Coating processes – Miscellaneous
Reexamination Certificate
2002-05-20
2004-03-09
VerSteeg, Steven H. (Department: 1753)
Coating processes
Miscellaneous
C204S192100, C204S298080, C118S7230ER, C118S7230IR, C156S345480, C216S067000, C330S199000, C330S123000, C330S127000
Reexamination Certificate
active
06703080
ABSTRACT:
FIELD OF THE INVENTION
the present invention relates to high power plasma processing system and more particularly to method and apparatus for providing VHF high power radio frequency energy to plasma processing systems under severe non-linear load mismatch conditions.
BACKGROUND OF THE INVENTION
Plasma processing systems are used worldwide in the semiconductor industry for thin film fabrication to create desired patterns on a semiconductor wafer.
Radio frequency (RF) power, produced and closely regulated by an RF generator, is used to create a high-energy environment inside a plasma chamber, which is a glass box filled with inert gas at low pressure. Within this environment, layers of silicon can be removed by etching and other chemical layers can be added by sputtering and chemical vapor deposition with microscopic precision, until a desired composition is achieved.
By changing electrical characteristics of a raw material such as a silicon wafer during plasma processing, integrated circuits (ICs) such microprocessors, random-access memories, etc., or compact discs are manufactured.
Chipmakers designing the next generation of ICs face tremendous technical challenges in plasma processing applications. These challenges include, but are not limited to, tighter control and precision measurements of film thickness, water temperature, gas and flow pressure, and distribution of contaminations inside the chamber. On the other hand, in the chip fabrication environment, the plasma etching process alone can account for up to half of the total wafer scrap. Therefore, reliability and stability are critical elements that determine plasma tool performance in reducing scraped wafers and eventually the cost of ICs. These challenges become even greater taking into consideration that plasma equipment makers are now implementing systems capable of producing 0.1 &mgr;m and lower devices on a 300-mm wafers.
A typical plasma processing arrangement used for chip fabrication generally includes as main equipment an RF power generator, an impedance matching network and a plasma chamber coupled to a system by RF power cables.
The highly automated plasma processing apparatus is controlled by the system controller. Many plasma processing systems use radio frequency (RF) energy in the LF (250 kHz-400 KHz), MF (2 MHz, 4 MHz) and HF (13.56 MHz, 27.12 MHz) ranges of the RF spectrum. VHF (as used herein, 40-300 MHz) high power generators are more complex, but provide better uniformity of the ion and radial flux across the wafer, lower variation in the rate of etching between wafers and within each wafer, higher chips productivity and repeatability, which result in higher production yield and lower cost. Thus, VHF plasma processing systems are attractive to system designers and chip manufactures.
Plasma etching reactors generally comprise two large parallel electrodes contained within an evacuated chamber. The chamber is filled with a small amount of etching gases mixed at precision flow rate and pressure, and a silicon wafer to be processed placed on a lower electrode of the two electrodes. A high power RF signal is applied between electrodes to convert the gas into a plasma, which is high energy, charged collection of ionized atoms and molecules. One advantage of the plasma is the ability to produce highly reactive particles-ions at a relatively low temperature. In all plasma techniques a shower of electrons generated by the electrodes strikes the low pressure gas between the electrodes. The collections fragment molecules into ions and radicals. In plasma etching these activated species strike the wafer and ablate the surface. After a predetermined sequence of such operations, electrical components are created.
The etching rate of the plasma system and the consistency of the final etched pattern depend greatly upon efficient and consistent coupling of RF power from the RF generator to the electrode plates of the plasma chamber. Efficient power coupling from the generator to the load (plate electrodes) occurs when the load impedance of the plate electrodes has a value equal to the complex conjugate of the generator's ouput characteristic impedance.
High power RF generators are usually designed so that their output impedance is a resistance of 50 Ohms and a reactance of zero Ohm. The input impedance of the reactor is determined by external conditions, namely, the magnitude of power transferred by the matching network from the RF generator, and a plurality of internal conditions. These conditions include, but are not limited to, type of gas mix, flow rate and pressure, and temperature of the raw gas. In the pre-ignition stage the gas in the chamber is not ionized and not conductive. Therefore, the load impedance of the plate electrodes is very high and extremely mismatched from the RF generator output impedance. At the time of ignition the raw gas in the reactor begins to ionize under the power from RF power supply and converts into a plasma, resulting in the load impedance of the chamber dropping dramatically. During this transient, because of significant deviation of the plasma load impedance, the generator will receive reflected power from the reactor. Any increase in RF forward power, due to low load impedance, may result in 100% of the generated power being reflected back to the generator. This leads to current and power dissipation or voltage overstress of the transistors in the power amplifier (PA) modules of the RF generator.
Even during the plasma-sustaining period, the plasma density may vary several orders of magnitude, resulting in a substantial impedance mismatch. There is thus general agreement that, in a plasma processing environment, the plasma reacts as a dynamic extremely nonlinear load. Furthermore, it has been generally agreed that repeatability and stability of the plasma processing almost entirely depends on the repeatability and stability of the plasma. Thus it was many times proven that poor uniformity and repeatability of the film thickness in the CVD process as well as overetching or underetching in the etching process are a consequence of poor reproduction of the plasma.
Plasma processing systems act as dynamic non-linear loads with a wide range of magnitude and phase, depending upon the type of chamber and plasma process, gas type and pressure, temperature, and other variables. Because of mismatched loads and reflected power, the performance of transistors in power amplifiers (PA) of RF generators feeding plasma processing systems changes, thereby resulting in RF current and power dissipation stress for some loads and overvoltage stress for other loads. In some instances, depending upon the output power of the RF generator and the severity of transient mismatch presented by the load, voltages across transistors (V
ds
) in the PA can exceed nine times their operating DC supply voltage and exceed 150% of the transistor breakdown voltage V
dss
. This stress drastically reduces the reliability of the entire plasma processing system.
In the past few years there has been a trend among RF power amplifier designers to use high voltage switch-mode MOSFET transistors instead of low voltage RF MOSFET or bipolar transistors. This has been described, for example, in U.S. Pat. No. 5,726,603, which is hereby incorporated by reference in its entirety.
RF power amplifiers utilizing high voltage switch-mode MOSFET transistors have operating DC voltages (e.g., B+ drain voltages V
ds
between 100 and 175 V) and have employed high voltage switch-mode MOSFETs with breakdown voltages Vd
ss
up to a 1000 V in a standard TO-247 package. The large associated RF breakdown voltage margins permit sustained operation near open circuit load mismatches, as may be required by the severe requirements of an RF plasma processing load. Unfortunately, because the internal capacitances C
ISS
, C
RSS
and C
OSS
of these transistors are all high, thereby affecting overall source and load impedances and performance at high frequencies, these transistors cannot be used in VHF generators. As a result, known VHF gener
Reyzelman Leonid E.
Sortor John E.
ENI Technology Inc.
Harness & Dickey & Pierce P.L.C.
VerSteeg Steven H.
LandOfFree
Method and apparatus for VHF plasma processing with load... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method and apparatus for VHF plasma processing with load..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method and apparatus for VHF plasma processing with load... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3217081