1629373397-425d4de58b7aea127ffc7c337418ea8d (846389), страница 66
Текст из файла (страница 66)
10.67 electrons flow from the source S to the drain D, so that the positivecurrent ID goes from D to S. The current is controlled by the voltage VG, which iscapacitively coupled through an insulator to change the width of the currentchannel.To make such transistors requires a series of steps of masking, etching, anddeposition. Silicon can be etched chemically with chlorine or fluorine, or physicallywith energetic beams of atoms or ions. Winters and Coburn showed that silicon canbe etched with XeF2, a gas, but that the etch rate is greatly enhanced by Ar+. Theirfamous graph is shown in Fig.
10.68. With only XeF2 or with only an argon ionbeam accelerated through the sheath, the etch rate is more than an order ofmagnitude lower than with both. This effect has been explained by moleculardynamics simulations by David Graves. By following the motion of each atom ina surface layer of hydrogenated amorphous carbon, he showed, in Fig. 10.69, thatan argon ion beam sputters away the H atoms near the surface to a depth increasingwith A+ energy.The patterns of Fig. 10.67 are built up layer by layer with photolithography, aprocess shown in Fig. 10.70. A photoresist layer is sensitive to UV light and areasexposed to light can be chemically dissolved or retained. The resist blocks theetching beams so that the desired pattern is etched in the Si layer underneath.
Theresist is then dissolved away. Figure 10.71 shows the etching process. A plasmacontaining Cl, F, and Ar ions and neutrals is at the top, and ions are accelerated bythe sheath potential down to the substrate. At the top of each column is the maskprotecting the layers below it.Fig. 10.68 The Coburn graph of symbiotic etching40210Plasma ApplicationsFig. 10.69 Molecular dynamics simulation of an aC-H (hydrogenated amorphous carbon) layerbombarded by argon ions at various energies [D.B.
Graves, Gaseous Electronics Conference,2011]Fig. 10.70 Patterning by photolithographyThe patterning of transistors requires light of wavelengths shorter than criticaldimensions of their features. As the CDs got smaller, ultraviolet light from excimerlasers had to be used. This involved new optics that can pass UV. Eventually,shorter wavelengths in the X-ray range will be needed.
In that case, only reflectiveoptics can be used, and this will require development of new technology. We have along way to go, however, to duplicate the computing ability of the human brain withan instrument of that size.10.5Semiconductor Etching403Fig. 10.71 Cartoon of semiconductor etching. The top layer of the columns is the maskFig.
10.72 Trenches formed with Deep RIEWhen a deep trench or via is required, a process called DRIE (Deep Reactive IonEtching, Fig. 10.72) is used. To keep the trench straight, etching and passivation areapplied alternately. Etching is done with, say, SF6; and, after a few seconds, an inert40410Plasma ApplicationsFig. 10.73 Notches at thebottom of a trenchpassivation layer like teflon is laid on with C4F8. This protects the walls, but theetching ions can cut through it at the bottoms to allow chemical etching to occur.This is sometimes called the Bosch process.The bottoms of the trenches can have defects, such as notches or micro-trenches.Notches can form from bombardment by ions with curved trajectories. In Fig.
10.73the thick shaded layer at the top is the mask, which is charged negatively at the topby electrons, which attract the ions, bending them towards the wall. The bottom ofthe trench is charged positively by ions which have collected on the wall. Theresulting electric fields determine the ion trajectories which etch the notches at thebottom. Figure 10.74 shows an SEM (Scanning Electron Microscope) image of anactual trench deformed with microtrenches.The mask shown in Fig.
10.73 is shown with rounded edges. Since the chargesthat build up on these edges create E-fields, the ion orbits vary with these charges.An example is shown in Fig. 10.75. This computation compares ion orbits withsharp trench entrances with those with rounded ones, taking into account thesurface charges. A sharp edge shields ions from the sidewalls, whereas even aslight rounding of the edge allows ions to deposit positive charge to the wall. Amore obvious electron shielding effect also occurs, with surface electron chargesrepelling electrons from the nanometer-size trench.This was a brief glimpse of the complex technology that makes modern electronics possible.10.6Spacecraft Propulsion405Fig.
10.74 Micro-trenchesat the bottom of a trench [R.J. Hoekstra, M.J. Kushner,V. Sukharev, andP. Schoenborn, J. Vac. Sci.Technol. B 16, 2102 (1998)]Fig. 10.75 Deflections of the orbits of ions from the plasma at the top caused by surface chargeson the mask, for a square trench entrance (left), and a rounded one (right) [T.G. MadziwaNussinov, D.
Arnush, and F.F. Chen, Phys. Plasmas 15, 013503 (2008)]10.6Spacecraft Propulsion10.6.1 General PrinciplesOnce a satellite has been put into orbit, it needs a small amount of thrust once in awhile to keep it there or to change its orbit. Chemical thrusters were formerly usedbut have been largely replaced by more efficient ion thrusters. These are of three40610Plasma Applicationskinds: gridded thrusters, Hall effect thrusters (HETs), and the new helicon thrusters,still under development.
Ion thrusters are characterized by their specific impulse Isp,defined byI sp vex =g sec ;ð10:22Þwhere vex is the exhaust velocity of the rocket, and g is the acceleration of gravity,of magnitude 9.8 m/s. Note that Isp has the units of seconds. What this meansphysically is that if a rock is dropped from a height, it will reach the velocity vex inIsp seconds.The total amount of thrust is limited by the amount of mass available forejection. Let M be the total mass of the load consisting of the intrinsic spacecraftmass ms plus the decreasing propellant mass mp:MðtÞ ¼ ms þ m p ðtÞ:ð10:23ÞIn the frame of the spacecraft, its velocity v is zero, and its acceleration dv/dt ispositive.
The thrust T is the negative of the rate of change of exhaust momentum:T¼ddMmp vex ¼ vex:dtdtð10:24ÞThe spacecraft’s acceleration due to T is given byddvðMvÞ ¼ M ;dtdt dv1 dMdðlnMÞ¼ vex¼ vex:dtM dtdtT¼ð10:25Þð10:26ÞIntegration from the initial velocity vi to the final one vf as mp goes from mp0 to0 givesvðfvidvdt Δv ¼ vexdtMðfMid ðlnMÞmsdt ¼ vex ln:dtms þ m p0ð10:27ÞThe mass of propellant needed for a given Δv is thenm p0 ¼ ms eΔv=vex 1 :ð10:28ÞThe amount of propellant needed to accelerate a given mass to a given velocitydepends exponentially on the exhaust velocity vex.10.6Spacecraft Propulsion40710.6.2 Types of Thrusters10.6.2.1Gridded ThrustersA schematic of a gridded thruster is shown in Fig. 10.76. A plasma is created by avoltage applied between a cathode and an anode grid.
The cathode could be a goodelectron emitter like lanthanum hexaboride (LaB6). Electrons are retained and ionsare accelerated by grids. The ion beam will not detach from the spacecraft unless itis neutralized, so electron emitters have to be added at the sides to inject electronsinto the accelerated ion beam and prevent the spacecraft from charging to a highnegative potential. The neutralizers are usually hollow-cathode discharges. Theejected plume of plasma can then be formed by nozzles into an optimal shape.Sputtering limits the lifetime of the grids, but they have been designed to last atleast 30,000 h.10.6.2.2Hall ThrustersElements of a Hall thruster are shown in Fig.
10.77, which is a cross section of acylindrically symmetric device. A plasma is formed between the coaxial cylindersby applying a high voltage to the anode ring at the left. This voltage accelerates theFig. 10.76 A gridded ionthruster40810Plasma ApplicationsFig. 10.77 A Hall effectthrusterions to vex. Holes in the ring also serve as the gas feed, as shown in the upper crosssection. Xenon is normally used because of it is an inert, monatomic gas with highmass and low ionization potential.
A radial magnetic field is created by coils (not asshown) in order to prevent electrons from following the ions. The electrons insteaddrift azimuthally with their E B drift, forming the Hall current. Here again,electron neutralizers have to be added to neutralize the ejected ion beam. Thefunction of the magnetic field is to prevent the electrons from moving axially andcollapsing the anode voltage into a thin sheath at the anode. Hall thrusters have afew intrinsic problems. One is secondary emission of cold electrons from theinternal surfaces which can upset the charge balance in the plasma. This can beminimized by coating the surfaces with carbon velvet.
A second problem isinstability, since many types of instabilities arise when there is a magnetic field(B-field). In spite of these problems, Hall thrusters have been engineered successfully to fly in space.10.6.2.3Helicon ThrustersHelicon plasmas, described in Sect. 9.5, could be advantageous for thrustersbecause of their high ionization efficiency and their thrust without an auxiliaryE-field. Elements of a helicon thruster are shown in Fig. 10.78. The required DCmagnetic field is generated by coils surrounding the tube.
![](https://s.studizba.com/z.php?f=/templates/si/i/noavatar.png&w=100&h=100&t=1"){kind=avatar}
