Planarization

Planarization is the process of increasing the flatness or planarity of the surface of a semiconductor wafer through various methods known as planarization techniques.

The starting raw wafers for semiconductor device fabrication are ideally flat or planar. However, as the wafer goes through the various steps of device fabrication, layers of different materials, shapes, and depths are deposited over the wafer surface through different growth and deposition techniques. Also, portions of materials already deposited over the wafer need to be removed from time to time. This series of material growth, deposition, and removal steps decreases the flatness or planarity of the wafer surface.

The recent trends towards device fabrication that involve four or more layers of metallization has aggravated the problem of wafer non-planarity. The shift to narrower and narrower metal lines also prompted the emergence of thicker metal lines in order to meet the current requirements of the device. Modern fabrication techniques that increase the number of metal layers on the wafer while decreasing the width of the metal lines will continue to make the problem of wafer non-planarity worse.

A decrease in the flatness of the wafer's surface introduces at least two problems to device fabrication. First, ensuring ample step coverage of very fine lines so that no breaks in the continuity of the lines arise becomes more difficult as the wafer becomes less flat. Second, progressive loss of planarity eventually makes the imaging of fine-featured patterns on the wafer challenging, if not impossible.

There are several planarization techniques used in wafer fabrication today. There are two categories for planarization techniques, namely, local planarization and global planarization. Local planarization refers to smoothing techniques that increase planarity over short distances. Global planarization, on the other hand, consist of techniques that decrease long-range variations in wafer surface topology, especially those that occur over the entire image field of the stepper.

Planarization techniques include: 1) oxidation; 2) chemical etching; 3) taper control by ion implant damage; 4) deposition of films of low-melting point glass; 5) resputtering of deposited films to smooth them out; 6) use of polyimide films; 7) use of new resins and low-viscosity liquid epoxies; 8) use of spin-on glass (SOG) materials; 9) sacrificial etch-back; and of course, 10) mechanical-chemical polishing of the wafer.

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