微电子应用中的旋涂技术

I. INTRODUCTION  

Spin coating of resist and polyimide films onto silicon  wafers is an important step in the fabrication of integrated  circuits. These films may be used for planarization and  photolithography (sacrificial layers), or as interlayer dielectric insulators (permanent layers). A complete review  of the process may be found elsewhere,’ and we will include only a cursory review that emphasizes the points  important for our discussion. The spin-coating process is  carried out by dispensing a sufficient amount of liquid onto  a wafer to flood it with the casting solution and then rapidly accelerating the wafer to the final spin speed. Rotation  proceeds for a specified amount of time during which the  film thins by a combination of fluid flow and evaporation.  Finally, the disk is decelerated to rest. This procedure may  then be followed by a baking step to remove residual solvent, or in the case of polyimide films, to cure the polymeric precursor to its desired final form.  

II. EXPERIMENTAL PROCEDURES .

 Typical conditions on which the spin-coating model is based.  Spin-coating experiments were performed with two Du  Pont polyimide precursor solutions, PI2525 and PI2545  Prior to spinning, the solutions were removed from a  freezer and allowed to equilibrate thoroughly to room temperature. Dilutions were made using pure N-methyl-Z  pyrrolidinone (NMP) solvent from Aldrich. Care was  taken to minimize the exposure of the solvent and solutions  to the atmosphere since NMP is extremely hygroscopic.  The stock bottle of solvent was stored under nitrogen and  small quantities were removed when required in a nitrogen  glovebox.  

Ill. SPIN-COAT MODELING

The spin-coating process can be divided into three  stages: deposition and spin up, spin off, and film drying. Although these stages overlap slightly, their physics can be  effectively modeled by separating them into three distinct  stages and treating their mathematics individually. This  approach differs from the “split model” in that mass fluid  flow and solvent evaporation are not restricted from occurring simultaneously.  

Fig1

A. Deposition and spin up 

The liquid solution may be deposited in several different manners ranging from pouring the entire solution at  the center of the disk and allowing it to spread radially  over the film, to delivering the liquid in a steady stream  moving radially over the rotating disk so that the deposited  liquid forms a spiral. In all cases the amount of fluid deposited is such that the disk receives a large excess of fluid  and is completely covered. Next, the disk is accelerated to  its final rotational speed. It is here where the liquid undergoes a great deal of change from its initial state as a thick  film essentially at rest to a thin film rotating with the disk.

B. Spin off  During this stage the film is thinned due to a combination of convection and solvent evaporation. The centrifugal forces act to drive the fluid radially off the edge of the  disk impeded only by the viscous resistance. This radial  flow quickly diminishes because the film has become exceedingly thin and evaporation of solvent has increased the  viscosity by several orders of magnitude. During fluid flow  the film is also thinned by solvent evaporation to the overlying atmosphere. It is the trade-off between these two  mechanisms that controls the film thickness, uniformity,  and the success of the spin-coating process.  

C. Film drying 

In this final stage of spin coating, fluid flow has essentially halted and further shrinkage of the film arises from  solvent loss alone. Previously, the solvent. concentration  profiles depended on fluid convection tlow through the  cross terms in the solvent conservation equation. However,  as the velocity components drop to zero, this dependence  becomes unimportant, and solvent conservation may be  considered independently. It is at this point where the spinoff stage ends and the film drying stage begins. We may  continue to track the solvent concentration via the conservation equation or simply flash off any remaining solvent  leaving a dry film behind. The former technique would be  required when the film or underlying topography is not  uniform because solvent diffusion may tend to modify existing free surface profiles.