques.   

TSI uses a latent image in the first 100 nm of the resist. This attractive techniqueincreases DOF, planarizes the surface and provides reflection control in a single,arbitrarily thick resist film. Subtle yet significant disadvantages occur, however, whensilicon is introduced, causing loss of CD control. In the presence of a latent image, thesilicon, by way of a gas phase silylation process, causes the volume of the system tochange. Features begin to swell, forming hillocks in the irradiated areas and imagetapering to "bird's beaks." The taper, in particular, makes it extremelydifficult to control the dimensions in the dry etch process.

Classic bi-layer systems optimize the top layer resist for imaging properties and thebottom layer for dry etch resistance. Therefore, an oxygen dry etch can be employed. Butwhile no silylation process is used, the resist itself contains >10% silicon, whichconverts to a silicon oxide, glass-like etch barrier. After exposure of the top layer, thewafer is developed in an aqueous solution. The hydrophobic nature of silicon can createdissolution problems, resulting in resolution limitations, poor sidewalls and potentialscumming from remnants of the dissolution.

The CARL process essentially uses a silylation process to eliminate the need to put allthe silicon into the resist film and uses a thin top layer to take advantage of thin layerimaging.

Fig. 1. The CARL process combines a silylation process with a thin top layer to take advantage of thin layer imaging.

Fig. 2. A 4Mb DRAM gate structure on high topography shows 0.20 µm contact holes using i-line exposure and standard illumination. Fig. 3. The chemically amplified system is sensitive at both 248 nm and 193 nm and is capable of resolving 0.12 µm structures.

Eliminating the need for fine process control and dedicated equipment, CARL's liquidsilylation process allows standard coating tracks to be used and silylation parameters tobe tuned to match different linewidth bias requirements. The high-aspect ratio andvertical profiles that can be achieved using the CARL technique makes its i-line version(AZ's CP365A) appropriate for 4Mb DRAM high-volume production, which is ongoing atSiemens.

"For the upcoming gigabit DRAM generations, in close cooperation with Clariant AZElectronic Materials, we're developing a chemically amplified dual-wavelength photoresistsystem that is sensitive at both the 248 nm and 193 nm exposure wavelengths and is capableof resolving 0.13 µm structures and below (Fig. 3)," said Dr. Michael Sebald,project manager at Siemens Corporate Technology.