Chapter 3 Experiment
3.5 DC and Power Measurements
3.5.3 Reliability Test
Fully Cu-metallized InGaP/GaAs HBT with Pd/Ge/Cu ohmic contact were tested by using current accelerated test for reliability evaluation. The high current test was performed at a high emitter current density of 100kA/cm2 at collector-emitter voltage of 1.5V for 24 hours.
Table
Table 3 The typical epitaxial layer structure of the InGaP/GaAs HBT
Layer Material Type Doping Thickness（Å）
Emitter Cap In0.6GaAs n+ 1×1019 800
Emitter Cap GaAs n+ 4×1018 1250
Emitter InGaP n 3×1017 500
Base GaAs p+ 4×1019 800
Collector GaAs n- 4×1016 7500
Etch stop layer InGaP n 1×1018 200
Subcollector GaAs n+ 4×1018 5000
Al0.3GaAs undoped 1800
Substrate GaAs
InGaAs Cap
InGaP Emitter GaAs Subcollector
GaAs Substrate
InGaAs Cap
InGaP Emitter GaAs Subcollector
GaAs Substrate
InGaAs Cap
InGaP Emitter GaAs Subcollector
GaAs Substrate
Figure
Fig 12.1 Emitter mesa etch
Fig 13.2 Base and collector mesa etch
Fig 13.3 Mesa isolation
InGaAs Cap
InGaP Emitter GaAs Subcollector
GaAs Substrate
InGaAs Cap
InGaP Emitter GaAs Subcollector
GaAs Substrate
Fig 13.4 Emitter and collector ohmic contact metal formation
Fig 13.5 Base ohmic contact metal formation
Fig 13.6 Silicon Nitride Deposition
InGaAs Cap
InGaP Emitter GaAs Subcollector
GaAs Substrate
InGaAs Cap
InGaP Emitter GaAs Subcollector
GaAs Substrate
InGaAs Cap
InGaP Emitter GaAs Subcollector
GaAs Substrate Fig 13.7 Nitride via etch
Fig 13.8 Interconnect metal line
Fig14. Illustration of transmission line methods (TLM) patterns
Fig15. Illustration of utilizing TLM identify ohmic contact resistance
Fig16. Distribution schematic of primary, backscattered and Auger electrons together with X-rays
Fig17. Schematic illustration of the operation of the AFM
Fig18. Illustration of four-point probes technique
Chapter 4
Results and Discussion
In this chapter, the results of Pd (150 Å)/Ge (1500 Å)/Cu (1500 Å) ohmic contact on n-type InGaAs are discussed. The formation mechanism of the Pd/Ge/Cu ohmic contact was evaluated based on the results of XRD, AES, AFM, TEM and EDX. In the last half of the chapter, the results of DC and Power measurements of fully Cu-metallized InGaP/GaAs HBT with InGaAs cap layer using Pd/Ge/Cu ohmic contact are presented.
4.1 Contact Resisitivity of The Pd/Ge/Cu Ohmic Contact
The Pd (150 Å)/Ge (1500 Å)/Cu (1500 Å) multilayer metals were
deposited on the InGaAs wafer with Si-doped epitaxial layer (1250 Å, 1x1019 cm-3). The results of the contact resistivities of the Pd(150 Å)/Ge(1500 Å)/Cu(1500 Å) ohmic contact extracted from the transmission line measurements (TLM) as a function of annealing temperature after annealing in a traditional tube furnace at different temperatures for 20 min are shown in Figure 19. Low ohmic contact resisitivity can be obtained when the Pd/Ge/Cu ohmic samples were annealed at 200°C ~ 300°C for 20 min. The lowest specific contact resistivity was 1.0 x 10-6 Ω-cm2 after the sample was annealed at 250 °C for 20 min.4.2 Formation Mechanism of The Pd/Ge/Cu Ohmic Contact
The formation mechanism of the Pd/Ge/Cu ohmic contact was investigated by results of XRD, Auger, AFM, TEM and EDX. Use X-ray diffraction for phase identification, Auger for interfacial elements material analysis, TEM image and EDX analysis for microstructure observation, and AFM for surface morphology observation. The results are shown below.
4.2.1 X-ray Diffraction for Phase Identification
Figure 20 shows the x-ray diffraction profiles for the Pd (150 Å)/Ge (1500
Å)/Cu (1500 Å) ohmic contact structure as deposited and after annealing at 150°C, 250°C, 350°C, and 450 °C for 20 min. It can be seen from the XRD spectra that the diffraction peaks of Ge and Cu remained observable for the as deposited sample. It indicated that the Pd/Ge/Cu multi-layers did not react with each other for the as deposited sample. However, it is obvious from these data that the diffraction peaks of the Cu3Ge compounds occurred and the diffraction peaks of Cu disappeared as the annealing temperature was higher than 250°C. The ohmic contact behavior was related to the formation of the Cu3Ge compounds as the annealing temperature was above 250°C
4.2.2 Auger for interfacial elements material analysis
Figures 21(a) to (c) show the AES depth profiles of the InGaAs/Pd/Ge/Cu/Cr samples as-deposited and after 250oC, and 450oC annealing for 20 minutes. As can be seen from figures 21(b), there is no obvious atomic inter-diffusions between Pd and the InGaAs layer after annealing at temperature of 250oC. However, Ge did diffuse into the InGaAs layer. It may
cause the high doping forming at the surface of InGaAs layer. The inter-diffusions occured between Cu and Ge, which can be seen for the XRD analysis at temperature of 250oC. However, as shown in Figure 21(c), after annealing at temperature of 450oC, the Cu atoms penetrated the Pd layer and diffused into the InGaAs layer, and the figure shows the serious intermixing of Cu, Ge, and the InGaAs layer. Furthermore, In atom diffused upward and only appeared on the surface of the sample. Supplementary evidence will be described from TEM and EDX data in the next section.
4.2.3 TEM image and EDX analysis
Figure 22 shows the TEM image of the as-deposited Pd (150 Å)/Ge (1500
Å)/Cu (1500 Å) structure deposited on InGaAs substrate. From this figure, the Pd, Ge, and Cu layers can be seen clearly. The thin Pd layer enhanced the adhesion of the ohmic metal and the Cr layer on the top was used as the anti-oxidation layer for Cu.Figure 23 ~ Figure 25 shows the TEM images and EDX profiles of Pd/Ge/Cu ohmic metal structure after annealing at 250°C and 450°C for 20min respectively. The TEM image of the Pd/Ge/Cu ohmic metal structure after annealing at 250°C for 20 min is shown in Figure 23(a). From the figure, the Cu/Ge compound started to form grains with vertical grain boundary and long range order [27]. From the EDX analysis, the grains were Cu3Ge compound as shown in Figure 23(b). Literature shows that the compound has low metallic resistivity and Ga has lower chemical potential in Cu3Ge than in GaAs compound [27]. On the other hand, Figure 24(a) shows the HRTEM image of the near-interface region between InGaAs substrate and ohmic compound after 250°C annealing. The PdxGaAs phases started to appear at the InGaAs surface
after 250°C annealing. Due to the PdxGaAs compound, it may create more Ga vacancies. The Ge atoms could easily diffuse into the Ga vacancies in the vicinity of the InGaAs surface, resulting in a heavy doping n+-InGaAs layer. So the ohmic contact characteristics of the Pd/Ge/Cu appeared after annealing at 250°C for 20 min.
The EDX profiles in Figure 24(b) show that there is still no Cu atom diffusing into GaAs the substrate near the Pd/InGaAs interface after 250 ℃ annealing.
However, after annealing at 450°C for 20 min, obvious atomic inter-diffusion and interfacial reactions started to occur as can be seen from Figure 25. The ohmic contact characteristics of the Pd/Ge/Cu ohmic system started to degrade after 450°C annealing, the possible reasons for ohmic contact degradation are As atoms diffused out and Ga atoms diffused into the Ge layer.
(Ga atoms acted as acceptors which reduced the donor concentration.)
Figure 26 shows the EDX profiles of sample’s surface after 450°C annealing. The out-diffusion of In atom would deteriorate the ohmic contact due to an increase in barrier height. The out-diffusion of As atom is also responsible for the degraded ohmic contact at 450℃ because it enhances the Ge atoms to occupy the As sites where they behave as acceptors.
4.2.4 AFM for surface morphology observation
Figure 27 is the AFM surface morphology of the as-deposited sample and samples subjected to annealing at temperature of 250oC and 450oC. The root-mean-square (rms) roughness of the sample as-deposited was 1.597 nm.
The roughnesses of the samples annealed at temperature of 250oC and 450oC
were 2.058 nm and 2.918 nm, respectively. From the results of AFM, the surface of sample is rougher when the annealing temperature increases.
4.3 Thermal Stability Test for the Pd/Ge/Cu Ohmic Contact
To study the thermal stability of the Pd (150 Å)/Ge (1500 Å)/Cu (1500 Å) ohmic contact, the Pd/Ge/Cu multilayer layer were annealed at 250°C for 24 hours and the specific contact resistivity was measured used TLM patterns.
Figure 28 shows the long time thermal stability test results. From this figure, it can be seen that there was no obvious degradation on the Pd/Ge/Cu ohmic system after annealing at 250 for 24 hours.℃ Figure 29 and Figure 30 show the thermal stability test of sheet resistance, and both of them have no obvious increase for high temperature test or log time annealing test.
4.4 DC and Power Measurements
4.4.1 DC Measurements
Figure 31 shows the optical microscope images of the fully Cu-metallized InGaP/GaAs HBTs with Pd/Ge/Cu ohmic contact after fabrication. Figure 32 shows the typical common emitter characteristics of HBTs with emitter area of 4 x 20µm2. In the figure, one group of curves belongs to the fully Cu-metallized HBTs and the other belongs to the traditional Au-metallized HBTs. It can be seen from Figure 32 that these two devices show similar knee voltage and offset voltage. We did not observe an increase in the knee voltage or the decay of the collector current, which indicates that the characteristics of the InGaP/GaAs
HBTs with Pd/Ge/Cu ohmic contact are reasonably good. The common emitter current gain is around 110 for both cases. Gummel plots of the HBTs with the traditional Au HBT and Au free fully Cu HBT were also compared. The results are shown in Figure 33. The two HBTs also showed similar behaviors.
To test the reliability of the Pd/Ge/Cu as the n-type ohmic metal for the Cu-metallized HBTs, both copper and gold metallized HBTs with 4x20µm2 emitter area were subjected to current accelerated stress test with high current density of 100 kA/cm2. It is much higher than 25kA/cm2 required for the normal device operation and the purpose is to shorten the stress time so that the stress tests could be performed at wafer level without using any package and the results could be obtained in a few hours [27]. Figure 34 plots the current gain (β) of the fully Cu-metallized HBTs with Pd/Ge/Cu ohmic contact after stressed at the high current density of 110 kA/cm2 with VCE of 2.5V for a period of 24 hours.
The measurements were made at an amibient room temperature of TA = 25°C. It can be seen from the data that the current gain of the device showed no significant change with time.
4.4.2 Power Measurements
The power performance of the HBTs was measured at 2 GHz by using a load–pull system. Measurements were carried out at collector current levels between 3 and 15 mA and VCE = 2.4 V. The results of power measurement were shown in Figure35. For Au-HBTs, turning for maximum power-added efficiency (PAE) match, the output power (Pout) was 12.46 dBm and the maximum PAE was 46.2%, with the DC bias conditions of VCE = 2.4 V and IC =13mA . With the same DC bias conditions, Cu-HBTs, the output power (Pout) was 11.70dBm and the maximum PAE was 42.1%.
Figures
Fig19. The specific contact resistivity of the Pd(15nm) / Ge(150nm) / Cu(150nm) contact on n-type GaAs as a function of annealing temperature.
0 100 200 300 400 500
1E-6 1E-5
as-dep.
Pd(15nm)/Ge(150nm)/Cu(150nm)/n+InGaAs
Specific Contact Resistivity (ohm-cm2 )
Annealing Temperature (oC)
Fig20. The X-ray diffraction patterns for the Pd (150 Å)/Ge (1500 Å)/Cu (1500 Å) contact after annealing at 250 °C for 20min, 450 °C for 20min, and the as-deposited sample.
30 40 50 60 70
Fig21. (a) AES depth profiles of the Pd (150 Å)/Ge (1500 Å)/Cu (1500 Å) contact for as-deposited sample
0 500 1000 1500 2000
0 20 40 60 80 100
A tomic percent (%)
Etch time (s)
In As
Ga
Pd
Cu Ge
O
Cr
Fig21. (b) AES depth profiles of the Pd (150 Å)/Ge (1500 Å)/Cu (1500 Å) contact for 250℃
0 500 1000 1500 2000
-10
Fig21. (c) AES depth profiles of the Pd (150 Å)/Ge (1500 Å)/Cu (1500 Å)
Fig22. The TEM image of the Pd(15nm) / Ge(150nm) / Cu(150nm) contact for the as-deposited sample.
Pd
InGaAs
Ge
Cu
Cr
(a) (b)
Fig23.
(a) The TEM image of the Pd/Ge/Cu contact cross section, (b) The EDX profiles of the Ge/Cu compound grains
after annealing at 250℃ for 20 min.
Cu:Ge=57.35%:20.89%
Cu
Ge
(a) (b)
Fig24. (a) The high resolution TEM image of the interface between the Pd metal layer and the InGaAs substrate after annealing at 250℃
for 20 min.
(b) The EDX profiles of the Pd/InGaAs interface after annealing at 250℃ for 20 min.
Element Ge Pd Ga As In
Atomic% 10.95 14.37 11.11 30.35 22.49
Weight% 9.31 17.91 9.07 26.64 30.25
InGaAs Pd
Element Cu Ga Ge As In Atomic% 6.19 19.81 3.77 43.07 27.16 Weight% 4.69 16.46 3.26 38.44 37.16
(a) (b)
Fig25.
(a) The TEM image of the Pd/Ge/Cu contact cross section, (b) The EDX profiles of near the surface of InGaAs
after annealing at 450℃ for 20 min.
InGaAs
Ge Cu3Ge
Cu
Fig26. The EDX profiles of sample’s surface after annealing at 450℃ for 20 min.
(a) (b) (c)
As-deposit @250℃ @450℃
Rms : 1.597 nm 2.058 nm 2.918 nm (Ra) : 1.151 nm 1.497 nm 2.344 nm
Fig27. AFM Surface Morphology
Fig28. The specific contact resistivities of the Pd/ Ge/ Cu ohmic contact on n-type InGaAs as a function of aging time.
0 10 20 30 40 50 60 70 80
1E-7 1E-6 1E-5
n+InGaAs/Pd(15nm)/Ge(150nm)/Cu(150nm)
Specific Contact Resistivity (ohm-cm2 )
Annealing Time (hr)
Fig29. Sheet Resistance of the n+InGaAs/Pd/Ge/Cu Structure annealed at different temperatures for 20 min.
0 100 200 300 400 500 600
0 1 2 3 4 5 6 7 8 9 10
n+InGaAs/Pd(15nm)/Ge(150nm)/Cu(150nm)
as-dep.
Sheet Resistivity (ohm/square)
Annealing Temperature (oC)
Fig30. Sheet Resistance of the n+InGaAs/Pd/Ge/Cu Structure annealed at 250℃
for long annealing time test.
0 10 20 30 40 50 60 70 80
0 1 2 3 4 5
as-dep.
Pd(15nm)/Ge(150nm)/Cu(150nm)/n+InGaAs
Sheet Resistivity (ohm/square)
Annealing Time (hr)
Fig31. The OM images of fully-Cu InGaP/GaAs HBT with Pd/Ge/Cu as n-type ohmic contact device.
Fig32. Comparison of the typical IC-VCE characteristics for the emitter area (4 x 20 μm2) HBTs with Cu and with Au metallizations.
0.0 0.5 1.0 1.5 2.0 2.5
0.000 0.005 0.010 0.015 0.020 0.025 0.030
Fully Cu HBT Traditional Au HBT
Collector current, IC(A)
Collector-emitter voltage, V
CE(V)
IB=50µA/step Emitter area: 4 x 20 µm2
Fig33. Comparison of Gummel plots for the emitter area(4 x 20μm2) HBTs with Cu and with Au metallizations.
1.0 1.1 1.2 1.3 1.4 1.5 1.6
Base-emitter voltage, VBE(V)
I
BFig34. The current gain (β) as a function of stress time at constant IB for the 4x20-μm2-emitter-area fully Cu-metallized HBT with Pd/Ge/Cu ohmic contact.
Emitter Area=4x20µm2 VCE=1.5V
JC=100KA/cm2
Stress Time(hr)
Current Gain(β)
(a)
(b)
Fig35. Measured output power as a function of input power for the 4 µm × 20 µm HBT at 2 GHz with bias VCE=2.4V, IC=7mA.
The device was tuned for maximum power added efficiency.
(a)Au-metallized (b)Cu-metallized HBTs.
Chapter 5 Conclusions
The fully Cu-metallized InGaP/GaAs HBTs using Pd/Ge/Cu ohmic contact to n-type InGaAs has been successfully fabricated and demonstrated. The optimized Pd (150 Å)/Ge (1500 Å)/Cu (1500 Å) metal structure forms a low contact resistivity ohmic contact to n-type InGaAs at a low annealing temperature. Low ohmic contact resistivity can be obtained when the Pd/Ge/Cu ohmic samples were annealed at 200°C ~ 330°C for 20min. The lowest specific contact resistivity achieved was 1.0 x 10-6 Ω-cm2 when annealed at 250 °C for 20 min.
From XRD, Auger, AFM, TEM, and EDX studies, the low contact resistivity was due to the formation of the Cu3Ge compound and the PdxGaAs compound in conjunction with the outdiffusion of Ga into the ohmic metal and the diffusion of Ge into the Ga vacancies. The contact resistivity of Pd/Ge/Cu ohmic contact was also very stable after annealing at 250°C for 24 hours.
Overall, the Pd/Ge/Cu ohmic contact has a low contact resistivity, good thermal stability, and good surface morphology.
The common emitter I-V curves, Gummel plot, and power characteristic of these Cu-metallized HBT using Pd/Ge/Cu ohmic contact on InGaAs cap layer showed similar electrical characteristics as those conventional Au-metallized HBT. The common emitter current gain for the 4x20-µm2-emitter-area fully Cu-metallized HBT using Pd/Ge/Cu ohmic contact and the traditional
Au-metallized HBT were both around 110. Current accelerated stress (100 kA/cm2 for 24 hrs) for the fully Cu metallized HBTs show almost no degradation.
The overall results show that the novel Pd/Ge/Cu ohmic contact can be used on the InGaP/GaAs HBTs with good electrical performance to achieve a Fully Cu-metallized device.
Reference
[1] K. Holloway and P. M. Fryer, “Tantalum as a diffusion barrier between copper and silicon,” Appl. Phys. Lett., vol. 57, no. 17, pp. 1736-1738, Oct.
22, 1990.
[2] K. Holloway, P. M. Fryer, C. Cabral, Jr., J. M. E. Harper, P. J. Bailey, and K.
H. Kelleher, “Tantalum as a diffusion barrier between copper and silicon:
failure mechanism and effect of nitrogen additions,” J. Appl. Phys., vol. 71, no. 11, pp. 5433-5444, 1992.
[3] D. S. Yoon, H. K. Baik, and S. M. Lee, “Effect on thermal stability of a Cu/Ta/Si heterostructure of the incorporation of cerium oxide into the Ta barrier,” J. Appl. Phys., vol. 83, no. 12, pp. 8074-8076, 1998.
[4] C. Y. Chen, L. Chang, E. Y. Chang, S. H. Chen, and D. F. Chang, “Thermal stability of Cu/Ta/GaAs multilayers,” Appl. Phys. Lett., vol. 77, no. 21, pp.
3367-3369, 2000.
[5] C. Y. Chen, E. Y Chang, L. Chang, and S. H. Chen, “Backside copper metallisation of GaAs MESFETs,” Electronics Lett., vol. 36, no. 15, pp.
1318-1319, 2000.
[6] Il-Ho Kim, ‘‘Pd/Ge/Pd/Ti/Au ohmic contact to n-type InGaAs,’’ Materials Letters 54 (2002) 323– 327.
[7] L. H. Chu, E. Y. Chang, L. Chang, Y. H. Wu, S. H. Chen, H. T. Hsu, T. L.
Lee, Y. C. Lien, and C. Y. Chang, ‘‘Effect of Gate Sinking on the Device Performance of the InGaP/AlGaAs/InGaAs Enhancement-Mode PHEMT,’’
IEEE ELECTRON DEVICE LETTERS, VOL. 28, NO. 2, FEBRUARY 2007
[8] D. Seghier, H.P. Gislason,Science Institute, University of Iceland, Dunhagi 3,IS-I07Reykjavik, Iceland, ‘‘Effects of Cu diffusion on electrical properties of GaAs,’’ IEEE, pp. 161-164, 1999.
[9] P. H. Wohlbier: Diffusion and Defect Data 10 (1975) 89.
[10] A. G. Baca and F. Ren, “A survey of ohmic contact to Ⅲ-Ⅴcompound semiconductors”, Thin solid films, vol. 308, pp. 599-606, 1997.
[11] M. Murakami, ”Development of refractory ohmic contact materials for gallium arsenide compound semiconductors,” S. and T. Advanced Materials, Vol.3, pp1~27, 1-27, 2002.
[12] James W. Mayer and S. S. Lau, “Electronic Materials Science: For Intergrated Circuits in Si and GaAs” pp102
[13] T. lalinsky, On the technology, electrical characterization and reliability of ohmic contacts on GaAs, Elektrotech Cas. 37 pp.354-370, 1986.
[14] A. K. Niessen, model predictions for the enthalpy of formation of transition metal alloys, CALPHAD 7 pp. 51-70, 1983.
[15] W. Hume-Rothery, Inst. Metals pp.205, 1937.
[16] E. D. Marshell, Nonalloyed ohmic contact to n-GaAs by solid-phae epitaxy of Ge, J. Appl. Phys. 62, 1987.
[17] E. D. Marshall et al.”Nonalloyed ohmic contacts to n-GaAs by soild-phase epitaxy of Ge,” J. Appl. Phys, Vol.62, No.3, 1987.
[18] E. D. Marshall et al., ”Nonalloyed ohmic contacts to n-GaAs by soild-phase epitaxy of Ge,” J. Appl. Phys, Vol.62, No.3, 1987.
[19] M. S. Islam et al., ”Thermal stability of the non-alloyed Pd/Sn and Pd/Ge ohmic contacts to n-GaAs,”Thin Solid Films,Vol. 308-309, pp.607-610, 1997.
[20] W. D. Chen et al., ”Microstructure studies of PdGe/Ge ohmic contacts to n-type GaAs formed by rapid thermal annealing,”Applied surface Science, Vol. 100-101, pp.530-533, 1996.
[21] P. Machac et al., “Raman spectroscopy of Ge/Pd/GaAs contacts,”
Microelectronic Engineering, Vol.71, pp177-181, 2004.
[22] M. O. Aboelfotoh et al., ‘Microstructure characterization of Cu3Ge / n-type GaAs ohmic contacts,” J. Appl. Phys. Vol. 76, 1994.
[23] H. Kroemer, “Theory of a wide-gap emitter for transistors”, Proc. IRE 45, 1535, 1957.
[24] P. Asbeck et al., “Heterojunction bipolar transistors for microwave and millimeter-wave integrated circuits”, IEEE Trans. Microwave Theory Tech.
vil, 1462, 1987.
[25] D. K. Schroder, Semiconductor Material and Device Characterization, Wiley-Interscience, Canada, 1998.
[26] W. C. Liu, J. H. Tsai, and S. L. Liu, IEEE Electron Device Lett. 13, 418, 1992.
[27] M. O. Aboelfotoh, C. L. Lin, and J. M. Woodall, “Novel low-resistance ohmic contact to n-type GaAs using Cu3Ge”, Appl. Phys. Lett. 65 (25), 1994.
[28] A. Gupta, A. Young and B. Bayraktaroglu, “InGaP makes HBT realibility a non-issue”, in GaAs Mantech Tech. Dig., 203, 2001.