Synchronous multiplexed near zero overhead architecture for...

Details Synchronous multiplexed near zero overhead architecture for... Synchronous multiplexed near zero overhead architecture for...

1998-04-14

2001-05-08

Kunemund, Robert (Department: 1765)

Semiconductor device manufacturing: process

Chemical etching

Vapor phase etching

C438S710000, C118S719000

Reexamination Certificate

active

06228773

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Fields of the Invention
The present invention relates generally to an apparatus and method for reducing workpiece handling overhead relative to active workpiece processing time in a manufacturing process, and, more particularly, to reducing the relative time spent pumping a vacuum in semiconductor wafer processing chambers, venting such chambers to atmosphere, and transferring wafers to and from such chambers while increasing the relative time spent actively processing the wafers in the chambers, such as, by actively etching, stripping, and depositing the semiconductive layers of the wafers, and, even more particularly, to the process of switching common RF or microwave power supply sources alternately between dual downstream or in-chamber plasma reactors and alternately actively processing in one reactor while performing the aforesaid overhead tasks on the other reactor thereby significantly increasing the throughput of the overall machine at overall reduction in equipment cost compared to conventional dual or multiple reactor systems.
2. Discussion Of Background And Prior Art
a. Generating A Plasma
The primary reason to generate a plasma is to generate an intense amount of heat in a localized area to excite atoms or molecules of gas to an elevated state. The energy can be so intense that some common compounds dissociate.
For example, CF
4
, a stable compound gas, can have some of the fluorine atoms stripped off to form monatomic F which is extremely reactive. In the case of oxygen, it exists in nature as O
2
and not O
1
. When oxygen is passed through an intense RF electric field, often produced by microwave radiation, a significant percentage actually separates into monatomic oxygen, O
1
, and is far more reactive than O
2
. The monatomic oxygen is also heated to temperatures as high or higher than 1000 degrees C when in the plasma field. Photoresist is a carbon based or organic compound. When the wafer is heated to typically 220 to 270 degrees C and subjected to the hot monatomic oxygen, the photoresist is combusted often within 10 to 15 seconds depending on thickness and other variables. The new compounds formed are generally CO
2
, CO, and H
2
O which are gaseous at ambient conditions. The photoresist effluent is then pumped off in the gaseous state through a vacuum pump. In etch applications, different gas volatiles are formed.
Although plasmas can be created at atmospheric pressure, they are very difficult to ignite, difficult to control, consume enormous power, generate intense UV light, and are awkward to confine. Nearly all semiconductor process applications operate at greatly reduced pressures relative to atmospheric conditions. As a general class, plasma processing is typically conducted from a low range of 1 milli-Torr to a high range of roughly 10 Torr. For example, photoresist ashing or surface cleaning and preparation pressure ranges operate from about 0.1 Torr to 2 Torr.
Examples of methods to generate a plasma at rarified pressures are capacitively coupled electrodes, parallel plate reactive ion etchers (RIE), inductively coupled plasma (ICP) methods involving a resonate RF coil, microwave cavities, and electron cyclotron resonance (ECR). Some embodiments also include magnetic fields for shaping the plasma.
b. Vacuum Processing
Semiconductor silicon wafer processes, such as, actively etching, stripping, and depositing the semiconductor layers of the wafers, are generally performed under greatly reduced atmospheric pressure conditions ranging generally from well under the 10
−3
Torr regime to a few Torr in order to readily establish plasma conditions exciting the process gas with RF or microwave energy. Plasma excited process gas species have greatly elevated reaction levels to speed combination with the intended semiconductor layer to be processed. This vacuum processing contributes substantially to the high cost of semiconductor processing equipment itself (vacuum pumps alone cost about $40,000.00) and the relatively excessive amount of time (typically 65-80% of a complete cycle) spent in the overhead tasks of pumping a vacuum, venting to atmosphere, transferring wafers to and from, opening and closing doors, metering gases in and out, and the like. Moreover, each of these subsystems is typically contained in a multi-chamber machine, providing 100% redundancy at great expense in terms of equipment cost and space. Nakane U.S. Pat. No. 4,483,651 (2:10-13).
c. Cost of Semiconductor Equipment
On one hand, when examining the cost of semiconductor wafer processing equipment, one quickly realizes that relatively few system components comprise the vast majority of the total equipment costs. The process gas mass flow controllers, the RF power sources, such as, microwave and lower frequency RF power supplies, and the vacuum pumps are among the largest high cost items followed by the actual process reactor. Significant savings can be immediately achieved simply by developing a process which increases throughput by using dual, side-by-side replicas of selected critical costly items, but one of everything else.
Accordingly, it is an object of the present invention to significantly reduce the overall costs of semiconductor processing equipment, and especially atmosphere to vacuum equipment, by eliminating all redundant components of a plural setup other than the dual, side-by-side, wafer processing chambers.
d. Overhead Time
On the other hand, when examining the typical overhead tasks performed by the semiconductor wafer processing equipment, including (1) wafer transfer from the cassette to the chamber entrance and later back to the cassette; (2) chamber door opening and closing; (3) wafer exchange to and from the process chamber; (4) wafer heating or cooling preparation; and (5) pumping or venting the process reactor chamber to the desired vacuum level, or conversely, to an elevated pressure, one quickly recognizes that a significant savings can be immediately achieved by coordinating and sharing , that is, synchronously multiplexing, common material support sub-assemblies which perform overhead tasks.
As one example of the relative speed of one type of semiconductor processing machine, such as a photoresist ashing machine, the fastest photoresist, strip, dual process chamber module unit in the marketplace today runs at typically 110-130 wafers per hour.
Accordingly, it is an object of the present invention, by adding redundancy of one critical stage of a single process machine at only a modest cost increase while synchronously multiplexing the remaining components of the processing system, virtually 100% of the wafer exchange overhead, the pump/vent overhead, and the wafer temperature conditioning overhead commonly associated with preparing an environment for wafer chemical processing to commence is eliminated or masked.
e. Current Semiconductor Atmosphere to Vacuum Continuous, Synchronously Multiplexed, Wafer Transfer Systems
(1). Solo Wafers, Early Attempts
Early attempts to solve the problem of high expense, high overhead wafer processing are seen in Uehara U.S. Pat. No. 4,149,923 in which single wafer, single processing chambers, each separately pumped, were replaced by multi-chamber, sequential processing of single wafers all in one common vacuum, and in the system by Nakane '651 referenced above in which a computer controlled a common main wafer transfer mechanism operating in parallel with plural plasma reactors each fed by its own branch shuttle wafer transfer mechanism and each having its own vacuum pump. While achieving some economies through a common vacuum and by an automatic parallel scheme, Uehara and Nakane left too much redundancy in their entire systems.
(2). Group Batch Process
Another method commonly in use reduces the pump and vent overhead time by pumping and venting an entire group of wafers in a load lock all at one time, typically a cassette of 25 or more wafers. In an early system by Blake in U.S. Pat. No. 5,019,233 plural, separately pumped, single process, chambers are serviced by a central interi

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