Roland A. - PVD for microelectronics (779636), страница 14
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3.6), [3.1]. Since the cross section is small, manysecondaries pass through the plasma region and impact the anode/wall62R. POWELL AND S. M. ROSSNAGELFIG. 3.5 The potential as a function of distance between the cathode and the anode. It should benoted that the position of 0 volts (ground) is not specified and can be set wherever desired.
In mostsputtering tools, the anode is grounded.without making a sizable number of ion-electron pairs. From the earlierdiscussion, the loss of some fraction of the secondary electrons (50%, forexample) due to the low collision probability results in the requirementthat the energy (or voltage) given to the remaining secondaries be increased by two to keep the same net ion-electron pair production rate.However, as the voltage (energy) is increased, the cross section for ionization of the background gas becomes even smaller, so the efficiency ofusing the secondaries can drop even farther.
The net result is a low plasmadensity and hence a small discharge current to the cathode at a rather highvoltage. This results in a low sputtering rate and a poor deposition rate ofsputtered atoms.One solution to this problem has been to increase the gas density to increase the number of gas-phase, ionizing collisions. This works to apoint, but the gas pressures required are often quite high m many hundreds of mTorr. At these pressures, the mean free path of the sputteredatoms is on the order of millimeters or less, and all of the sputtered atomsPLASMA SYSTEMSFIG. 3.663Electron-impact ionization cross sections as a function of electron energy 13.1].are effectively stopped very close to the cathode. They can then moveabout by diffusion, but the cathode face provides a very large sink for thediffusing particles, and the net number deposited onto a nearby surfacecan be very low.Another problem that occurs with DC diode plasmas occurs when a reactive gas, such as oxygen, is added to the gas mixture during sputtering.This might be done, for example, to deposit films of aluminum oxide whensputtering a cathode made of AI.
Unfortunately, the cathode can becomerapidly oxidized in this case, and as a result the net DC current through theinsulating, oxidized layer can be quite small. This means that the deposition rate will be too small to be useful.Therefore, although DC diode plasmas can be easily made and operated,from a practical point of view they are rarely (if ever) used for the deposition of films.
In terms of semiconductor fabrication, DC diode plasmas arenot used in any production cases.R. POWELL AND S. M. ROSSNAGEL643.6 RF PlasmasUsing an RF potential (typically 13.5 MHz) in place of DC on a cathodecompletely solves the insulating-oxide problem and helps to solve the lowdeposition-rate problems discussed above. An RF diode system, shown inFig. 3.7, is quite similar to the DC diode with the exception of the additionof impedance-matching networks between the power supply and the electrodes. In an RF diode, it is more common that the second electrode (theanode in the DC case) is actually powered by the power supply and not justgrounded.The potential on the RF-powered electrode oscillates positive andnegative.
Because of the much higher mobility of the electrons, the RFpowered electrode can pick up a much greater electron current than ioncurrent for one cycle, and this begins to charge the electrode negative.The capacitor in the impedance network (to be discussed below) blocksthis DC potential from the power supply. On each succeeding RF cycle,the electrode charges more negatively, which causes the effective ioncollection time during each cycle to increase and reduces the electroncollection time. The net result after several cycles is a negative DC potential on the electrode at a value that can approach one-half the appliedRF peak-to-peak voltage.
For very short fractions of each RF cycle, theFIG. 3.7RF diode system with nongrounded sample electrode.PLASMA SYSTEMS65electrode goes positive and collects electrons. For the remainder of thecycle, the electrode is negative and repels the electrons, collecting ionsinstead at an average potential roughly equal to the net DC negative bias(Fig. 3.8). The average DC bias is usually equal to one-half of the applied RF peak-to-peak voltage.Since there is no net current flow (the ion current and the electron currents cancel out), there is no charging of an insulating surface; thus oxidized or nonconducting surfaces can be sputtered at modest rates.
In addition, because of the rapidly changing potentials in the plasma due to theRF-powered electrode, secondary electrons as well as other plasma electrons are mildly trapped within the plasma and can cause additional ionization, compared to the DC diode case [3.6]. This results in a higherplasma density at the same pressure and a larger ion current to the cathode.It also results in a lower applied voltage (compared to a DC diode at thesame applied power), which means that the secondary electrons will havea higher probability of being used efficiently for ionization.RF diodes are routinely used for sputtering and sputter deposition of insulating m a t e r i a l s - such as silicon dioxide, aluminum oxide, and othero x i d e s - as well as magnetic materials such as Ni and Fe.
The etchingand deposition rates are modest (tens to a few hundred angstroms/minute).RF diode tools can be scaled up to rather large electrode size, and toolswith diameters of 1 meter are routinely used for batch processing. Unfortunately, much like the DC diode case described above, increasing the RFFIG. 3.8Applied RF voltage as a function of time to the powered electrode.66R. POWELL AND S. M.
ROSSNAGELpower to the cathode does not linearly increase the sputtering and deposition rate. This is, again, due to the decrease in the ionization cross sectionas a function of increases in electron energy. Therefore, as the power is increased, more and more secondary electrons pass through the plasma without collisions and can strike and heat the anode/sample. The practical limitfor RF diode systems appears to be a few kilowatts, and as such, RF powersupplies larger than 3 kW are uncommon.3.7 RF MatchboxesRF plasmas have a complex impedance that has capacitive, inductive, andresistive components.
Most power supplies, though, are designed to bemost efficient powering a simple 50-ohm load. The role of the matchingnetwork m often called the m a t c h b o x~is to optimize the impedance ofthe plasma-and-matchbox system to maximize the amount of power thatcan be delivered to the plasma. The most common network is the "L" network, shown in Fig. 3.9 [3.7].
In this matchbox, the incoming power issplit upon entering the matchbox. One side goes through a variable capacitor to ground, known as the load or shunt capacitor. The second side goesthrough a multiple-turn inductor, a second capacitor (the series or tuningcapacitor), and then to the plasma electrode. The circuit is then completedthrough either the chamber walls (ground) or sometimes through the second electrode, which may be impedance-matched to ground.Operationally, the role of the matchbox is to match the net impedance ofthe plasma side of the circuit to the shunt capacitor within the matchbox.Under these conditions, half of the applied power goes into the plasma andthe remainder goes through the shunt capacitor. The power reflected backto the power supply will be a minimum at this point, and most matchboxescontain feedback control circuits that systematically adjust the two capacitors to minimize the reflected power.Matchboxes are engineered for the appropriate impedance and powerlevels for each given system.
At powers of a few hundred watts, it is possible to use variable capacitors constructed of intersperced plates in air. Asthe applied RF power approaches 1 kW, it becomes necessary to use vacuum-gap capacitors, which typically are rated to 5 to 6 kV and have capacitances of 10 to 2000 pF.
The inductor in the matchbox is often a silverplated Cu coil, which might be 3 to 7 turns. For powers greater than 1 kW,this coil is configured for water cooling. The capacitors are generallymotor-driven, and a feedback circuit is organized to minimize the reflectedpower to the power supply. The inductor is generally fixed and permanently mounted.PLASMA SYSTEMSFIG. 3.967Common L-type matching network.3.8 Magnetic FieldsThe other significant way to alter the plasma is to impose a magnetic fieldinto the plasma region. The force on a charged particle moving in a magnetic field is justF = qv • B(3.11)where q is the particle charge, v is the velocity, B is the magnetic field, andthe term v x B is the vector cross product of the velocity and the magneticfield. If the charged particle is moving in the same direction as the magnetic field, this cross product is zero and there is no magnetic force.
If theparticle is moving at right angles to the magnetic field, the particle is constrained to move in a circular path, as though it were circling a magneticR. POWELLANDS. M. ROSSNAGELfield line (Fig. 3.10). The net result for most particles is m o v e m e n t in a circular or helical path in which the helix axis is the same as the magneticfield direction. This can greatly increase their net path length within theplasma chamber. The radius of curvature for this motion is mass dependentand is given asr =mveB(3.12)where e is the electron charge, B is the magnetic field, m is the particle(electron or ion) mass, and v is its velocity (v = ( 2 K E / m ) 1/2, where K E isthe kinetic energy).
Since the ion mass is many orders of magnitude largerthan the electron mass, the effect of the magnetic field on ions is minimaland only the electrons respond in any practical way.In practical units for electrons, r (cm) = 2.4(KE)~/Z/B, where K E is thekinetic energy of the electron in eV and B is the magnetic field in gauss. Inpractical terms for ions, r (cm) = 104 (W)1/2 (KE)I/Z/B, where W is theweight of the ion in A M U (Ar = 40), K E is the kinetic energy of the ion ineV, and B is the magnetic field in gauss.Example: The radius of curvature for a 100-eV electron in a magnetic field of 100 gauss (0.01 T) is 0.3 cm.
For an Ar (40 AMU)100-eV ion in the same 100-gauss field, r = 66 cm. At energies of1000 eV, the electron radius is 0.7 cm and the Ar + radius is 203 cm.Magnetic Field: perpendicular to pageXXXXXXXXXXXXXXXXXXXXX,". X--.---X-IX",XXXX,XXXX ~\X~Xxxx"-__~xxxxxxxxxxxxxxxElectron motionperpendicular tol/~XxFIG. 3. I0 Motionof an electron in a magnetic field.magnetic fieldPLASMA SYSTEMS69Forcing the electrons in a plasma to move in helical, rather than straight,paths results in a great increase in the probability that an electron will havea collision with a gas atom, leading to either exciting or ionizing the atom.This effect can be used to great advantage to form a very dense, lowimpedance plasma.It should be noted here that the frequency of rotation of the charged particle, which is called the cyclotron frequency, is proportional to B/m, where m isthe mass of the electron or ion.
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