Defects in silicon wafers

Introduction

Defects in Silicon are one of the principle causes for rejection of Silicon wafers both by the large Silicon manufacturers and the IC manufacturers. A lot of the time if you buy wafers from a Silicon Vendor and not a manufacturer they might be called "Prime wafers", but chances are they have some faults. There are many defects associated with Silicon wafers and most are caught atthe Silicon Manufacturer, some are not. Wafer defects range from pits in the Silicon surface and tiny scratches, to things that are Hidden from view, buried below the Silicon surface like the picture at the RIGHT of Copper Precipitates (a common defect). Found just below the Silicon surface by means of a SIRTL etch for 60 sec. This defect exhibits a crows foot like appearance and would cause a badGate Oxide over this region, causing a failed device(s). If this defect was under a few points on your 49 point Thermal Oxide Measurement map, you would definitely see non-uniform Oxides in these areas (Thermal Oxide growth can accelerate or decelerate over defects in the Silicon).

There are many Hidden defects in Silicon wafers and to date no one has made a perfect one.

Types of defectsThe silicon crystal inherently contains many crystallographic imperfections. The presence of most of these crystalline defects is undesirable in silicon wafers, although certain types of 'defects' are essential in semiconductor manufacturing. The main type of defect present in the silicon crystals during the 50s were dislocations primarily formed by thermomechanical stresses in the vicinity ofthe melt/crystal interface. However, elimination of thermomechanically induced dislocations introduced microdefects.

Crystalline defects may be classified into four categories according to their geometry:

Table 1. Examples of Crystalline Defects
|Defect Type |Examples |
|Point or Zero-Dimensional Defects|Vacancy Defects |
| |Interstitial Defects |
| |Frenkel Defects |
| |Extrinsic Defects |
|Line or One-Dimensional Defects|Straight Dislocations (edge or screw) |
| |Dislocation Loops |
|Area or Two-Dimensional Defects |Stacking Faults |
| |Twins |
||Grain Boundaries |
|Volume or Three-Dimensional Defects |Precipitates |
| |Voids |

There are many forms of crystal point defects. A defect wherein a silicon atom is missing from one of thesesites is known as a 'vacancy' defect.  If an atom is located in a non-lattice site within the crystal, then it is said to be an 'interstitial' defect.  If the interstitial defect involves a silicon atom at an interstitial site within a silicon crystal, then it is referred to as a 'self-interstitial' defect.  Vacancies and self-interstitial defects are classified as intrinsic point defects.
   
Ifan atom leaves its site in the lattice (thereby creating a vacancy) and then moves to the surface of the crystal, then it becomes a 'Schottky' defect.  On the other hand, an atom that vacates its position in the lattice and transfers to an interstitial position in the crystal is known as a 'Frenkel' defect. The formation of a Frenkel defect therefore produces two defects within the lattice - a...