Minimal Dummy Wafer Load for Doped Silicon Films in an LPCVD Furnace: Reduced Operating Costs and Improved Tool Throughput
Publication Date: 2003-Sep-04
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In general, uniformity of deposition in an LPCVD process is improved when the boat is full, with a wafer in every slot. If fewer than a full boat load of wafers are to be processed, unused slots are often filled with dummy wafers. Loading and unloading dummy wafers is time-consuming, however, and dummy wafers must be replaced regularly, increasing cost. Identifying the minimum number of dummy wafers required for uniform deposition and optimizing loading of the boat improves throughput and decreases cost.
- What is the problem solved by the invention?
One method to deposit silicon in thin film form is by low pressure chemical vapor deposition (LPCVD). Wafers are heated in a pressure vessel to between 450 and 650°C while atmospheric gases are pumped out. When the pressure in the vessel (usually a quartz tube) reaches 200-600 mTorr and the wafers have reached high temperature, a silicon containing gas such as silane (SiH4) is flowed into the chamber, resulting in silicon deposition (fig. 1). Excellent control of the thickness of the film, thickness uniformity across a wafer, and step coverage (ability to cover a wafer surface with varied topography) is achieved by adjusting the temperature, pressure, and gas flow in the tube.
Many steps in integrated circuit (IC) manufacture not only require the deposition of thin films of silicon, but also require that the thin film be doped to make it conductive. Doping describes intentionally contaminating the silicon with specific foreign atoms, such as boron or phosphorus. One method to dope silicon thin films to codeposit dopant atoms whilst depositing silicon atoms. Precursor gases such as boron trichloride (BCl3) or phosphine (PH3) are admitted to the quartz tube simultaneously with SiH4, resulting in boron- or phosphorus- doped silicon films. This is called in-situ doping. This methods allows for better control of the depth profile concentration in the silicon film relative to ion implantation. Cost is also reduced as silicon film deposition and doping is done using the same furnace.
Typical LPCVD furnaces can hold many wafers (176 in our case). However, rarely are exactly 7 lots (cassette of 25 wafers that forms the basic unit of wafers, 7 x 25 = 175 wafers) loaded into the furnace at the same time. For example, a deposition on only 25 wafers may be needed. It has been discovered that film thickness nonuniformity and dopant concentration nonuniformity within a wafer can be negatively affected by the total number of wafers loaded into the boat. This inconsistency can be caused by changes in the total surface area available for the chemical reaction to occur, changes in the thermal profile of mass in the boat, and changes in the aerodynamics of the chemical gas flow in the tool. To maintain consistency from deposition to deposition, “dummy” wafers are loaded along with the product wafers to maintain a full load each time. These dummy wafers serve only to maintain deposition uniformity, and are an extra overhead cost to LPCVD operation in a fab. A cache of 175 dummy wafers is maintained in the tool (yes, we sometimes deposit on only 1 product wafer). However, the dummy wafers need to replaced periodically as they fracture when the deposited Si film exceeds a certain thickness. Dummy wafer replacement can also be trigg...