Arthur Sherman - Chemical Vapor Deposition for Microelectronics, страница 11

Second, the chemical composition of the gas will differ because theplasma region will be carbon rich due to the retention of CF 3 + species, whilefluorine atoms are free to drift away. The excess carbon can lead to depositionrather than etching.Results shown in Figure 10 ill ustrate this behavior where deposition isseen at zero bias in the plasma, but etching is observed when -40V bias is applied.10000 .........0 - ; - .

. . 0 ,600IISi0'l0IPICF4 ) = 10 ~mI1cui200cu,I~i81~i0'10060c'E~rouWc'2011-20-60-100H = 30 De0"0"0'"0 .........-40 V,._e_.__.--'-.-.-.-. ."0'~IwiI<a:.c:~I"IIII0II20-0-0_00VI41r (em)III•IFigure 10: Etch and deposition behavior in a magnetically-confined cylindricalplasma. 2356Chemical Vapor Deposition for Microelectronics2.4 PLASMA-ENHANCED CVD (PECVD) REACTORSHaving covered some of the elements of plasma behavior and how it relatesto reactors, it is appropriate to consider the plasma-enhanced CVD reactorspecifically.

This is where the plasma is created in an appropriate gas mixtureso that a suitable thin film will grow on a chosen substrate. As discussed in theprevious chapter, a gas mixture can react thermally, both in the gas phase andon the surface, to grow films. A similar process occurs with plasma enhancement, except that the gas mixtu re presented to the surface has many morespecies due to decomposition of the starting gas by high-energy electron impact, and there can be a high density of such species.There are basically three ways to create such a plasma for purposes ofthin film deposition. These are shown in Figure 11.

In the first, a pair of conducting electrodes are exposed to the low-pressure reacting gas and a DC orAC glow discharge is created. If a metallic film is being deposited, either a DCor AC discharge can be used. If a dielectric is being deposited, one must usean AC discharge, because the metal electrodes will become coated and a DCdischarge would extinguish. The second approach uses a coil wound arounda tube containing the reacting gas. When an AC current flows through the coil,an alternating electric field is induced within the tube and causes the gas tobreak down. Finally, if a pair of conducting electrodes are situated outside thetube, as in Figure 118, and an AC potential is applied to them, the electricfield is felt within the tube and again a discharge is created.RFPOWEREDELECTRODETUBE WI THoIELECTRIC WALLSA - PLANAR ELECTRODE SYSTEMB - CLAM SHELL ELECTRODE SYSTEMRFELECTRODEC - COIL TYPE ELECTRODEFigure 11: Geometries of plasma-assisted CVD reactors: (A) parallel-plate discharge, (8) tube with capacitive coupling, (C) tube with inductive coupling.

13Tube reactors are generally used for resist ashing or less critical depositions. In resist ashing, wafers are inserted into an oxygen plasma which reactswith (ashes) the hydrocarbon-based resist to form gaseous products (CO, CO 2 ,etc.). These reactors are simple and relatively inexpensive to build. It is difficult to have them etch uniformly on many wafers, but this is not a criticalissue for resist ashing.Fundamentals of Plasma-Assisted CVD572.4.1 Cold-Wall, Parallel-Plate PECVD ReactorsThe original plasma-enhanced CVD reactor was developed by Reinberg 24and is illustrated in Figure 12.

This was a parallel-plate reactor of circular symmetry, where the wafers sat on a heated platen. The reactants were introducedat the outer periphery, and the exhaust was at the center. Reinberg theorizedthat the discharge intensity would be higher in the center, tending towardhigher deposition rates there. Offsetting this would be the higher flow velocities in the center (shorter residence times), leading to uniform depositionrates from center to outer edge.

Based on this concept, a patent for this reactorwas issued. 25-?)iSourceCii-I+--Va.cuumFigure 12: Radial-flow, plasma-enhanced CVD reactor after Reinberg. 24In an attempt to develope their own concept, Applied Materials built areactor which introduced the reactants in the center and exhausted at theperiphery.2 This design is shown in Figure 13.The Applied Materials reactor is fabricated of aluminum (including thewafer-holding susceptor), and the susceptor is rotated by a magnetic coupling.Because of th is rotation, the susceptor must be heated by radiation, typicallyto 325°C.

Reactant gases enter at the center and flow outward where theyare exhausted. Since the electrodes are 2 inches apart and approximately 26inches in diameter, there is a relatively uniform glow discharge between them.In spite of Reinberg's predictions, deposition is quite uniform with radius.58Chemical Vapor Deposition for MicroelectronicsShieldedRF PowerInput~HeaterRotating ShaftOut toVAC PumpDuttoVAC PumpMagneticRotationDrive~~!!~tGases InFigure 13: Radial-flow, plasma-enhanced CVD reactor?The platen is grounded, and the upper electrode is powered with a lowfrequency rf power supply ('""50 kHz) at a power level of 500 to 1000 watts.The reactor operates in the batch mode with a wafer load of twenty-two 4-inchwafers, for example. For larger wafers, the load size is less.

Although the loadsize is restricted, high qual ity films are produced. 7A recent modification of the Appl ied Materials system incorporates aperforated upper electrode for more uniform gas distribution, as shown inFigure 14. Such a design change was necessary when deposition with 2% SiH 4in N 2 was attempted. Because of the low concentration of Si H 4 , the reactantbecame depleted as gases flowed outward and deposition became quite nonuniform. This problem was corrected when the reactant gases were introducedmore uniformly with radius. The motivation behind the use of 2% SiH 4 in N 2was safety.

It is felt that such a dilute mixture cannot sustain an explosivereaction in a gas cabinet.Fundamentals of Plasma-Assisted CVD59ShieldedRF PowerInput~HeaterRotating ShaftOut toVAC PumpOut toVAC PumpMagneticRotationDrivenv-.........A r................... ntGases InFigure 14: Radial flow plasma-enhanced CVD reactor with perforated upperelectrode for uniform reactant gas introductions.2.4.2 Hot-Wall, Parallel-Plate PECVD ReactorsThe two reactors just described are parallel plate reactors. However, theyare also cold wall reactors.

In other words, the electrode holding the wafers ishot, but all other surfaces exposed to the plasma are cold, or at least not heated.This is done to minimize the deposition on other surfaces so that down timefor cleaning can be kept as short as possible.The same reactor concept would be valid in a hot wall system, if the entirereactor were placed in a furnace. In this way, temperature uniformity wouldbe excellent by definition. Obviously, this would be awkard and not economically attractive. However, if our electrode geometry were of two parallel, narrowrectangular electrodes, the structure could conveniently fit into a hot tube(much like a diffusion furnace).

In this way, a large batch load could be run ina relatively hot furnace tube. Such a hot wall system is shown in Figure 15.60Chemical Vapor Deposition for MicroelectronicsFigure 15: Hot-wall, parallel-plate reactor for plasma-enhanced CVD. (Courtesyof Pacific Western Systems, Inc.)Multiple reactangular electrodes are arranged so that they fit down thelength of a tube and are alternately powered by a 400-kHz power supply. Theelectrodes are fabricated of graphite. A major attraction of the hot wall systemis the large wafer load that can be run (i.e., 84 4-inch wafers) at one time. Thisis offset to some extent by the fact that the electrode structure cools off eachtime wafers are unloaded, and the time needed to reheat upon insertion intothe furnace detracts from wafer throughput.2.5 NOVEL PLASMA-ENHANCED CVD REACTORSCurrent commercial plasma-enhanced CVD reactors operate with only twophysical concepts.

In one case, we have the inductively-excited discharge ina tube, which is used for plasma ashing of resist. The other is the parallel plateariangement using high-frequency RF power to create a low-pressure glowdischarge, where the wafers to be coated sit on one of the electrodes.In reality, these are only two of many arrangements that could be devised to create and deliver to a substrate large quantities of reactive speciesusing a plasma. Since there are many shortcomings to existing commercialplasma-enhanced CVD reactors, it will be useful to explore other reactor concepts that are under development, but have yet to be widely developed commercially. Whether or not they will lead to practical production systems remains to be seen.2.5.1 Electron Cyclotron Resonance (ECR) CVD ReactorOne approach being pursued by Japanese investigators makes use of theelectron cyclotron resonance phenomena discussed earlier.

24 In this case, aFundamentals of Plasma-Assisted CVD612.45-GHz microwave generator feeds microwave energy into a rectangular waveguide and then through a quartz window into a plenum chamber, as shown inFigure 16.2.45 GHzGasN2'CoolingwaterMagnetcoilsPlasmaGas (2)SiH4~,"Plasmastream'1-'1+\'.~\\ Plasma. ow IIextractionWln d[/_:-. _".~ speClren1'----..-,-------,1VacuumsystemFigure 16: ECR (Electron Cyclotron Discharge) reactor for plasma-enhancedCVD (after Matsu0 26 ).A very low pressure gas is introduced into this plenum and a magneticfield is established by a solenoidal magnetic coil placed outside this chamber.The very high frequency electric field established by the microwave sourceionizes the reactant gas to a small extent. However, when th? steady magneticfield is applied, a resonance condition is achieved (electron cyclotron resonance)and the "energy transfer from the microwave source to the plasma is maximized.Such strong resonant interaction causes a much higher degree of ionization,dissociation, and excitation than would be possible with the microwave energyalone.It is important to recognize that in this system the intense degree of ionization and dissociation is established in a region away from the wafer.