Atmospheric downstream plasma- a new tool for semiconductor processing
07/01/1998
EQUIPMENT FRONTIERS
Atmospheric downstream plasma - a new tool for semiconductor processing
Sergey Savastiouk, Oleg Siniaguine, Tru-Si Technologies, Sunnyvale, California
Martin L. Hammond, TMX International, Cupertino, California
Plasma processing is critical to semiconductor manufacturing because it provides the means to create and control chemical reactions at the wafer surface that would otherwise not be available. Conventional anisotropic plasma-etch systems operate at low pressure, require sophisticated vacuum systems, and have low etch rates. Conventional isotropic etch systems operate at higher pressures and have somewhat higher etch rates. Neither approach provides the etch rates necessary for the economic removal of large amounts of silicon, as required for backside damage/stress removal or for wafer thinning.
Higher pressure plasmas provide the surface flux necessary for high etch rates, and are used for ashing and cleaning. However, such plasmas are characterized by relatively low plasma temperatures and low etch rates of silicon compounds. Atmospheric pressure plasmas have the energy density and flux necessary for high etch rates, and are used for powder spray-coating and plasma cutting, but they are not used in fabs because of damage and contamination concerns. These concerns include possible overheating due to the high power density, contamination from chemical attack and erosion of electrodes, and etching nonuniformity.
A new Atmospheric Downstream Plasma (ADP) system was developed by Tru-Si Technologies that isotropically etches uniform layers of silicon from wafers. Operating at ambient pressure, the etch rate is at least two orders of magnitude greater than in vacuum plasmas. In addition, charged species are quickly quenched so that only isotropic chemical reactions occur. The downstream configuration eliminates potential ion impact damage.
There is immediate demand for ADP technology to overcome the limitations of existing wafer backside thinning techniques, including backgrinding and wet etching. Backside damage/stress is easily removed by the ADP system, and the very thin dice that can be produced meet the demands of smart cards, handheld devices, and high-speed communications.
ADP source design
This new ADP source overcomes the limitations of high-pressure plasma processing for microelectronic processing with a magnetically controlled, high-density, inert gas DC arc plasma discharge. Chemical reactions are created by injecting reactant gases into a precisely controlled atmospheric pressure plasma, where they are 100% decomposed by the high plasma temperature. Due to the short mean-free-path at ambient pressure, charged species quickly recombine downstream from the plasma region to eliminate the possibility of ion damage.
The ADP source consumes and releases power in a precisely regulated process. The power required to maintain the plasma is provided using a relatively small electrical current and a long arc. The resultant lower power density allows a reduction in the heat flux to the electrodes and, as a result, minimizes thermal erosion and plasma contamination while increasing electrode life. Magnetic fields control both the location and power density of the plasma. The plasma is spread over a zone of several square centimeters but the footprint is still smaller than a wafer. Therefore, wafers move through the chemical reaction zone to achieve uniform etching, and short but repeated exposures to the reaction zone keep wafer temperatures below 125?C.
The ADP source contains two electrode units directed upward and toward each other with an angle of about 90? between their axes (Fig. 1). Each unit consists of an electrode placed inside a mini-chamber with a water-cooled orifice. The orifice and electrode are located along the mini-chamber axis so that the main plasma gas (usually argon) exits through the orifice. The mini-chambers are electrically isolated from the electrodes.
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Figure 1. The ADP source uses neutral Ar gas and a DCarc to strike the plasma. A chimney effect draws the reactants into the plasma, where they are directed toward a wafer.
When a DC field is applied between the two electrodes, a plasma arc is formed that exits the orifice of one electrode unit and enters the orifice of the other. The plasma is kept from the chamber walls by the flow of mainstream gas. External magnetic fields direct the plasma arc from its initial path so that a portion can be extended vertically. The resultant plasma region is further heated by the increased resistance of the extended path length. Visually, the arc appears to exist as two plasma jets extending from the electrodes with a central combined plasma region.
The vertical orientation of the plasma region creates a chimney effect that captures the injected cold reactant gases along its vertical axis. Because the plasma arc is bent at a small angle, there is a bottom region of reduced pressure that assists in effective reactant capture. The reactant gases are heated, activated, and completely decomposed in the central plasma region far from the electrode units. Since this reaction occurs at a distance from the electrode units, chemical erosion of the plasma generators is eliminated.
Reactant capture and distribution depends on the location of the plasma region relative to the reactant injection. The location of the plasma region is tracked by a computer-controlled vision system that corrects the jet`s direction by changing the external magnetic fields. This closed-loop monitoring system provides real-time, highly reproducible control of the plasma jet properties.
ADP characteristics
The ADP source has a plasma temperature of about 10,000?K and a velocity of about 10 m/sec. The Reynolds Number is estimated to be less than the critical value; therefore, the plasma flow is laminar. Like a well-controlled flame, there is a central plasma region that is surrounded by a quenching or recombination zone where both physical (recombination) and chemical reactions occur (Fig. 2). A chemical reaction zone - where activated chemical species are available for reaction - surrounds the two inner regions.
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Figure 2. Schematic of the downstream plasma region. Wafers are moved through the several cm2 chemical reaction zone to provide uniform etching and to prevent excessive wafer heating.
High energy, activated species are created in the plasma region. The plasma at atmospheric pressure is thermal due to its small mean-free-path (<1 mm). The kinetic energies of the ions, electrons, and neutrals are approximately the same and very low (see table), so species in the chemical reaction zone surrounding the ADP source also have low kinetic energy. These characteristics, and the correspondingly low floating potentials on wafers-in-process, explain how the ADP plasma source (unlike vacuum plasmas) does not produce high-energy electron/ion bombardment and electrostatic charging of dielectric surfaces.
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Because of the high (atmospheric) pressure, there is a high species flux to the wafer surface. The species density in an atmospheric pressure plasma with a temperature of 6-10,000?K is about 1018/cm3; therefore the flux will be about 1020 species/cm2/sec. ADP etch rates are at least two orders of magnitude greater than conventional vacuum plasma systems.
One of the biggest advantages of the ADP source is reduced equipment complexity. By operating at atmospheric pressure, complicated vacuum systems are not required. Many of the applications being considered can be accomplished in a clean air minienvironment. Complex hazardous waste disposal equipment is not required because the plasma decomposes 100% of the reactant gases (including fluorocarbons), and a simple water-based effluent scrubber easily removes any reaction by-products.
Process versatility
The nature of the inner plasma region is controlled by selection of the inert gas and the electric power density. The characteristics of the outer chemical reaction zone are controlled by the reactant gases, allowing for a wide range of plasma chemical reactions.
Well-known reactions of silicon with activated oxygen and fluorine species easily achieve the etching of silicon compounds. The isotropic process results in equal etch rates for both single-crystal and polycrystalline silicon. Selectivity to silicon oxide and silicon nitride depends on the wafer temperature and the choice of reactant gases. Potential applications of the ADP source include isotropic etching to remove wafer backside damage/stress post-backgrinding, photoresist, and flat panel films. ADP etching can produce strong, super-thin wafers.
The demand for thinner and stronger wafers is driven by the need to create ever-thinner chips for the packages required for portable devices such as computers, cell phones, and smart cards. Existing packaging technologies backgrind bulk silicon from the wafer backside, and wet etch to remove the damage/stress caused by grinding. An all-dry etching system overcomes wet etching nonuniformity, and eliminates the costs of handling hazardous wet materials.
The ADP source is part of the new "Tru-Etch" series of atmospheric pressure plasma systems for the removal of bulk silicon from 100- to 300-mm wafers. Tru-Etch systems also incorporate a new, noncontact wafer-handling system, "NoTouch," and an automated cassette-to-cassette handler that provide the following:
all-dry backside damage/stress removal,
all-dry wafer thinning with high uniformity,
no frontside wafer surface protection needed,
no additional wafer bow,
no subsurface damage and no chipping.
Uniformity and reproducibility are assured by controlling the location and power density of the plasma region and by moving wafers through the chemical reaction zone. In removing 20 ?m of silicon from 200-mm wafers, the ADP system added total thickness variation of <2% with a throughput of 60 wph.
Acknowledgments
Atmospheric Downstream Plasma (ADP), NoTouch, and Tru-Etch are trademarks of Tru-Si Technologies Inc.
For more information, contact Sergey Savastiouk, Tru-Si Technologies Inc., 657 N. Pastoria Ave., Sunnyvale, CA 94086; ph 408/720-3333, fax 408/720-3334, e-mail [email protected].