Single Wafer Cleanings

Single-Wafer Spin Cleaner Another type of single-wafer cleaning involves a spinning wafer next to a megasonic transducer as shown in Fig. 16.3. 

FIGURE 16.2 Triple transducer megasonic single-wafer cleaning system (from Ref. 2). 

M. Olesen, C. Franklin, A Single Wafer Non-Contact Post CMP Cleaning Technology, Third International Chemical-Mechanical Polish (CMP) for VLSI Multilevel Interconnection Conference, pp. 375-378, Feb. 19-20, 1998. 

The old scrubber technique is in fact very attractive for post-CMP cleaning as the same mechanical effect is active for all the materials present at the surface (insulators, metal barriers). Doubled-sided scrubbers for cleaning the frontside and the backside of the wafer and lateral brushes to take care of the wafer side are now proposed on the market. Furthermore, the implementation of megasonic sprays in the scrubber can sometimes help for difficult cases. The major limitation is in terms of cost of ownership (COO) as a single-wafer process is involved. Indeed according to Witt et al. [17] who used the standard SEMATECH COO model, brush cleaning is more than three times more expensive than wet cleaning, which was confirmed by other economic studies [18]. 

FIGURE 3.6 Schematic illustration of various CMP tool designs including multihead system (left), single head and platen system (center), multitable system (right), built-in single-wafer post-CMP cleaning system (bottom), and a small table and linear belt model (top). 

Wet cleaning of wafers during the semiconductor production process often requires uniform removal of a few nanometers of material. Ideally, a single cleaning chemistry can be found that etches all exposed features at a comparable rate. Etch rates near 1 nm/min are desired for batch process and near 10 nm/min for single-wafer processes. A mixture of 500 1 DHF (dilute HF) with dissolved oxygen controlled near parts-per-million (ppm) levels has been found to meet these requirements for post copper CMP (chemical-mechanical polishing) cleans with exposed SiOj and Cu metal. 

Fig. 14. Cross-sectional views of post-CMP cleaning using double-sided roller brushes. The two brushes roll in opposite directions to keep the wafer pressed against two rollers. The rollers are used to rotate the wafer and are withdrawn to allow the wafer to pass once the wafer is clean (a) a single-brush configuration and (b) a multiple-brush configuration.

Gaffney (USA) 2008 Cleaning semiconductor wafers simulation study results compared to steady-state, single compartment model Methanol Short-term (15-min) and long-term (2 to 4-hr) personrd and area samples (ppm)... 

Single crystal (100) silicon wafers were used for cleaning experiments (provided by the Wacker Siltronic Corporation). The thickness of the wafers was typically 0.16 mm. The surface roughness Ra is 0.06 pm. 

Mechanical stimulation triggers the chemical dissolution of certain single-crystal metals, as has been reported by Dickinson.15 Therefore, in cleaning, the continued mechanical impact through brush or liquid on particles will activate the bonds between particles and the wafer surface. 

A commercial zincation bath is used. Single zincation time is varied between 0-30 s, and a second zincation time of 5-30 s. CMOS wafer chip samples are retained after each designed stage. Process steps like degreasing, soak clean and rinsing is the same for all experiments. The following outlines the typical zincation process for electroless nickel plating ... 

---