Chapter 4 Results and Discussions
4.6 RF Characteristics
4.6.4 The Unity-Current-Gain Cut-off Frequency
The unity-current-gain cut-off frequency, fT, is defined as the frequency at when the short-circuited current gain (H21) becomes equal to 1 (0 dB) and it is a dependable indicator for high frequency transistor operation. Figure 4-19 shows the cut-off frequency of the LN-PHEMTs before and after thermal annealing at 200℃
for 3 hours. The values of fT were measured under a DC supply-voltage of Vds=1.5 V and the bias voltage Vg = -0.2 V. The H21 is almost the same before and after thermal annealing. Their cut-off frequencies are similar with a value of 70 GHz.
This shows that it doesn’t impact the cut-off frequency too much after thermal annealing.
The cut-off frequencies of LN-PHEMTs fabricated using various airbridge processes were shown in Figure 4-20. All the curves have the similar cut-off frequency of about 70 GHz. These results show that the unit current gain performance of copper-metallized LN-PHEMTs is comparable to those with gold-airbridged devices.
Chapter 5 Conclusions
In this study, WNx layer was used as the diffusion barrier for Cu airbridge fabrication. The Ti layers were added in the Au/WNx and WNx/Cu structures, respectively, to improve the adhesion of the metal structure in the airbridge for solving the problem of the copper-plated airbridge peeling off the gold-contacts. We also developed a optimal selective etching processes for etching the thin metal system of Ti/WNx/Ti/Cu in the copper-airbridged PHEMT fabrication.
From the results of AES depth profiles and XRD patterns, WNx sustained as the diffusion barrier between Au and Cu even after thermal annealing at 300℃ for 30 minutes. Ti adhesion layers did not make a great impact on the diffusion barrier property of the WNx layer.
When using Ti/WNx/Ti/Cu as the thin metal system, the peelings of the plated metal no longer occurred on the LN-PHMTs even though it was immersed in a tank with ultrasonic vibration for removing the first via photoresist. The yield of the devices with Ti/WNx/Ti/Cu structure was about 85.5% and was better than those with the Au/WNx/Cu scheme which had the yield of just 62.5%. The results show that the addition of Ti adhesion layers in the Au/WNx and WNx/Cu is necessary to improve the yield of the device.
Low Noise PHEMT with copper airbridges using of Ti/WNx/Ti/Cu as thin metal layers had the saturated drain current of 200 mA/mm and the maximum transconductance was up to 449 mS/mm when tested at VDS = 1.5 Volts and VGS = -0.05 Volts. The noise figure of the fabricated device was 0.96dB and the associated gain was 10.68dB when tested at 17 GHz under Vds=1.5 V and Vg = -0.5 V. The noise performance decayed very little after thermal annealing at 17GHz. The DC characteristics and NF performance were thermally stable even after the 200 ℃ annealing for 3 hours. From the above results, the DC and NF characteristics of copper-airbridged devices with Ti/WNx/Ti/Cu as thin metal system were better than those with only WNx/Cu structure and were comparable to that of gold-airbridged PHEMTs. The plating metal peelings impact the noise performance and the addition of Ti adhesion layers sufficiently improved the noise performance of the copper-airbridged LN-PHEMTs.
The values of S-parameters changed very little before and after thermal annealing. This means the Si3N4 passivation and the thermal annealing treatment do not influence the gate-drain negative feedback. There is not much attenuation during the signal propagation after Si3N4 passivation and the thermal annealing.
The equivalent circuit-modeling diagram was reasonable for these s-parameters measured. Because of the poor adhesion between WNx and Au, the
LN-PHEMT using only WNx/Cu as thin metal system had higher resistance. The insertion of Ti layer can improve the adhesion and there was no significant impact on the source resistance after thermal annealing. The source resistance of the copper-airbridged PHEMTs with Ti/WNx/Ti/Cu multilayer was comparable to that of gold-airbridged devices. The values of the short-circuited current gain (H21) were almost the same before and after thermal annealing. Their cut-off frequencies were similar with a value of about 70 GHz. This shows that it did not impact the cut-off frequency too much after thermal annealing. These results show that the copper-metallized LN-PHEMTs have the comparable unit current gain performance to the LN-PHEMTs with gold airbridges.
All these results show that LN-PHEMTs with copper-airbridge using of Ti/WNx/Ti/Cu multilayer as the thin metal system have been successfully developed. The electrical performance of these devices was better than those using only WNx/Cu scheme and was comparable to those with the conventional gold airbridges. And the best benefit is that the use of Ti/WNx/Ti/Cu scheme improved the yield of the devices because the Ti insertion layers improved the adhesion with the diffusion barrier layer and solved the copper-airbridge peeling problem.
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TABLES
Table 2-1: Performance of Barrier Layers.
Sample Barrier Stability Deposition Notes
Si/TiW (100 nm)/Cu 725 °C, 30 sec. In-situ Cu on TiW Si/TiW (100 nm)/Cu 775 °C, 30 sec. Air between Cu and TiW
Si/TiNx (50 nm)/Cu 600 °C, 1 hr. Sputtering
Si/TiN (50 nm)/Cu 550 °C, 1 hr. CVD
Si/TiN (50 nm)/Cu 650 °C, 1 hr. Plasma treated CVD Si/Ta (60 nm)/Cu 600 °C, 1 hr. Sputtering Si/Ta (50 nm)/Cu 550 °C, 30 min. Sputtering Si/Ta2N (50 nm)/Cu > 650 °C, 30 min. Sputtering Si/TaN (100 nm)/Cu 750 °C, 1 hr. Sputtering
Si/TiSi2 (30 nm)/
Ta–Si–N (80 nm) /Cu
900 °C, 30 min. Sputtering
Si/W (25 nm)/Cu 650 °C, 30 min. Sputtering Si/W2N (25 nm)/Cu 790 °C, 30 min. Sputtering Si/WN (25 nm)/Cu 500 °C, 30 min. Sputtering Si/WNx (20 nm)/Cu > 550 °C, 30 min. PECVD
Table 2-2：Surface energy (J.m-2) at melting temperature of gold, tungsten, titanium and chromium
Melting
temperature (℃)
Surface energy
(J.m-2)
Gold (Au) 1063 1.1
Tungsten (W) 3410 2.5
Titanium (Ti) 1668 1.7
Chromium (Cr) 1875 1.5
Table 3-1 Comparison between gold airbridges and copper airbridges process
Gold airbridges Copper airbridges
Sample Number S01A362B36 S01A362B9 S01A362B25E
Nitride via Plasma etch
Plating via photolithography
S1818 PR. coated
Thin metal deposition Ti/Au/Ti WNx/Cu Ti/WNx/Ti/Cu Plating photolithography S1818 PR. coated
Pre-plating etching (Ti) Diluted HF ×
Airbridges electroplating Au 2μm Cu 2.5μm
Top photo resist strip (second PR.)
Flood exposure + flood development
Thin metal removal 1.Diluted HF 2.KI/I2 Bottom photoresist strip
(first PR.)
1.Acetone (1hr) 2.ICP (O2)
1.Acetone (1hr)
2. Acetone ultrasonic 20sec.
Airbridges passivation × PECVD Si3N4
DC and RF measurement
Table 4-1 Cu surface roughness change with various solution ratios Ⅰ
Etching solution ratio H2SO4: H2O2: H2O
Cu surface
Table 4-2 Cu surface roughness change with various solution ratios Ⅱ
Etching solution ratio H2SO4: H2O2: H2O
Cu surface
1:1:50(5:5:250) 130.2 10
1:1:100(5:5:500) -100.4 30
Figures
Figure 1-1 The peelings of the plating metal of LN-PHEMT with only WNx/Cu as the thin metal.
Figure 1-2a Au-W phase diagram
Figure 1-2b Cu-W phase diagram
Figure 2-1 Phase evolution of amorphous WNx films with annealing temperature for various nitrogen contents.（a）Amorphous phase WNx.
Figure 2-2 Solder-joint pull-strength values measured for test samples with various BLM systems.
WAFER CLEAN
FIRST PR. COATING
THIN METAL DEPOSITION Ti/WNx//Ti/Cu Multilayer
COATING SECOND PR.
for AIRBRIDGES
Cu AIRBRIDGES ELECTROPLATING
SECOND PR. STRIP
THIN METAL REMOVE----ETCHING Ti/WNx/Ti/Cu
FIRST PR. STRIP
PECVD Cu AIRBRIDGES PASSIVATION Si3N4
DC MEASUREMENTS RF MEASUREMENTS
Figure 3-1 Process flow for copper airbridges
Etch CuO Wafer preparation
Wafer cleaning
First PR. coating
Exposure
Second PR. remove
Thin metal etching
First PR. remove WAFER
Figure 3-2 Major steps of the Cu airbridge process
Airbridge
Figure 3-3 The SEM photo of the finished Cu airbridge
Figure 3-4 Distribution diagram of electroplating additives
Current density: 2 A/dm2 Current density: 1A/dm2
Current density: 0.5 A/dm2
Figure 3-5 The copper surface by various current density (Metal Thickness: 3 µm and Magnification: 5000X)
a1
(l
1)
a2(l
2)
Two-port device
b1
(l
1)
b2(l
2)
Port 1 x1=l1
Port 2 X2=l2
Figure 3-6 Incident and reflected waves from a two-port network
Source Gate Drain
Lg
Ls Ld
Rg
Rs Rd
Cgd
Cgs+
Figure 3-7 Small signal equivalent circuit physical relationship to the pHEMT structure
Cds
gd
Ri
－ gmve-jωτ
Intrinsic
gm v
Lg Ld
Ls Cdg
Cgs V
Cds Rg
Ri
gd
Rs
Rd
+
- jωτ
mVe
g −
Figure 3-8 The pHEMT small signal equivalent circuit
Surface roughness (Å)
Figure 4-1a The Cu metal surface roughness differences (Å angstrom) with various ratios of copper etchant,
(△Ra1_av1 is the average roughness of copper surface after etched by 5:3:100, 5:6:100, 5:9:100 ratios of H2SO4: H2O2: H2O---change H2O2;
△Ra1_av2 is the average roughness deviation of copper surface after etched by 5:6:100, 10:6:100, 15:6:100 ratios of H2SO4: H2O2: H2O---change H2SO4;
△Ra1_av3 is the average roughness deviation of copper surface after etched by 1:1:20, 1:1:50, 1:1:100 ratios of H2SO4: H2O2: H2O---change H2O)
Surface roughness (Å)
1：1：20 1：1：30 1：1：50 1：1：100
5：4：100 5：5：100 5：6：100 5：7：100 5：8：100
Pre_rough_1 Ra1 Figure 4-1b The Cu metal surface roughness (Å angstrom) with various ratios of
copper etchant,
(Pre_rough_1 Ra1 is the Cu surface roughness before etched, and Post_rough_1 Ra1 is the Cu surface roughness after etched by 5:4:100, 5:5:100, 5:6:100, 5:7:100, 5:8:100 ratios of H2SO4: H2O2: H2O---change H2O2;
Pre_rough_1 Ra2 is the Cu surface roughness before etched, and Post_rough_1 Ra2 is the Cu surface roughness after etched by 1:1:20, 1:1:30, 1:1:50, 1:1:100, ratios of H2SO4: H2O2: H2O---change H2O;
Dra_11 is the roughness difference of copper surface after etched by 5:4:100, 5:5:100, 5:6:100, 5:7:100, 5:8:100 ratios of H2SO4: H2O2: H2O---change H2O2;
Dra_12 is the roughness difference of copper surface after etched by 1:1:20, 1:1:30, 1:1:50, 1:1:100 ratios of H2SO4: H2O2: H2O---change H2SO4)
• AB2：Acetone @30sec.
Various layers etching process parameters：
Gate
Figure 4-2 Copper airbridge schemes with the thin metal removed process parameters
Figure 4-3 WNx protruded at the bridge edges by using NH4OH/H2O2/H2O etching
Figure 4-4 The profile of Cu airbridge used of H2SO4/ H2O2/ H2O etching
0 5 10 15
Intensity (arb. unit)
Sputter Time (min.) W
Ti Cu
Au
Figure 4-5a AES depth profiles of the as-deposited Ti/WNx/Ti/Cu multilayer scheme.
0 5 10 15 20 25 30 35 40 45
Intensity (arb. unit)
Sputter Time (min.) W
Cu
Ti
Au
Figure 4-5b AES depth profiles of the Ti/WNx/Ti/Cu multilayer scheme after 300°C annealing for 30min.
0 5 10 15 20 25
Intensity (arb. unit)
Sputter Time (min.) Au Cu
Ti
W
Figure 4-5c AES depth profiles of the Ti/WNx/Ti/Cu multilayer scheme after 400°C annealing for 30min.
20 25 30 35 40 45 50 55 60 65 70
1400 Annealing Time: 30 mins.
Au(220)
CuO (110) AuCu (420) CuO (-113)
Cu 3Au 2 (211)
XRD patterns of the Ti/WNx/Ti/Cu multilayer system on Au substrate
As_dep.
500 oC
AuCu 3 (100) Cu(200)
Figure 4-6 XRD patterns of the Ti/WNx/Ti/Cu multilayer system after thermal annealing of different temperature for 30min.
Figure 4-7 Au-Cu phase diagram
0 100 200 300 400 500 600
Average: 316 mS/mm σ: 97 mS/mm
Copper Airbridged PHEMTs without Adhesion Layers (WNx/Cu)
Counts
Gm (mS/mm)
4-8a). The average transconductance of Cu airbridged devices using WNx/Cu as thin metals
Copper Airbridged PHEMTs with adhesion layers (Ti/WNx/Ti/Cu)
Counts
Gm (mS/mm)
4-8b). The average transconductance of Cu airbridged devices using Ti/WNx/Ti/Cu as thin metals
0 100 200 300 400 500 600 0
5 10 15 20 25 30 35
Average: 344 mS/mm σ: 45 mS/mm
Au-Airbridged PHEMTs
Counts
Gm (mS/mm)
4-8c). The average transconductance of Au-airbridged devices using Ti/Au/Ti as thin metals
Figure 4-8a~c The uniformities of transconductance with various thin metal devices
-1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0
Copper Airbridged PHEMTs without Adhesion Layers (WNx/Cu)
Counts
Vp (V)
4-9a). The average Vp of Cu airbridged devices using WNx/Cu as thin metals
-1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0
Copper Airbridged PHEMTs with Adhesion Layers (Ti/WNx/Ti/Cu)
Counts
Vp (V)
4-9b). The average Vp of Cu airbridged devices using Ti/WNx/Ti/Cu as thin metals
-1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0
5 10 15 20 25 30
Average: -0.61 V σ: 0.11 V
Au-Airbridged PHEMTs
Counts
Vp (V)
4-9c). The average Vp of Au airbridged devices using Ti/Au/Ti as thin metals Figure 4-9a~c The uniformities of pinch-off voltage with various thin metal devices
0.0 0.5 1.0 1.5
Drain Current, I DS (mA/mm)
Drain-to-Source Voltage, V (V)DS
Figure 4-10a The drain I-V characteristics of copper-airbridged LN-PHEMTs with Ti/WNx/Ti/Cu as the thin metal system before and after 200℃ 3 hours thermal annealing in the air. (Ti as the adhesion layer)
-2 -1 0 1 2
Gate-to-Source Voltage, VGS (V) Drain Current, I DS (mA/mm)
0
Figure 4-10b The transconductance (Gm) of copper-airbridged LN-PHEMTs with Ti/WNx/Ti/Cu as the thin metal system before and after 200℃ 3 hours thermal annealing in the air. (Ti as the adhesion layer)
0.0 0.5 1.0 1.5
Drain Current, I DS (mA/mm)
Drain-to-Source Voltage, VDS (V)
Figure 4-11a Drain I-V characteristics for copper-airbridged LN-PHEMT with only WNx/Cu as the thin metal system.
-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0
Gate-to-Source Voltage, VGS (V) Drain Current, I DS (mA/mm)
0 100 200 300 400 Drain-to-Source Voltage,VDS=1.5V
Transconductance, Gm (mS/mm)
Figure 4-11b The dependence of the transconductance (Gm) on the gate bias voltage for copper-airbridged LN-PHEMT with only WNx/Cu as the thin metal system.
0
Bias: Vds=1.5V, Vg=-0.2V S21_before annealing S12_before annealing
Figure 4-12a S21, S12 polar chart of Cu-airbridged LN-PHEMT with Ti/WNx/Ti/Cu before thermal annealing
0.2 0.5 1.0 2.0 5.0
Figure 4-12b S11, S22 Smith chart of Cu-airbridged LN-PHEMT with Ti/WNx/Ti/Cu before thermal annealing
0
Bias: Vds=1.5V, Vg=-0.2V S21_after annealing S12_after annealing
Figure 4-12c S21,S12 polar chart of Cu-airbridged LN-PHEMT with Ti/WNx/Ti/Cu after thermal annealing of 200℃ for 3 hours
0.2 0.5 1.0 2.0 5.0
Figure 4-12d S11,S22 Smith chart of Cu-airbridged LN-PHEMT with Ti/WNx/Ti/Cu after thermal annealing of 200℃ for 3 hours
0
Figure 4-13a The comparison of S21, S12 between before and after thermal annealing (polar chart)
Figure 4-13b The comparison of S11, S22 between before and after thermal annealing (Smith chart)
0 5 10 15 20 25 30 35 40 45
S21 before annealing withoout Si3N4 S21 after annealing at 200 ° C with Si3N4
Figure 4-14 The magnitude of S21 before and after thermal annealing
0 10 20 30 40
S12 before annealing without Si3N4 S12 after annealing at 200 ° C with Si3N4
Figure 4-15 The magnitude of S12 before and after thermal annealing
Linear FET model
Figure 4-16 The small signal equivalent circuit model of LN-PHEMT
Figure 4-17a The calculated s-parameters of equivalent circuit model are fitted to the measured s-parameters of Au-airbridged devices. (Bias point：Vg：0 V，
Vd：1.5 V)
Figure 4-17b The calculated s-parameters of equivalent circuit model are fitted to the measured s-parameters of Cu-airbridged PHEMT with WNx/Cu. (Bias point：Vg：-0.3 V，Vd：1.2 V)
Figure 4-17c The calculated s-parameters of equivalent circuit model are fitted to the measured s-parameters of Cu-airbridged PHEMT with Ti/WNx/Ti/Cu before thermal annealing. (Bias point：Vg：0.2 V，Vd：2 V)
Figure 4-17d The calculated s-parameters of equivalent circuit model are fitted to the measured s-parameters of Cu-airbridged PHEMT with Ti/WNx/Ti/Cu after thermal annealing. (Bias point：Vg：-0.2 V，Vd：1.5 V)
0 5 10 15 20 Thin Metal System: Ti/WNx/Ti/Cu
Frequency (GHz)
Vg:-0.500 V, Ig=0.000 mA Vd: 1.500 V, Id=14.584 mA
Noise Figure (dB)
Figure 4-18a The thermal stability of the noise performance of the copper-airbridged LN-PHEMT with Ti/WNx/Ti/Cu multilayer as the thin metal system.
(Bias: Vd=1.5V, Vg=-0.5V)
Vg/Vd:-0.4/ 1 V, Ig/Id: 0/9.82 mA
Frequency (GHz)
Figure 4-18b Noise figure and associated gain against frequency for copper-airbridged LN-PHEMT with only WNx/Cu as the thin metal system.
1E8 1E9 1E10 1E11 1E12
Cu airbridge (Ti/WNx/Ti/Cu) with Si3N4 passivation after 200 °C annealing. Vd = 1.5 V, Vg = -0.2V
Cu airbridge before 200 °C annealing.
fT= ~ 70 GHz
Figure 4-19 The cut-off frequency of the LN-PHEMTs before and after thermal annealing at 200℃ for 3 hours. (Bias: Vd=1.5 V, Vg= -0.2 V)
1E8 1E9 1E10 1E11 1E12
0
Cu airbridges with WNx/Cu Cu airbridge with Ti/WNx/Ti/Cu.
H 21 (dB)
Freq. (Hz)
Figure 4-20 The cut-off frequency of the LN-PHEMTs with various airbridges processes.