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A TITANIUM NITRIDE COATING TESTED IN PASSIVE INT
ERMODULATION

F. Suárez (1), C. Palacios (2), C. Montesano (3)
and F. Rueda (1) (1) Universidad Autónoma d
e Madrid (2) Distcom Antenas, S.L. (3) Constru
cciones Aeronáuticas S.A. (CASA)-División
Espacio
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• INTRODUCTION

• Titanium Nitride (TiN) is the most used
  coatingto prevent Multipactor (MP) due to its
  stability in RF field they have solved the MP
  problems of high RF power (MegaWatt) in particle
  accelerators (Los Alamos, Fermilab, DESY), at
  wave guides and wave guide windows, with metallic
  and ceramic surfaces (W.D.Cornelius and R.J.
  Grieggs J. Lorkiewickz et al.).

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• TiN has metallic conduction, due to one electron
  per Ti atom not entering the covalent bond.
  
• A thin layer of TiN (40nm) is enough to reduce
  the SEE coefficient.
  
• The sublimation of Ti by heating a simple
  filament inside the RF element in N2 or NH3
  atmosphere is used as anti-multipactor remedy.

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• PROPERTIES OF THE TINX COMPOUNDS

• Metallic electrical conductivity ( max. for
  x1).
  
• Thermalmand chemical stability.
• NaCl Crystal structure (0.6
• Lattice Parameter 4.22 - 4.25 A (0.6 1.2).
  
• Absorción in visible and near IR.
• Híbryd bond of N and Ti.
• Color of broze.
• Hardness similar to diamond.

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• In previous work in our laboratory and in ESTEC
  several coatings were tested for multipactor
  threshold power. when deposited with ion
  assistance TiN has shown the best results.
• Alodine has also shown a considerable
  threshold power, as shows the next transparecy.
  

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• TiN ion assissted coating presents 2 times the
  Alodine threshold power (Final Report ESTEC
  Contract No. P.O. 162594 /1996).
  

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• For satellite applications a coating such as TiN
  has to fulfill a complementary requirement not
  applicable in particle accelerators to be PIM
  compatible.
• The present work had as objective to test two
  types of TiN coatings for PIM.
  

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• The difficulty in obtaining a coating to prevent
  MP lies both in compositional reproducibility and
  stability of the SEE coefficient after long
  exposures to air and contamination at the vacuum
  chamber during subsystem or satellite
  qualification tests. The approach of this work
  has been

1) To obtain a relatively thick coating of TiN
so that its crystal structure and bulk c
omposition can be
measured.
2) To test this coating for PIM.
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• Among the proposed PIM generating processes,
  leading to non linear interaction function
  between field and induced current, the tunnel
  effect at localized electrical contacts in
  flanges or connectors has been the most studied.

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• The absence of surface oxides does not seem to be
  sufficient to guarantee a PIM free circuit, as
  saturated non ohmic contacts can develop even in
  gold coated connectors (Weibel and Hügel).

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• PIM level as a function of the applied voltage
  to a piezoelectric crystal by Weibel and Hügel.
  (1998).
  

Voltaje
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• COATING DEPOSITION - 1

• The TiN coatings were deposited by evaporation of
  Titanium with a Varian e-gun in a N2 atmosphere
  of 10-4 torr at a bakeable metal bell jar pumped
  by 500l/s turbo molecular and ionic pumps,
  reaching after baking at 250ºC for 12h a base
  vacuum of 2x10-8 torr. This deposition was
  preceded by 500eV Ar ion cleaning of the
  substrate and subsequently by an Ion Beam
  Assisted Deposition ( IBAD) of Ti. This was done
  either by Ar ions ions resulting from feeding a
  Commonwealth Scientific Co Kaufman 3 cm
  diameter ion gun either with Ar or with an Ar
  N2 mixture at a current of 15mA and 200-300 eV.

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• COATING DEPOSITION - 2

• The Indium of 99.999 purity (Fa. Haart Co) was
  evaporated at 1 10-7 torr up to about 1
  micrometer thickness.
  
• The Ti and In sources were near the normal of the
  substrate at about 20 cm distance and a shutter
  collected the first fractions. The ion beam angle
  with the sample normal was about 45º for TiN
  deposition.

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• COATING DEPOSITION - 3

• AA 2017 Al alloy gaskets with different surface
  finish, glass microscope slides and graphite were
  used as substrates During the evaporation the
  substrates attain about 50oC (thermocouple on
  substrate). The film thickness, in the range of
  0.12-1.0?m, was monitored with a quartz crystal
  oscillator (APT) and verified with a stylus type
  apparatus (Talystep, Taylor ? Hobson, U. K.). The
  titanium based coating deposition rate was
  between 25 and 35Å/min.

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• COATING DEPOSITION - 4

• The surface morphology and EDAX bulk composition
  measurements were performed in a Philips XL 30
  scanning electron microscope in as-grown films
  on graphite substrate.

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• PIM TEST DESCRIPTION - 1

• The purpose of subjecting the different coated
  gaskets to the PIM test is to check the behaviour
  of these coatings in terms of PIM-free generation
  and confirm that this type of coating is valid to
  be used in space antenna elements (horns,
  orthomode-transducers, polarisers) with
  restrictive specification both for multipaction
  and PIM.
• Hence, it seemed interesting to perform the test
  according to the most restrictive requirements
  requested for a real component designed and
  manufactured by EADS Casa Espacio.

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• PIM TEST DESCRIPTION - 2

• The test consisted in applying two carriers of 84
  W (49.24 dBm) each at Ku band. The selected TX
  frequency band was 11.5GHz to 12.3 GHz. The
  combination of this carriers produced a 5th order
  PIM falling into the RX frequency, ranging from
  13.5 GHz to 14.0 GHz. 
• To guarantee the proper reception of the PIM
  level, the receiving system must present a good
  noise floor, providing during the test
  performance a SNR of 10 dBm. 
• Table 1 summarises the main parameters which
  define the PIM test performed on the gaskets
  under study. 

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Figure 1. Type 1 gasket layout
Figure 2. Type 2 gasket
layout

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• SAMPLES DEFINITION

Two types of geometry gaskets have been designed
based on WR75. Type 1 corresponds to a piece of
wave-guide of 4mm-length with a PIM-free flange
design which presents lips, hatched in black
colour, at both sides. Type 2 corresponds to a s
imilar piece but without PIM-free flange. The
purpose of this Type 2 is to see if a proper
coating can or cannot replace a PIM-free flange
mechanisation.
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Conventional ALODINE coating on AA1200
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Gasket surface prior to treatment .
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Gasket after Alodine treatment .
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Gasket surface after Alodine treatment .
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Gasket after TiN ion assisted deposition
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• SEE film properties

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•  
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• Crystalline TiN phase with high nitrogen content
  is reproduced in ion assisted depositions.
  
• The TiN coated flanges showed similar PIM levels
  to those of Alodine 1200.
  
• This indicates that ion beam assisted TiN
  coatings may also be accepted as PIM compatible
  for mechanical contacts in RF wave-guides.

• CONCLUSIONS