Chapter III: III: Sputtering and secondary electron emission

Transcription

1 References [1] Handbook of putter deposition technology, Kiyotaka Wasa, Noyes publications, NJ IN: [2] old Plasma in Materials Fabrications,. Grill, IEEE Press, NY(1993). IN: [3] The Materials cience of Thin Films, M. Ohring, cademic Press, an Diego,1992. IN: X [4] asic data of plasma Physics,.. rown, IP Press, N.Y., IN: X hapter III: III: puttering and secondary electron emission III.1 Plasma,. Lucas III.2 Plasma,. Lucas

2 1. 1. Interactions of of particles with with surfaces putter yield # atomsremoved # incidentions is function of: - energy of incident particles, - target material, - incident angle of particles, - crystal structure of the target. 2.1 Ion energy. First observed by Grove in a D gas discharge in The cathode surface of a discharge tube was sputtered by energetic ions and deposited on the inner walls of the tube. t that time, it was regarded as an undesired phenomena. Today, it is widely used for surface cleaning and etching. nother phenomenon resulting from the particles bombardment of a surface is the secondary electron emission. It is observed for bombardment by ions, electrons, photons and neutrals. It plays a significant role in the glow discharges. Threshold energy is in the range of ev. It depends on the particular collision sequence involved. E th means primary recoil is produced in the first collisions and is ejected directly. E th is for multiple sputtering collisions. Maximum sputter yields are seen in region of about 10 kev. E E 2 (E < 100 ev) (E > 100 kev) (E > 100 ev) Energy III.3 Plasma,. Lucas III.4 Plasma,. Lucas

3 2. 2. putter yield putter yield 2.2 Incident ion, target material. Threshold values and sputtering yield for r bombardment [2,3] Threshold (ev) r + energy (ev) l r u Fe Ni Ge i Ti V puttering yield versus incident atom and target (normal incidence) [2] Ne r Zr l u III.5 Plasma,. Lucas III.6 Plasma,. Lucas

4 2. 2. putter yield 2.3 ngle of incidence and angular distribution putter yield 2.4 rystal structure of the target puttering yields and the angular distribution of the sputtered particles are affected by the crystal structure of the target. In very first approximation, this is related to the interplanar distance and the binding energy in the different planes. When target is in liquid state, of %. III.7 Plasma,. Lucas III.8 Plasma,. Lucas

5 2.5 putter yield of alloys Experience have shown that chemical composition of deposited films sputtered from an alloy is very closed to that of the target. This suggests that sputtering is not governed by thermal processes, but by momentum transfer processes. uppose sputtering on a binary alloy target of atoms and atoms (n and n, nn + n ) with sputter yields and. Target concentration: n / n and n / n. Flux of atoms : F, Flux of atoms : F. F F.. If n g atoms of gas impinge on the target, the total number of atoms sputtered is n g. ame for The surface concentration of the target is modified as: putter yield ( 1- n ( 1- n g g / n) / n) 2.5 putter yield of alloys If >, the surface is enriched in atoms which now begins to sputter in greater profusion, i.e. Ex: F F ( 1- n ( 1- n / n) / n) Progressive change in the surface target composition will alter the sputter flux ratio until it is equal to /, same as initial: F F g g Target Permalloy film (80% Ni, 20 % Fe) at 600 ev: putter yield 80 (1.26) (1.52) which is equivalent to 78 Ni and 23,6 Fe. Remarks: This is in complete contrast to evaporation where deposit stoichiometry is totally lost. III.9 Plasma,. Lucas III.10 Plasma,. Lucas

6 2.6 puttered atoms putter yield Nature of sputtered species 2.6 puttered atoms putter yield Most sputtered material is ejected in the neutral atomic state. The fraction of the charged particles sputtered from clean metal and semiconductors is on the order of 10-4, becoming larger for surfaces contaminated with strongly electropositive or electronegative species. Polyatoms or clusters can also be formed, but in very low amount (< 1 %) % of sputtered particles are neutral Energy distribution of sputtered particles. E mpe E E : urface binding energy. urface binding energies for selected metals Element E (ev/atom) Element E (ev/atom) Mg 1.51 g 2.94 l 3.39 d 1.16 Ni 4.44 W 8.79 u 3.48 Pt 5.85 Zn 1.35 u 3.81 Nb 7.44 Pb 2.03 III.11 Plasma,. Lucas III.12 Plasma,. Lucas

7 2.7 puttering mechanisms putter yield 2.7 puttering mechanisms putter yield The maximum energy transferred T m is: It is max when M 1 M 2. 4 M1 M ( M 1+M 2 T m 2 2 ) E (b) imple collisions: Low energy regime: incident ion stays under the surface, energy can be transferred to sputtered particles by incident ion (c) Linear cascade: puttered particles have been mainly knocked by recoils from the targets. ppears for incident particles of several kev, and mean atomic weight. (d) Thermal spike: Large amount of defaults are created inducing very high temperature ( evaporation T ). ppears for heavy ions of hundreds kev. Regime (c) and (d) are found in Ion Implantation. Regime (b) is found in plasma sputtering. In the energy range relevant to sputter deposition (< 1 kev), typical collision cascades are of radius 10 nm, and sputter ejection are due to collision cascades initiated within five atomic layers below the surface. igmund has established for E < 1 kev that: 3 (E) T 2 4π U igmund has established for 1 < E < 10 kev that: Where m α n(e) (E)0.420α U U : heat of sublimation (ev), α is a measure of the efficiency of momentum transfer in collisions n (E): Nuclear stopping power (fct Z 1,Z 2,M 1,M 2,orn- Mayer interaction potential,...) III.13 Plasma,. Lucas III.14 Plasma,. Lucas

8 2.7 puttering mechanisms putter yield 2.8 ome data putter yield III.15 Plasma,. Lucas III.16 Plasma,. Lucas

9 2. 2.putter putteryield yield 2.8 ome data 2. 2.putter putteryield yield 2.8 ome data III.17 Plasma,. Lucas III.18 Plasma,. Lucas

10 2. 2. putter yield econdary electron emission 2.8 ome data 3.1 Electron bombardment # of δ # of emittedelectrons primary electrons Rem: 2 nd Electron energy: few ev. III.19 Plasma,. Lucas III.20 Plasma,. Lucas

11 3. 3. econdary electron emission econdary electron emission 3.1 Electron bombardment 3.2 Ion bombardment # of emittedelectrons γ # of primary ions Element/ompounds δ max and V δmax [4] δ max g 1.5 l u 1.3 Fe 1.3 Ni 1.3 W 1.4 Zr 1.1 V δmax (800 V) Rem:-urface contamination plays a major role. This is very important in D sputtering with a contaminated target (V-I characteristics. continuously changing). -2 nd electron energy: few ev. - γ for metals << 1 (typical: 0.2). - γ for insulators > 1 (difficult to measure). III.21 Plasma,. Lucas III.22 Plasma,. Lucas

12 3. 3. econdary electron emission 3.3 Neutral bombardment Rem: - In sheath of an electrode, energetic neutrals are produced through collisions of ions with neutrals. The neutral energies are a few hundred ev at most. Emission due to neutral collisions is probably negligible in cold plasma. 3.4 Photon bombardment Typical photoelectric yield is 10-4 to 10-3 electron per photon, negligible in sputtering and cold plasma discharges. III.23 Plasma,. Lucas