Gimbaled carriers

The simplicity of early carrier designs was complicated by their use as part of the robot used to move wafers from the load cassette to the unload cassette. On a so-called gimbaled carrier, the down force was applied to a central point on a plate behind the wafer, and it was assumed that the applied force was transferred through the wafer backing plate to be distributed uniformly across the wafer. Lateral motion of the pad then caused a torque to be applied to the carrier. To compensate for this rotation, a gimbal was built into the carrier at the point where the down force was applied. 

These carriers were generally capable of providing 5-7% nonuniformity at 6- to 7-mm edge exclusion. The nonuniformity is more precisely called the within-wafer nonuniformity (WIWNU) and is defined by the standard deviation of a set of film thickness measurements on a wafer divided by the mean of that set. Smaller numbers denote better process control. Better performance was limited in part by the manner in which the carrier held the wafer during polish. 

Even in the absence of the edge effect, several other issues limited the ability of these earners to produce very flat wafers. First, the application of pressure to the center of the wafer, in spite of the rigid structure of the 

There are two basic methods used to address these effects back pressure and the use of curved carriers. Back pressure consists of increasing the pressure applied to the wafer via a port behind the wafer plate. The use of back pressure is preferred since it is readily adjusted to compensate for a broad spectrum of issues. Adequate performance is attainable in most cases. However, in cases of extreme film stress, it is often necessary to use a curved carrier. Such carriers have a wafer plate with a small amount of curvature built into them. The maximum deflection is in the center of the carrier, and is typically about 5 to 15 iim. 

---

FIGURE 3.11 Schematic illustration of the cross section of different carrier models gimbal type (A), floating type (B and C), and actual example of a carrier (D). 

Carrier A The carrier is supported by gimbals and can be swung around its gimbals. The backside pressure is adopted in this carrier example to control the polishing pressure (rate). 

Runnels and Eyman consider the wafer, with some finite curvature, to be held by a carrier which is mounted via a gimbal mechanism as shown in Figure 4.2. The wafer then glides at an angle of attack, 6, upon a slurry fluid layer of thickness h. Runnels... 

The carrier mainly comprises Carrier head-1 (2), Carrier head-2 (3), Wafer guide (4), as well as Gimbal (5), and is held as well as rotated by Shaft (6). 

The Gimbal (5) mechanism is originally designed to allow the carrier to smoothly follow the table and pad regardless of their surface asperity. Whether Gimbal (5) is necessary or not has been a topic of debate for a long period. As a result, Gimbal (5) has been used so far. This is because it has more merits than demerits. 

---