Low loss high definition conductor line in LTCC

III MEASUREMENT APPROACHIII-1 TEST STRUCTURERF measurements were performed on stripline (embedded line). With this kind of structurethe field is confined [5]-[7]. This transmission line is particularly adapted for conductor losscharacterisation [8]. So, it was possible to study the following parameter effects on theconductor losses: DC electrical resistivity, line edge ripple and dielectric-conductor interfaceroughness.70 µmAIR180 µmGroundplaneLTCCEmbeddedline180 µmFigure 1: stripline structure used for electrical resistivity, line edge ripple and interfaceroughness, as well as electrical losses characterisation.III-2 DC ELECTRICAL RESISTIVITYThe DC electrical resistivity has a significant impact on RF losses. The line resistivity is givenby ρ = R*(S/L) with R the measured resistance, L the conductor length and S the conductorcross section area. In order to be in a similar configuration to that of a stripline, a buriedmeander line having 70 µm width and 25 cm length was included in the test vehicle. Theresistance was measured using a high accuracy four wire ohmmeter Hy-track 100D.S is the average section area, determined by making 10 cross sections of the line and using anappropriated image processing tool to calculate the section area.

III-3 LINE EDGE RIPPLEIn addition to the conductor DC electricalresistivity, the non regular shape of the lineintroduces electrical losses in RF. Wecalled this phenomenon “line edge ripple”.See fig 2. To quantify the line edge ripple,thirty 70 µm buried lines were included.Successive sections of these lines wereperformed to measure the correspondingwidths (W 1 , W 2 , …W i, … Wn). The lineedge ripple is defined as the standarddeviation of Wi.Perfect lineTOP VIEWWidth W 1Width W 2...Width W iFigure 2: ideal and real linesLine edge ripple affectingelectrical lossesIII-4 INTERFACE ROUGHNESSThe topography of the interface between conductor and dielectric also contributes in RFlosses. Most of EM simulation tools takes into account this parameter. In order to characterisethe topography of the conductor-dielectric interface, the quadratic roughness Rq has beenmeasured. Since the transmission line is buried, the substrates were polished following the z-axis down to internal conductors. Then, the silver conductors were chemically dissolved.Thus, it was possible to really measure the roughness (Rq) with a Talysurf Series, RANKTAYLOR HOBSON. A cut-off of 0.8 mm was used and measures were performed on 8 cutoff(6.4 mm).Internal lineInternal line afterpolishingInternal line –dielectricinterface after dissolutionFigure 3: Method for roughness measurement of internal lines.

IV CHARACTERISATION RESULTSIV-1 PHYSICAL CHARACTERISATION RESULTSDC electrical ResistivityStd Adv Photo Solid silverresistivity (10 -8 Ω.m) 2.1 1.8 2.0 1.6The DC electrical resistivity is affected by the glass phase ratio and the grain structure of thepaste. The DC electrical resistivity increases with the glass phase ratio. Finer grains increasethe contact surface and consequently improve the conductivity.Paste grain structureStandard (glass + silver) Advanced (pure silver) Photo-imageable (glass + silver)scale (x 5000) : 2 µmPhoto 1: grain structure of the silver powdersThe advanced paste, which has the best conductivity, contains pure silver and features thefinest grains. The photo-imageable paste has a similar grain structure but contains glass phase.Its DC electrical resistivity is 11% higher than the advanced one. The standard paste has theworst granulometry and contains also glass phase. It has the highest resistivity: + 17%referring to the advanced material.Line edge rippleLine edge ripple quantified by thestandard deviation (µm)Std Adv Photo6.43 2.37 2.29

Cross section viewThe smallest interface roughness is obtained with thephoto-imageable paste while the standard paste givesthe highest roughness. The measured values areconsistent with the optical analysis of the cross sections.Photo 3: conductor-dielectric interface for the Advanced paste.IV-2 RF CHARACTERISATIONRF measurements were performed with a vector network analyser and two GSG millimeterwavecoplanar probes. Measurements were calibrated with a coplanar SOLT standard kit.0,35Stripline attenuationv. frequency & pastesStdPhotoAttenuation(dB/cm)0,300,250,200,150,10Adv0,051 2 3 4 5 6Frequency(GHz)Figure 4: Attenuation versus frequency for 3 conductive materials.

ATTENUATION (dB/cm)1 GHz 2 GHz 3.5 GHzStd 0.13 0.17 0.24Adv 0.08 0.12 0.18Photo 0.096 0.13 0.2Table 1: Attenuation versus frequency and conductive materials.The measured losses are relatively high due to the fact that the characterisation is based on astripline structure with very fine line (70µm).Over the 1 to 6 GHz frequency range, the two none standard pastes show significantimprovements. Compared to the standard one, the advanced paste reduces RF losses by 29.4%@ 2GHz and the photo-imageable paste reduces RF losses by 23.5% @ 2GHz.IV-3 SYNTHESIS OF PHYSICAL AND RF MEASUREMENTSSome important physical and electrical parameters for line losses were measured:• The DC resistivity gives a measure of the “intrinsic” material conductivity.• According to RF current locations, line edge irregularities may have a strong influence online losses.• In the upper frequency range, when the skin depth begins to be very influential, theconductor surface roughness is also a key parameter.These features are summarised in the following table:Std Adv PhotoResistivity (10 -8 Ω.m) 2.1 1.8 2.0Line edge ripple (µm) 6.43 2.37 2.29Mean Rq (µm) 0.81 0.73 0.63Attenuation (dB/cm @ 2 GHz) 0.17 0.12 0.13Table 2: Measured stripline features.

V ANALYSIS OF RESULTSV-1 ADV AND PHOTO PASTES VERSUS THE STD ONEAs shown in table 2, all electrical and physical parameters are most favourable in the twonone standard paste case. The most significant difference appears on the line edge ripple.Advanced and photo-imageable materials give far better RF losses in comparison with thestandard one. So the line edge ripple seems to be the most sensitive parameter concerning RFlosses.V-2 PHOTO VERSUS ADVPhoto-imageable conductor RF losses are not as good as expected. The photo-imageableprocess is intended to give very sharp line edges in comparison with the screen-printingprocess. Nevertheless, high-resolution lines achieved with this process suffer from edgedeformations introduced during the laminating phase. In fact, photo-imageable line edges areonly slightly better than the advanced one.The interface roughness is better for the photo-imageable process as well. Neither thisparameter can explain the RF losses difference.The remaining advantage of the advanced material is the DC resistivity. Table 3 shows howRF losses increase with 10% more resistivity.10 % resistivity increaseLoss increaseFreq(GHz)v.frequency1 2.4 %4 1.9 %10 1.5 %Table 3: Result on stripline simulation. Loss increase versus frequency for 10 % resistivityincrease.

As shown on table 3, the resistivity difference alone cannot explain the RF loss differencebetween advanced paste and photo (+10 % measured at 2 GHz). This conclusion opens thequestion on the impact of other parameters on RF losses. We think the glass phase included inthe photo-imageable paste may have a significant influence on the RF performance. Indeedthe diffusion of glass first occurs on the conductor-dielectric interface. As the frequencyincreases, the skin depth decreases. Therefore, a larger fraction of the total current is disturbedby the interface roughness and could be disturbed by the glass phase as well. Further analysesare necessary to study in details the impact of this glass phase.VI CONCLUSIONThis study demonstrated that the direct screen printing process with an advanced paste allowto produce conductor lines featuring:• 70µm ± 10µm width,• line edge ripple similar to the photo-imageable process one,• better DC electrical resistivity,• significant reduction of RF losses: 0.12dB/cm @ 2GHz compared to 0.17dB/cm forstandard paste.Moreover, in order to study the correlation between physical parameters and RF properties, anew methodology was established allowing to characterise the DC electrical resistivity, theinterface roughness and the edge ripple of conductor lines.The line edge ripple appears to have the most significant impact on RF losses in the 1 to 6GHz range.The considered physical parameters do not allow to fully explain the less good RFperformance of the photo-imageable process. The glass phase included in the photoimageablepaste may have a significant impact. Further work is planned to verify thisassumption.

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