Vacuum deposition of TiN

Transcription

1 J.Lorkiewicz DESY Vacuum deposition of TiN (TiN coating of high power coupler elements as an anti-multipactor remedy at DESY) The scope of the project: - reducing secondary electron emission and multipactor effects in TESLA couplers by TiN layers generation on the vacuumfacing surfaces. TESLA Test Facility (TTF) couplers: Coupler Frequency Pulse Repetition Beam Cold coax length rate power diam. TTF 2 TTF GHz 1.3 ms 5 Hz 250 kw 40 mm TTF GHz 1.3 ms 5 Hz 845 KW 80 mm Ca. 90 components were treated for anti-multipactor protection : 70 K (cold) cylindrical windows for TTF2 and TTF 3 couplers, 300 K (warm) cylindrical windows for TTF2, planar wave guide windows for TTF2, whole surface TiN coating of a single TTF 2 coupler, (planar coaxial windows for LAL ORSAY), (ceramic discs for planar windows (for DESY and SINS)

2 1. Coating method: - Ti deposition on a substrate from vapor phase in a (reactive) atmosphere of low-pressure ammonia. Metal vapor is released by sublimation from electrically heated titanium filament. 1.1 Apparatus: - a standard vacuum vessel (20 cm diam., 80 cm long) equipped with a turbo-pump unit, two gas feeding lines for pure nitrogen ( %) and ammonia (99.98%) with typical vacuum-handling equipment. - a sublimation setup with insulated filaments of (99.8% purity) 1 mm diam. titanium wire shaped as loops or vertical lines, a system of supports and shields for substrates. - diagnostic: - pyrometer, - mass spectrometer, - quartz crystal film thickness monitor, - thermoelements and thermometers. 1.2 Coating procedure: Substrate preparation: - preliminary cleaning (sandblasting ), - (ultrasonic bath in a degreasing agent-soap)+ ultrasonic bath in utlra-pure water, - drying with pure nitrogen blow, - sublimation setup assembly.

3 Coating procedure (see time-schedule plots): - venting with nitrogen and pumping to reach a base vacuum of low 10-6 mbar (mass-spectrometric measurement of residual gas composition shows 90% of H 2, 8 % of H 2 O and 1% of O 2 in terms of partial pressure), - filament preheating with dc current at o C (impurities desorption + preheating the substrate), - intermediate venting with NH 3 and further pumping, - raising the NH 3 pressure to mbar under continuous pumping, ammonia pressure is matched to the average filament substrate distance L using the formula: L(m)=2.33x T(K)/(2.54. p(torr). (4.42x10-8 ) 2 ) - After min. Ti sublimation (coating) is started by raising the current to A (wire temperature 1790 K), the coating is completed in 30-90s (possible surface resistivity monitoring on separate ceramic samples, its value should reach kohm/square at the end of deposition), - after coating - the vacuum pumping is interrupted and ammonia pressure is raised (step-by-step) to mbar and left for more than 10 h (so-called afterprocessing in ammonia). - exposition to air - surface resistivity grows up to GOhm values within hours/days.

4 Typical coating procedure - time schedule for TTF3 windows pressure (mbar) 10 3 Total pressure vs time 10 1 NH 3 admission 10-1 Afterprocessing in NH 3 for > 10 h time (min) current (A) substrate preheating Filament current vs time Coating for s Filament preheating Time (min) temperature ( o C) 160 Substrate temperature vs time Time (min) Time (min.)

5 2. Measurement and control of layer thickness, layer uniformity. - The on-line indications of the quartz thickness monitor are effected by heat and radiation during deposition (due to space restrictions). Therefore, it was not used to determine the absolute value of layer thickness. -The final layer thickness is controlled and established by deposition time at a given filament temperature T fil. Metal vapor pressure p and sublimation rate w (the mass of metal sublimated from 1 m 2 of a filament surface in 1 s) are related to filament temperature via the formulas Log p(torr) = ( T fil (K) 19.72) w(kg m -2 s -1 ) = 0.585p(M Ti /T fil ) 1/2 Deposition rate distribution is derived from the geometry of source - substrate system. It is computed from basic emission laws. (inverse square law for a point source was verified for a filament which is positively biased ( +10V) respectively substrate holders. The calculated deposition rate distribution is directly checked during an extra preliminary test with an array of strategically exposed glass samples (instead of the real substrate) coated up to a layer thickness of some hundreds of nm (after a deposition time extended to 30 min.). The apparent mass increase of a sample, of the order of 1 mg, is weighed and the deposition rate distribution on the substrate surface is derived. The relative change in time of the deposition rate is checked using a quartz crystal thickness monitor.

6 - The layer thickness uniformity depends on the sublimation setup geometry, substrate shape complexity, possible shadows and space restrictions and stability of filament position. - The thickness of the surface layers deposited from 6+1 vertical filaments on cylindrical surfaces od TTF3 windows varied within +/-8% of the average value. They looked optically uniform. 16 cm Ti filaments (1 mm diam.) RF windows Sublimation setup for Ti-coating of TTF3 coupler windows. Typical processing parameters for cold windows (outer diam. 45 mm, inner diam 35 mm, ceramic ring length 33 mm): filament (dc) current 16A, deposition rate on the outer surface 6.2 nm/min, layer thickness uniformity on the outer surface % ± 8 deposition rate on the inner surface 13 nm/min, ammonia pressure ca 10-3 mbar

7 - A single-operation coating of TTF2 planar wave guide windows was done using a Ti-wire loop installed inside the pillbox container of the window located 25 mm off the ceramic disc surface. The layer thickness achieved on the disc varied by a factor of 4 depending on the position on the disc. The thickness distribution was matched to the RF electrical field distribution: thick layer ( 15 nm) close to the filament position was reached in the region largely overlapping the areas of high rf electrical field component perpendicular to the disc where strong multipactor effects are likely at TE 01 propagation mode, thin layer (of 3 4 nm) was deposited in the center of the disc where maximum of the tangent component of rf electrical field is reached. Thus the achieved thickness distribution reduces ohmic losses within the layer. medium thickness layer (of ca 7 nm) is deposited at the rim of the disc (rf electrical field close to 0) where multipactor is likely to be initiated by lowenergy electrons. 3 nm ca 15 nm Filament inside the pillbox pillbox ca 6-7 nm ceramic disc inside the pillbox isolated support structure Sublimation setup for TTF2 wave guide window

8 The rf behavior of the wave guide window was largely improved after coating (3 x shorter conditioning time, proper power transmission and insensitivity of a previous exposure to air). Coverage of metallic rf power components should be complete. Tle layer thickness can be as high as several tenths of nm. 3. TiN layer testing: DESY test resonator for multipacting current measurements of metal surfaces in RF fields. Test resonator for complex dielectricity constant measurement of aluminum oxide windows and loss tangent determination (inv. Moscow Engineering Physics Institute). Secondary electron yield (SEY) measurement (Soltan Inst. Nucl. Studies (SINS), Dpt. 4, Swierk, Poland). TiN layer chemistry studies: - SIMS tests (Fraunhofer Inst., Braunschweig) - XPS measurements (Institute of Physical Chemistry, Warsaw, Poland) 3.1 TiN layer chemistry - SIMS depth profiles of TiN layers on copper and alumina (elemental analysis):

9 - A ca 1nm-thick carbon layer is present on the surface. Ti:O ratio of about 1:0.85 is reached below. There is evidence of oxygen incorporation and chemisorption from the substrate into the layer on the layer-al 2 O 3 Interface, resulting in good layer adhesion to ceramic surface and higher oxygen contents (in the layer) compared with metallic substrate. TiN surface layers on ceramic are not removed by ultrasonic bath or etching in citric acid. TiN adhesion to a metallic substrate is much weaker (the layer can be scratched). - XPS depth profile of TiN layer on a ceramic sample (the plot below): - high TiO 2 contents in top layer (left side of the plot), - TiN, TiO x N y (oxinitrides) and TiO 2 mixture beneath. contents (at %) 16 TiO 2 12 TiO x N y TiN Sputtering time (ion current 5 microa) [min.] XPS analysis of a TiN layer on ceramic, titanium forms (by J. Sobczak et al, Institute of Physical Chemistry, Warsaw)

10 3.2 Multipactor suppression, ohmic losses, secondary emission: - The choice of the layer thickness of 6 9 nm for evaporation coating was done taking into account the measured values of multipacting time, secondary electron yield and loss tangent. multipactor time (s) A. Brinkmann, DESY electrode of: Al Cu loss tangent (10-4 ) Z. Yu, DESY temperature: 300 K 70 K J. Kula, S. Pszona, SINS, Swierk secondary electron yield (SEY) Promary electrons energy: at maximum SEY 1 kev 5 kev layer thickness (nm)

11 - One can expect higher ohmic losses for sputtering TiN coated ceramic substrate due to a higher contents of conducting TiN. In this case the layer thickness is often restricted to 1 nm. 4. Impact of the whole surface coating of a TTF2 coupler Apart from the rf windows the following TTF2 coupler components were TiN coated: - the terminal wave-guide section with doorknob junction, - the inner and outer warm coax conductors, - the complete cold coax. The preconditioning time on a test stand of TTF2 coupler dropped drastically from over 130 to 24 hours for normal conditioning parameters (peak power of 1 MW, pulse length 1.3 ms and repetition rate equal 2 Hz) as a result of coating. Preconditioning time at rf pulse length of 20 µs alone was reduced from 36 to 4.5 hours. In this case TiN coating proved to be more effective in multipactor elimination than inner conductor bias in coax line or rf glow discharge cleaning in argon performed earlier. After rising the maximum input power to 1.8 MW electron activity started in a strictly localized region of the coupler antenna with layer thickness below 5 nm, at a pulse length of 800 µs which could not be removed by further conditioning. To avoid such effects, increase of layer thickness on metallic components in cold coax is recommended (up to nm) and improvement of its uniformity. The latter can be reached by a better stabilization of filaments positions during surface processing (against thermal stresses ).

12 5. Sputtering TiN deposition vs evaporation Sputtering Deposition of TiN molecules from discharge plasma The sputtering rate is not easy to determine Source-substrate distance may become critical for energy distribution and stability of discharge Substrate rotation often needed to reach uniform deposition rate The risk of ohmic losses due to high TiN contents Evaporation Deposition of thermal energy ( 0.1 ev) Ti atoms + chemical conversion, non-discharge process Advantages The process is slow, easy to control and to interrupt Deposition rate is easily calculable Can be used in restricted volume No rotational elements Low ohmic losses due to partial oxidation Simplicity and low cost Drawbacks Discharge run in inert gases (N 2 +Ar) Long and incomplete Ti TiN conversion, (the risk of high residual conductivity due to the presence of pure Ti) Usage of reactive atmosphere (NH 3 ) and its impact on vacuum system. No possibility of using cold traps