Magnetic suppression of secondary electrons in plasma immersion ion implantation

Abstract

In this work, magnetic suppression of secondary electrons in plasma immersion ion implantation is demonstrated experimentally in a vacuum arc system. Secondary electrons emitted normally to a copper sample surface were detected by a Faraday cup, whose signal exhibited large negative spikes coincident with high voltage pulses when aluminum ions of an unmagnetized plasma were implanted. When a 12.5 mT magnetic field parallel to the sample's surface is applied, these spikes are not seen, showing that secondary electrons were magnetically suppressed. Another cup, oriented to detect electrons that flow along the field lines, does not exhibit such negative spikes in either unmagnetized or magnetized plasmas, indicating that a virtual cathode was formed by the trapped secondary electrons. Plasma immersion ion implantation (PIII) is a non-lineof-sight technique that eliminates the necessity of target manipulation. It consists of applying negative high voltage pulses to a target immersed in a plasma medium, from where ions are extracted directly and accelerated into the target, resulting in a homogeneous surface implantation from all sides. Several works have demonstrated the successful metallurgical and semiconductor implantation using this technique.1,2 The use of ion implantation using plasmas made of metallic ion specie 3,4 has also been demonstrated to have numerous applications, when vacuum arc sources proved to be excellent devices for the production of metallic plasmas. 5Secondary electrons emitted due to high energy ion bombardment of material surfaces in PIII can decrease significantly the efficiency of this technique, since a large portion of power is lost into electron energy. Secondary electron emission coefficient can be as large as 20 in metal surfaces, reducing efficiency to values as low as 5%. Furthermore, for electron energies higher than about 40 keV, electron impact with chamber walls produces hazardous x-ray radiation. 6 One technique used to suppress x-ray generation is based on electrostatic confinement of secondary electrons, which are trapped within a metal enclosure biased to the same potential as the target.7 Secondary electrons are repeatedly reflected within this enclosure, and are prevented from impacting the chamber walls. Efficiency could be improved if electrons dissipate their energy into the plasma during reflections. Despite the success in reducing x-ray levels, to our knowledge efficiency improvement was not demonstrated.Suppression of secondary electrons has been proposed by Rej et al. 8 using an externally applied magnetic field, parallel to the target surface. Secondary electrons would be confined by the field forming an electron layer near the surface, which acts as a virtual cathode, reducing the local electric field so that subsequent emitted electrons can be reabsorbed by the surface. Since electrons are free to move in the direction parallel to the magnetic field lines, the virtual cathode will be maintained if it is formed faster than the axial transit time. If this condit...

Magnetic field effects on secondary electron emission during ion implantation in a nitrogen plasma

“…2͑b͔͒. Furthermore, the total suppression obtained during the same experiments made in aluminum vacuum arc plasmas, 16 in which plasma densities are even larger ͑n ϳ 10 11 cm −3 when the magnetic field is turned on͒, corroborates this explanation. From the expression = / ␥J i = 0 E / ␥J i , it is seen that at higher voltages ͑when x rays become important͒, the electric field will increase proportionally to V and make larger, but at the same time the incident ion current J i ͑Child-Langmuir space charge limited current͒ will increase proportionally to V 3/2 , so that suppression should also occur at higher voltages.…”

“…15 Since it was published, this magnetic suppression method in PIII systems has not been tested experimentally until recently when we demonstrated in a vacuum arc aluminum plasma that secondary electrons were indeed suppressed by a magnetic field parallel to the target surface. 16 In our equipment, the plasma streams along a straight magnetic duct and a target electrode is positioned with its surface parallel to the duct axis. Two Faraday cups monitoring secondary electrons emitted along and across the field lines did not detect any electrons when the magnetic field was turned on.…”

Graphene-quantum-dot nonvolatile charge-trap flash memories

“…To check the existence of the GQDs, PL was measured before the deposition of the control oxide. As shown in The amount of charge stored in the GQDs can be estimated by the relation Q = CΔV MW , where C is the capacitance density and V MW is the memory window [23]. In this work, C and ΔV MW are estimated to be 1.6 × 10 −8 , 1.1 × 10 −8 , 9.5 × 10 −9 F cm −2 , and 2.8, 3.2, 4.4 V, taken in the linear region of figure 4(d), for d = 6, 12, and 27 nm, respectively.…”

“…The graph decreases after reaching ∼3 V, saturates at ∼4 V, or continues to increase up to ∼8 V for 6, 12, or 27 nm GQDs, respectively. The MOS structures without GQDs showed just a few mV of memory window, meaning almost no memory effect.The amount of charge stored in the GQDs can be estimated by the relation Q = CΔV MW , where C is the capacitance density and V MW is the memory window[23]. In this work, C and ΔV MW are estimated to be 1.6 × 10 −8 , 1.1 × 10 −8 , 9.5 × 10 −9 F cm −2 , and 2.8, 3.2, 4.4 V, taken in the linear region of figure4(d), for d = 6, 12, and 27 nm, respectively.…”

Aluminum Implantation in Kapton® for Space Applications: Magnetic Field Effects on Implantation in Vacuum Arcs

“…Another possible effect of the magnetic field would be the confinement of electrons near the substrate, neutralizing the charging effect of the ions. We demonstrated recently 21) that secondary electrons emitted during PIII in metals are confined by a magnetic field parallel to the substrate, forming an electron layer near the surface. This effect, however, would only confine secondary electrons and neutralize the extra charge caused by their emission, which in any case is not significant in polymers.…”