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The Sputtering Process

Sputtering is a momentum transfer process in which atoms from a cathode/target are driven off (or sputtering) by bombarding ions. In this process the momentum of the bombarding particles is more important than the energy. For example , a hydrogen or helium ion accelerated to 3,000 eV will cause very little sputtering compared to an ion of argon (which is chemically inert) with the same 3,000 eV energy, simply because the much higher hydrogen or helium ion has much less momentum.
Sputtered atoms travel until they strike a substrate , where they are deposited to form the desired thin film. As individual atoms, they can be chemically active and form compounds with the ions and atoms of the bombarding gas. For this reason, inert argon typically is used as the bombarding gas. In some applications, however, a reactive gas is intentionally added to the argon to alter the chemical composition of the deposited film (e.g., nitrogen gas in combination with tantalum sputtering to form tantalum nitride, TaN).
When argon atoms strike the target, their electrical charge is neutralized and they return to the process as atoms. If the target is an insulator, the neutralization process results in a positive charge on the target surface. The charge may grow so large that the bombarding ions are repelled and the sputtering process stopped. To allow the process to continue, polarity of the target must be reversed, attracting enough electrons from the discharge to eliminate the surface charge. This periodic reversal of polarity is accomplished automatically by applying RF voltage to the target assembly ( hence the term “RF” sputtering).
Of interest here is the diode rectifier-like behavior of the target and discharge systems. This results from the vast difference in mobility of ions and electrons. Electrons , being so much fast, are attracted in greater numbers to the target during the positive half-period of the RF voltage than are ions during the negative half-period. Thus, the target develops a negative DC bias.
Perkin-Elmer 4400-Series Production Sputtering Systems perform a number of sputtering process, each of which is ideal for a different application.

RF Diode Sputter Deposition
When the vacuum set point is reached, sputtering gas (typically argon) is introduced in the process chamber at a pre-selected rate (typically 40 sccm). A plasma, or self-sustaining glow charge , initiated by an automatic plasma igniter, appears when RF power is applied between the target and electrical ground, ionizing the argon gas.
A negative (-) potential applied to the target, as a result of the applied RF power, attracts the ionized argon at a momentum determined by a) the magnitude of applied potential and b) the mass of the ion. The momentum of the incoming ion is transferred to the target material, causing surface atoms or molecules of target material to be ejected (sputtered). These sputtered atoms travel across the gap separating the target (cathode) and substrate table (anode), and are deposited on the substrate (wafers) which are arranged on the substrate pallet.
RF Magnetron Sputter Deposition
RF magnetron and RF diode sputtering are very similar, except that during RF magnetron sputtering a magnetic field deflects the secondary electrons (which are produced during normal sputtering operation) away from the substrates. The sputtering process, which is cooler than RF diode sputtering, permits materials to be sputter deposited on substrate at lower temperatures and greatly reduces the chance of radiation damage to delicate substrates.
Because the impedence of a magnetron is lower, higher power densities are possible at lower potentials, effecting high sputter rates.
DC Magnetron Sputter Deposition
DC magnetron targets enhance the plasma density and increase the sputtering rate, by trapping electrons in an electromagnetic “envelope”. This “envelope” is formed when lines of the magnetic field enter and exit the target face and when the loci of maximum transverse magnetic fields form a closed figure. Because the currents involved are very large, a separate, positively-biased anode (a dark space shield) is used to collect the electrons. A similar dark space shield is used in RF diode and RF magnetron deposition, This dark space shield prevents the sides of the target and target backing plate from sputtering.
RF Sputtering-Etching
Essentially the reverse process of RF diode sputter deposition, in which the substrate table becomes the cathode (negative pole) and the target assembly becomes the anode (positive pole). Under these circumstances, surface material from the substrates is ejected. Surface impurities are ejected along with substrates material, making this process useful for pre-cleaning substrate prior to sputter deposition. In order to prevent ejected material from contaminating the target, s shutter is positioned between target and substrate.
Bias Sputter Deposition
Bias sputtering combines the DC or RF sputtering and the RF etching operations. While DC or RF power is applied to the target, a small amount of RF is also applied to the substrate table. As a result, the substrate and target are both bombarded by ions ( the substrate to a lesser extent than the target ). In many applications this process yields superior quality films than can be attained using DC or RF sputtering with grounded substrates. Bias sputtering influences the crystal structure, and tends to re-sputter trapped argon from the growing film during deposition and rearrange individual atoms of the sputtered material; this improves stoichiometry and step coverage. Bias sputtering can be used to adjust film resistivity and film stress to desired levels.
Reactive Sputtering
Some metals, such as nitrides and oxides, are best deposited by this method: the target is the parent metal and a small amount of nitrogen or oxygen is introduced into the process chamber along with the argon sputtering gas. Because ionized gases are typically highly reactive, a film deposited in a mixture of argon and a reactive gas will often form a compound with the reactive gas (e.g., a nitride or an oxide).
Co-Deposition
Sometimes called co-sputtering or dual deposition, co-deposition is identical in principle and practice to other types of sputter deposition, except that two targets (typically of different materials) are simultaneously activated. Substrates passing sequentially and repeatedly beneath the targets are coated with alternating, very thin films of two materials. Under certain circumstances, the resultant film can be equivalent to or better than one formed using a composite target. During co-deposition, both targets may be RF, both DC, or one RF and one DC.