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High-resolution transmission electron microscopy on aged InP HBTs
Microelectronics Reliability 44 (2004) 1055–1060 www.elsevier.com/locate/microrel
High-resolution transmission electron microscopy on aged InP HBTs Bruce M. Paine a
a,*
, Timothy J. Perham a, Stephen Thomas III
b
Boeing Satellite Systems Inc., El Segundo, CA 90245, USA b HRL Laboratories LLC, Malibu, CA 90265, USA
Received 12 January 2004; received in revised form 25 February 2004 Available online 10 May 2004
Abstract We have used high-resolution transmission electron microscopy (HRTEM), with focused-ion-beam preparation of foils, to study InP HBT devices, both as-fabricated and after aging in life tests. The technology is HRL’s G1 process. We found that even after aging for the equivalent of 2 · 107 h under normal operating conditions, the visible damage is extremely benign: the base and emitter contacts are completely intact, there is no evidence of significant Au or Pt migration into the semiconductor, and minor crystal disorder that develops does not extend more than 100 nm below the metal–semiconductor interfaces. Except for these regions, the intrinsic devices are completely devoid of any anomalous features, to within the 0.3 nm resolution limit of the measurements. These observations provide strong evidence that failure mechanisms involving migration of metal from the ohmic contacts, or extended crystal defects, do not limit the reliability of this technology. The small disorder under the base contacts can probably explain the increases of gain (b) observed early in our life tests, and small increases in the base-collector leakage occurring later in the life tests. The minor disorder under the emitter contacts can probably explain the moderate increase of emitter resistance observed late in our life tests. But the mechanism for the eventual decline in b, observed after a long period of stress, for the equivalent of 107 h of normal operation, is not apparent from the HRTEM images. 2004 Elsevier Ltd. All rights reserved.
1. Introduction To have confidence in a reliability prediction based on life testing, it is desirable to have an understanding of the mechanisms responsible for the degradation. This way, one can check the plausibility of the analysis, check that the experiment was sufficiently sensitive to the changes that were occurring, and look for evidence of multiple mechanisms. For heterojunction bipolar transistors (HBTs) these checks are particularly important because potentially-fatal changes such as growth of extended defects, and migration of contact metals are known to occur without being fully quantified in traditional accelerated life tests [1]. Yet in most HBT reli-
*
Corresponding author. E-mail address: [email protected] (B.M. Paine).
ability studies, researchers typically invoke one of a small number of popular mechanisms, but fail to provide any direct physical evidence. This is because the drastic electrical changes are caused by relatively small and subtle atomic displacements that are difficult to observe. For HBTs, simple inspection by scanning electron microscopy (SEM) is rarely fruitful because they are entirely covered by contact metal. So cross sectioning is often done by focused ion beam (FIB) milling, and then SEM inspection can be extremely useful. But this technique is still limited to resolution of the order of 50 nm, by straggling of electrons in the samples. New techniques of FIB milling, to prepare ultra-thin slices through a device (known as ‘‘foils’’), followed by highresolution transmission electron microscopy (HRTEM) allow inspection with superb resolution (less than 1.0 nm), with the added advantage of providing information
0026-2714/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.microrel.2004.03.011
1056
B.M. Paine et al. / Microelectronics Reliability 44 (2004) 1055–1060
on crystal structure from diffraction data. Since this capability has become commercially available, we have begun applying it to HBT structures, both as-grown, and after aging in life tests, to observe physical changes with the best-possible resolution, and hence to conduct the checks described above. We believe this is the first published report of HRTEM analysis of aged HBTs. It was applied to HRL Laboratories’ first generation (G1) InP-based HBTs. These have already been shown to have excellent reliability [2–5] (e.g. the predicted mean time to failure at 100 C is about 107 h). For specimens aged for the equivalent of about 2 · 107 h in a typical application the results show only minor degradation under the contact metals on the base and emitter. There is no sign of significant extended defects that would cause degradation, and no sign of significant migration of contact metals in to the semiconductor layers. A similar study has been conducted on HRL’s second generation devices (G2) with very similar results. There are very few published transmission electron microscopy studies on aged III–V transistors. Magistrali et al. [6] studied Ti/Pt/Au gates on life tested GaAs MESFETs by TEM with conventional sample thinning, and saw evidence of metal diffusion from the gate, into GaAs. Takahashi et al. [7] conducted TEM inspection of GaInP/GaAs HBTs and found evidence of carbonrelated precipitates near the emitter–base interface.
2. Life test The parts were discrete HBT devices fabricated with the HRL Laboratories LLC G1 process [2]. The basic material structure is shown in Fig. 1. The base doping is beryllium. Nominal emitter dimensions were 2 · 5 lm. For the life tests the die were attached to Kovar carriers
with Au–Sn eutectic. Substrates with DC conductors, chip resistors and chip capacitors were also attached for electrical connections and stabilization. Connections on the carrier were made with Au wirebonds, and connections from the carrier to biasing circuitry were made with Au ribbons. The carriers were bolted to blocks containing electrical heaters. These fixtures were enclosed in dry boxes with flowing dry nitrogen. Biasing (in common-emitter mode), heating, and monitoring were conducted under computer control. DC characterization was done manually, at room temperature, with the biases off. The measurements included collector IV curves, Gummel plots, DC gain (b) vs. Ic , and forward and reverse junction characteristics. The life test was similar to those we have described earlier [2,3]. Junction temperature was 225 C (as measured electrically), current density was 0.233 mA/lm2 and Vce was 3.0 V. The evolution of b at Ic ¼ 0:06 mA/ lm2 for the duration of the life test is shown in Fig. 2. We see that all parts experience an increase in b during the first hours of life test, by up to 35%, followed by a relatively long period of stability. They then degrade between 200 and 400 h of life test, i.e. the equivalent of 1–2 · 107 h of normal operation at 100 C [2,3]. Similarly, the evolution of Vbe , i.e. the voltage for Ib ¼ 5 · 105 mA/lm2 , with collector floating, is shown in Fig. 3. Here there is no significant change until about 200 h of life test, when the Vbe ’s start to increase. The evolution of a Gummel plot for a typical device is shown in Fig. 4. From this we see that the initial increase of b is due to a decrease in Ib at all voltages (accompanied by a smaller decrease of Ic ). Following this there is minimal change, until after 200 h when Ic drops at all voltages
100
Contact
GaInAs
Emitter Contact
AlInAs
n+
Emitter
AlInAs
n n
GaInAs/AlInAs SL Spacer
GaInAs
Base GaInAs GaInAs Collector GaInAs Subcollector GaInAs Buffer InP Substrate
p
Beta
80 n+
60 40 Equivalent to 107 hrs of normal operation
20
p p+ nn+
Fig. 1. Schematic diagram of the InP HBT structure. The base dopant is beryllium. The superlattice is graded with the thickness of the GaInAs layers being reduced as successive layers are deposited.
0 0
100
200 300 Time (hrs)
400
500
Fig. 2. Drift plot for b (for Vce ¼ 1:0 V and Ic ¼ 0:06 mA/lm2 ) in the accelerated life test. b increased in the first hours of the life test, and then remained stable for the equivalent of 107 h under normal operation. Then it eventually declined. Dashed curves are for control samples that remained unstressed. The samples studied by HRTEM are represented by the and open triangle symbols.
B.M. Paine et al. / Microelectronics Reliability 44 (2004) 1055–1060
the adjacent base contacts. They were imaged at 400 kV, with the electron beam parallel to the [1 1 0] zone axis of the InP substrates, to ensure that all interfaces were viewed edge-on. They were also tilted slightly off-axis to satisfy various 2-beam diffraction conditions to better image any crystalline defects and compositional modulations within the semiconductor layers. Resolution as good as 0.3 nm was obtained in the phase contrast imaging mode. Energy-dispersive X-ray microanalysis (EDX) was conducted in a separate electron microscope with a non-field-emission electron gun.
Vbe(turn-on) (V)
1.0 0.8 0.6 0.4
Equivalent to 107 hrs of normal operation
0.2 0.0 0
100
200 300 Time (hrs)
400
500
Fig. 3. Drift plot for Vbe in the life test. (Ib ¼ 5 · 105 mA/lm2 , with the collector floating.) It remained stable for well beyond the period of normal operation, but eventually increased. Dashed curves are for control samples that remained unstressed.
1.E+04 Increasing Lifetest Time
Ic, Ib (A/cm2)
1.E+03
Ic
1.E+02 Ib
1.E+01 1.E+00 1.E-01
t = 0, 24, 171, 407 hrs Vb = Vc
1.E-02 1.E-03 0.3
0.4
0.5
0.6 Vbe (V)
0.7
1057
0.8
0.9
4. Microscopy results and discussion 4.1. As-fabricated Fig. 5 shows details of the side of the emitter mesa on an as-fabricated specimen. The emitter metal and selfaligned base metal are clearly visible. Within each of them, the dark material is a layer of Au, on top of a layer of Pt. The light material is Ti, which forms the first layer of the metallizations. The nþ GaInAs contact layer is clearly visible at the top of mesa. The lighter AlInAs ‘‘emitter contact’’ and emitter are below it, and the darker GaInAs base structure is below that, also extending laterally, under the base contact. The transition between the emitter and base spacer layer contains a superlattice of alternating layers of AlInAs and GaInAs, with the thickness graded so the GaInAs layers are thin on the emitter side and thick on the base side. Some of these layers are resolved. Beneath them the transition
Fig. 4. Gummel plots with Vbc ¼ 0, for various points in the life test: 0, 24, 171, and 407 h. For both Ic and Ib , the curves are lower at each interval.
(accompanied by a smaller decrease of Ib ). There is no sign of increase in series resistances, although moresensitive RF modeling has shown that the emitter resistance increases moderately [8]. The life test was stopped at 400 h, and specimens were sent for HRTEM analysis.
3. High-resolution transmission electron microscopy Sample preparation and inspection were conducted at NanoTEM Inc. The FIB preparation technique was similar to those described in the literature (a good review is Ref. [9]). Cross-sectional specimens with a 150–200 nm thick electron-transparent membrane were prepared. These cut through the emitter contact and the edges of
Fig. 5. HRTEM image of an as-fabricated specimen. The emitter metal consists of the dark Au and Pt, on top of Ti (light appearance). Below this, one can see (with discernable boundaries): the nþ GaInAs contact layer, the nþ AlInAs contact layer, the AlInAs emitter, the superlattice, a spacer, and the pþ GaInAs base layer.
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from the base spacer to the doped base layer is faintly visible as a darker line. There are no disordered regions in any areas of semiconductor, and no voids. Also there is no evidence of any metal migration. Thus there is negligible reaction of the metal contacts with the underlying semiconductor. But we note that not all of the base spacer has been removed before deposition of the base contact metal. Numerical modeling [10] has shown that this may allow for lateral flow of electrons to the base contacts, leading to a lower initial b than the optimum (see Fig. 6). The slightly darkened areas under the emitter contact are strain fields, as evidenced in the Fig. 7. Fig. 7 shows a close-up of the emitter metal– semiconductor contact area. This is typical of the entire contact region visible in this cross section. The contrast fringes indicate strain fields, which are most likely caused by thermal expansion mismatches remaining from the metal deposition process. The fringes vary due to local variations in the strain. The 3–4 nm line at the interface may indicate a layer of In-rich semiconductor.
Fig. 7. HRTEM image of the region under the emitter contact, in an as-fabricated specimen. The interface may contain a thin layer of In-rich semiconductor at the interface. There is also darkening caused by strain fields, probably from thermal expansion mismatches following metal deposition.
4.2. After life tests Fig. 8 shows an overview of a specimen with the largest degradation seen in this life test. This is the specimen represented by * symbols in Figs. 2 and 3: the b degradation was about 90%. A small number of defects are visible at the collector–sub-collector inter-
Fig. 8. HRTEM image of almost the entire emitter mesa in a specimen that has been life tested to the point where b dropped by 90%. There are small defects at the sub-collector–collector interface. There is also minor disorder, up to 100 nm thick, under the emitter and base contacts. Otherwise there are no anomalous features.
Fig. 6. Schematic diagram of an InP HBT, showing electron current flow that is allowed when the emitter etch is incomplete, leaving part of the spacer layer between the base and the base metal. This path may be blocked by disorder under the base metal, early in the life test, causing the observed increase of b.
face. It is not known whether they occurred during the initial epitaxial growth or during the aging. They are not expected to have any effect on device performance. The figure encompasses almost the entire width of the emitter
B.M. Paine et al. / Microelectronics Reliability 44 (2004) 1055–1060
mesa, and no disorder or extended defects are visible anywhere near the emitter–base or base–collector interfaces. Another specimen from the life test was also inspected: the one represented by open triangles in Figs. 2 and 3, with b degradation at 400 h of about 30%. This specimen did not display the defects at the collector– sub-collector interface. Otherwise the features of the images were the same in all respects. Fig. 9 shows detail of the base metal contact from Fig. 8. There is disorder under the metal, extending up to 100 nm into the semiconductor. It includes extended defects such as stacking faults and partial voids. These may result from some interaction between the metal and the semiconductor. But there are no well-defined phases with dark appearance, so we rule out any extensive reaction of Au or Pt with the semiconductor. The voids may arise from volume change due to local phase formation near the interface. Note that the disorder and voiding may effectively eliminate the small layer of spacer material that was inadvertently left below the base contacts. This could cut off the lateral flow of electrons from the emitter; with the result that b increases. This may well be the mechanism for the b increase observed early in the life tests. (But of course with this severely-aged specimen, one cannot tell if there was significant disorder under the base metal after the first few hours of life test.) We also note that although the base–collector interface is not visible, the disorder and voiding under the base contact almost certainly extend through the base layer and into the collector. This may lead to base–collector leakage, which indeed has been measured in our life tests, although with magnitude that is not significant relative to the normal operational currents [2,4]. Fig. 10 shows detail of the emitter metal contact from Fig. 8. This is the worst-case disorder of all that was observed along the entire widths of the mesas in the two specimens that were sectioned. But it is still extremely
Fig. 9. HRTEM close-up image of the region under the base contact in the specimen that had been aged such that b dropped by 90%.
1059
Fig. 10. HRTEM close-up image of the region under the emitter contact. The specimen is the same as that used for Figs. 8 and 9. The base–emitter junction is slightly below the superlattice, faintly visible at the bottom of the picture. No disorder or anomalous features of any kind are observed anywhere near the junction.
benign. There is disorder under the emitter metal, extending up to 100 nm in depth in some locations. Since the depth to the base–emitter junction is greater than 250 nm, this is no risk of it affecting the device operation. In fact the disorder remains in the nþ GaInAs contact layers––the emitter, spacer and base layers beneath them are completely featureless in all our HRTEM images. The disorder includes extended defects such as stacking faults, and probably results from interaction between the metal and the semiconductor. But we again observe no well-defined phases with dark appearance, and therefore rule out any significant reaction of Au or Pt with the semiconductor. The disorder may be responsible for the moderate increase in emitter resistance observed late in our life tests [8], and may explain part of the increase of Vbe shown in Fig. 3. But the disorder under the emitter contacts does not appear to explain the large drops in the b’s, or the drops in the Gummel plots at all voltages. An EDX image was taken of the disordered region under the emitter metal (see Fig. 11), and it showed no significant signals from Au or Pt. Note that the Cu peaks arise from the support grid while the Fe peaks arise from the iron pole piece in the microscope. Thus we find no evidence for any mechanisms involving migration of metal from the ohmic contacts, or extended crystal defects that might limit the reliability of this technology. And we rule out low-activation energy mechanisms since none was observed in extremelylong-duration low-temperature life tests reported earlier [4,5]. But no mechanism is observed for the eventual decline of b in the life tests, after the equivalent of 107 h of operation under normal conditions. We, therefore, speculate that the decline of b arises by diffusion of Be atoms from the base layer, through the superlattice and
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Wafer 564-054, device 5348-76, carrier s/n 005 EDX, disordered region under contact Cu
Counts
100
As 50
Ga
As Ga
We acknowledge Dr. Roger J. Graham and Dr. Fred Shaapur of NanoTEM, Inc. for the sample preparation, high-resolution transmission electron microscopy work, and help with interpretation. We also acknowledge the staffs of both the BSS Technology Qualification Laboratory and HRL Clean Room Operations for their contributions to this work.
In
In Cu Fe In Ti Ti Al Au 0
Acknowledgements
5
Fe Au Cu Ga Energy (keV)
As
10
Fig. 11. Energy-dispersive X-ray analysis of the disordered region under the emitter contact. The specimen is the same one from which Figs. 8–10 were taken. Peaks from Pt and Au are negligible, indicating insignificant migration of these elements from the contact, into the semiconductor.
into the emitter. Since Be atoms are known to migrate interstitially, this requires no defects in the host lattice. Also Be has a very low atomic number so the migration would not be expected to leave any signature that is detectable by HRTEM. This mechanism, which is known as Recombination-Enhanced Impurity Diffusion (REID) would also explain the increase of Vbe that is observed at about the same time (see Fig. 3). Finally we note that early reductions of base current have also been attributed to REID, by annihilation of recombination centers [11]. So this may be an alternative explanation of the increases of beta observed in the first hours of the life test.
5. Conclusion HRTEM, together with FIB-preparation of sample foils, has allowed us to see direct physical evidence of the changes induced in InP HBTs during life testing. We found no evidence of anomalies of any type near the transistor junctions. But small amounts of disorder under the base and emitter contacts can explain most of the observations in our life tests. The exception is the eventual major degradation of b, observed late in the life tests, at times equivalent to 107 h of normal operation: this may occur by migration of Be dopant atoms, which does not create any structures that are visible with HRTEM.
References [1] Henderson TS. The GaAs heterojunction bipolar transistor: an electron device with optical device reliability. Microelectron Reliab 1996;36:1879–86. [2] Kiziloglu K, Thomas III S, Paine BM, Williams Jr F, Fields CH. Investigation of process related reliability for InP-based heterojunction bipolar transistors (HBTs). Electrochem Soc Proc 1999;4:95–102. [3] Kiziloglu K, Thomas III S, Williams Jr F, Paine BM. Reliability and failure criteria for AlInAs/GaInAs/InP HBTs. In: Proceedings of the Indium Phosphide and Related Materials Conference. 2000. p. 294–297. [4] Paine BM, Thomas III S, Delaney MJ. Low-temperature, high-current life tests on InP-based HBT’s. In: Proceedings of the International Reliability Physics Symposium. 2001. p. 206–213. [5] Paine BM, Thomas III S, Delaney MJ. Very-Low-temperature life tests on InP-based HBT’s––An update. In: Proceedings of the GaAs Reliability Workshop. 2002. p. 119–136. [6] Magistrali F, Sala D, Turner J, Valli P, Vanzi M. Experimental evidence of Ti/Pt/Au–GaAs interaction obtained by means of TEM technique. In: Proceedings of the GaAs IC Symposium. 1991. p. 199–202. [7] Takahashi T, Sasa S, Kawano A, Iwai I, Jujii T. Highreliability of InGaP/GaAs HBTs fabricated by self-aligned process. In: Technical Digest of the International Electron Devices Meeting. 1994. p. 191–194. [8] Thomas III S, Fields CH, Madhav M. RF modeling approach to determining end-of-life reliability for InPbased HBTs. Microelectron Reliab 2001;41:1129–35. [9] Giannuzzi LA, Stevie FA. A review of focused ion beam milling techniques for TEM specimen preparation. Micron 1999;30:197–204. [10] Hussain T, Shi BQ, Nguyen C, Madhav M, Sokolich M. A numerical analysis to study the effects of process related variations in the extrinsic base design on dc current gain of InAlAs/InGaAs/InP DHBTs. In: Proceedings of the Indium Phosphide and Related Materials Conference. 2001. p. 216–219. [11] Zanotti Fregonara C, Salviati G, Borgarino M, Lazzarini L, Fantini F. Low-temperature spectrally-resolved cathodoluminescence study of degradation in opto-electronic and microelectronics devices. Micron 2000;31:269–75.
High-resolution transmission electron microscopy on aged InP HBTs Bruce M. Paine a
a,*
, Timothy J. Perham a, Stephen Thomas III
b
Boeing Satellite Systems Inc., El Segundo, CA 90245, USA b HRL Laboratories LLC, Malibu, CA 90265, USA
Received 12 January 2004; received in revised form 25 February 2004 Available online 10 May 2004
Abstract We have used high-resolution transmission electron microscopy (HRTEM), with focused-ion-beam preparation of foils, to study InP HBT devices, both as-fabricated and after aging in life tests. The technology is HRL’s G1 process. We found that even after aging for the equivalent of 2 · 107 h under normal operating conditions, the visible damage is extremely benign: the base and emitter contacts are completely intact, there is no evidence of significant Au or Pt migration into the semiconductor, and minor crystal disorder that develops does not extend more than 100 nm below the metal–semiconductor interfaces. Except for these regions, the intrinsic devices are completely devoid of any anomalous features, to within the 0.3 nm resolution limit of the measurements. These observations provide strong evidence that failure mechanisms involving migration of metal from the ohmic contacts, or extended crystal defects, do not limit the reliability of this technology. The small disorder under the base contacts can probably explain the increases of gain (b) observed early in our life tests, and small increases in the base-collector leakage occurring later in the life tests. The minor disorder under the emitter contacts can probably explain the moderate increase of emitter resistance observed late in our life tests. But the mechanism for the eventual decline in b, observed after a long period of stress, for the equivalent of 107 h of normal operation, is not apparent from the HRTEM images. 2004 Elsevier Ltd. All rights reserved.
1. Introduction To have confidence in a reliability prediction based on life testing, it is desirable to have an understanding of the mechanisms responsible for the degradation. This way, one can check the plausibility of the analysis, check that the experiment was sufficiently sensitive to the changes that were occurring, and look for evidence of multiple mechanisms. For heterojunction bipolar transistors (HBTs) these checks are particularly important because potentially-fatal changes such as growth of extended defects, and migration of contact metals are known to occur without being fully quantified in traditional accelerated life tests [1]. Yet in most HBT reli-
*
Corresponding author. E-mail address: [email protected] (B.M. Paine).
ability studies, researchers typically invoke one of a small number of popular mechanisms, but fail to provide any direct physical evidence. This is because the drastic electrical changes are caused by relatively small and subtle atomic displacements that are difficult to observe. For HBTs, simple inspection by scanning electron microscopy (SEM) is rarely fruitful because they are entirely covered by contact metal. So cross sectioning is often done by focused ion beam (FIB) milling, and then SEM inspection can be extremely useful. But this technique is still limited to resolution of the order of 50 nm, by straggling of electrons in the samples. New techniques of FIB milling, to prepare ultra-thin slices through a device (known as ‘‘foils’’), followed by highresolution transmission electron microscopy (HRTEM) allow inspection with superb resolution (less than 1.0 nm), with the added advantage of providing information
0026-2714/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.microrel.2004.03.011
1056
B.M. Paine et al. / Microelectronics Reliability 44 (2004) 1055–1060
on crystal structure from diffraction data. Since this capability has become commercially available, we have begun applying it to HBT structures, both as-grown, and after aging in life tests, to observe physical changes with the best-possible resolution, and hence to conduct the checks described above. We believe this is the first published report of HRTEM analysis of aged HBTs. It was applied to HRL Laboratories’ first generation (G1) InP-based HBTs. These have already been shown to have excellent reliability [2–5] (e.g. the predicted mean time to failure at 100 C is about 107 h). For specimens aged for the equivalent of about 2 · 107 h in a typical application the results show only minor degradation under the contact metals on the base and emitter. There is no sign of significant extended defects that would cause degradation, and no sign of significant migration of contact metals in to the semiconductor layers. A similar study has been conducted on HRL’s second generation devices (G2) with very similar results. There are very few published transmission electron microscopy studies on aged III–V transistors. Magistrali et al. [6] studied Ti/Pt/Au gates on life tested GaAs MESFETs by TEM with conventional sample thinning, and saw evidence of metal diffusion from the gate, into GaAs. Takahashi et al. [7] conducted TEM inspection of GaInP/GaAs HBTs and found evidence of carbonrelated precipitates near the emitter–base interface.
2. Life test The parts were discrete HBT devices fabricated with the HRL Laboratories LLC G1 process [2]. The basic material structure is shown in Fig. 1. The base doping is beryllium. Nominal emitter dimensions were 2 · 5 lm. For the life tests the die were attached to Kovar carriers
with Au–Sn eutectic. Substrates with DC conductors, chip resistors and chip capacitors were also attached for electrical connections and stabilization. Connections on the carrier were made with Au wirebonds, and connections from the carrier to biasing circuitry were made with Au ribbons. The carriers were bolted to blocks containing electrical heaters. These fixtures were enclosed in dry boxes with flowing dry nitrogen. Biasing (in common-emitter mode), heating, and monitoring were conducted under computer control. DC characterization was done manually, at room temperature, with the biases off. The measurements included collector IV curves, Gummel plots, DC gain (b) vs. Ic , and forward and reverse junction characteristics. The life test was similar to those we have described earlier [2,3]. Junction temperature was 225 C (as measured electrically), current density was 0.233 mA/lm2 and Vce was 3.0 V. The evolution of b at Ic ¼ 0:06 mA/ lm2 for the duration of the life test is shown in Fig. 2. We see that all parts experience an increase in b during the first hours of life test, by up to 35%, followed by a relatively long period of stability. They then degrade between 200 and 400 h of life test, i.e. the equivalent of 1–2 · 107 h of normal operation at 100 C [2,3]. Similarly, the evolution of Vbe , i.e. the voltage for Ib ¼ 5 · 105 mA/lm2 , with collector floating, is shown in Fig. 3. Here there is no significant change until about 200 h of life test, when the Vbe ’s start to increase. The evolution of a Gummel plot for a typical device is shown in Fig. 4. From this we see that the initial increase of b is due to a decrease in Ib at all voltages (accompanied by a smaller decrease of Ic ). Following this there is minimal change, until after 200 h when Ic drops at all voltages
100
Contact
GaInAs
Emitter Contact
AlInAs
n+
Emitter
AlInAs
n n
GaInAs/AlInAs SL Spacer
GaInAs
Base GaInAs GaInAs Collector GaInAs Subcollector GaInAs Buffer InP Substrate
p
Beta
80 n+
60 40 Equivalent to 107 hrs of normal operation
20
p p+ nn+
Fig. 1. Schematic diagram of the InP HBT structure. The base dopant is beryllium. The superlattice is graded with the thickness of the GaInAs layers being reduced as successive layers are deposited.
0 0
100
200 300 Time (hrs)
400
500
Fig. 2. Drift plot for b (for Vce ¼ 1:0 V and Ic ¼ 0:06 mA/lm2 ) in the accelerated life test. b increased in the first hours of the life test, and then remained stable for the equivalent of 107 h under normal operation. Then it eventually declined. Dashed curves are for control samples that remained unstressed. The samples studied by HRTEM are represented by the and open triangle symbols.
B.M. Paine et al. / Microelectronics Reliability 44 (2004) 1055–1060
the adjacent base contacts. They were imaged at 400 kV, with the electron beam parallel to the [1 1 0] zone axis of the InP substrates, to ensure that all interfaces were viewed edge-on. They were also tilted slightly off-axis to satisfy various 2-beam diffraction conditions to better image any crystalline defects and compositional modulations within the semiconductor layers. Resolution as good as 0.3 nm was obtained in the phase contrast imaging mode. Energy-dispersive X-ray microanalysis (EDX) was conducted in a separate electron microscope with a non-field-emission electron gun.
Vbe(turn-on) (V)
1.0 0.8 0.6 0.4
Equivalent to 107 hrs of normal operation
0.2 0.0 0
100
200 300 Time (hrs)
400
500
Fig. 3. Drift plot for Vbe in the life test. (Ib ¼ 5 · 105 mA/lm2 , with the collector floating.) It remained stable for well beyond the period of normal operation, but eventually increased. Dashed curves are for control samples that remained unstressed.
1.E+04 Increasing Lifetest Time
Ic, Ib (A/cm2)
1.E+03
Ic
1.E+02 Ib
1.E+01 1.E+00 1.E-01
t = 0, 24, 171, 407 hrs Vb = Vc
1.E-02 1.E-03 0.3
0.4
0.5
0.6 Vbe (V)
0.7
1057
0.8
0.9
4. Microscopy results and discussion 4.1. As-fabricated Fig. 5 shows details of the side of the emitter mesa on an as-fabricated specimen. The emitter metal and selfaligned base metal are clearly visible. Within each of them, the dark material is a layer of Au, on top of a layer of Pt. The light material is Ti, which forms the first layer of the metallizations. The nþ GaInAs contact layer is clearly visible at the top of mesa. The lighter AlInAs ‘‘emitter contact’’ and emitter are below it, and the darker GaInAs base structure is below that, also extending laterally, under the base contact. The transition between the emitter and base spacer layer contains a superlattice of alternating layers of AlInAs and GaInAs, with the thickness graded so the GaInAs layers are thin on the emitter side and thick on the base side. Some of these layers are resolved. Beneath them the transition
Fig. 4. Gummel plots with Vbc ¼ 0, for various points in the life test: 0, 24, 171, and 407 h. For both Ic and Ib , the curves are lower at each interval.
(accompanied by a smaller decrease of Ib ). There is no sign of increase in series resistances, although moresensitive RF modeling has shown that the emitter resistance increases moderately [8]. The life test was stopped at 400 h, and specimens were sent for HRTEM analysis.
3. High-resolution transmission electron microscopy Sample preparation and inspection were conducted at NanoTEM Inc. The FIB preparation technique was similar to those described in the literature (a good review is Ref. [9]). Cross-sectional specimens with a 150–200 nm thick electron-transparent membrane were prepared. These cut through the emitter contact and the edges of
Fig. 5. HRTEM image of an as-fabricated specimen. The emitter metal consists of the dark Au and Pt, on top of Ti (light appearance). Below this, one can see (with discernable boundaries): the nþ GaInAs contact layer, the nþ AlInAs contact layer, the AlInAs emitter, the superlattice, a spacer, and the pþ GaInAs base layer.
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from the base spacer to the doped base layer is faintly visible as a darker line. There are no disordered regions in any areas of semiconductor, and no voids. Also there is no evidence of any metal migration. Thus there is negligible reaction of the metal contacts with the underlying semiconductor. But we note that not all of the base spacer has been removed before deposition of the base contact metal. Numerical modeling [10] has shown that this may allow for lateral flow of electrons to the base contacts, leading to a lower initial b than the optimum (see Fig. 6). The slightly darkened areas under the emitter contact are strain fields, as evidenced in the Fig. 7. Fig. 7 shows a close-up of the emitter metal– semiconductor contact area. This is typical of the entire contact region visible in this cross section. The contrast fringes indicate strain fields, which are most likely caused by thermal expansion mismatches remaining from the metal deposition process. The fringes vary due to local variations in the strain. The 3–4 nm line at the interface may indicate a layer of In-rich semiconductor.
Fig. 7. HRTEM image of the region under the emitter contact, in an as-fabricated specimen. The interface may contain a thin layer of In-rich semiconductor at the interface. There is also darkening caused by strain fields, probably from thermal expansion mismatches following metal deposition.
4.2. After life tests Fig. 8 shows an overview of a specimen with the largest degradation seen in this life test. This is the specimen represented by * symbols in Figs. 2 and 3: the b degradation was about 90%. A small number of defects are visible at the collector–sub-collector inter-
Fig. 8. HRTEM image of almost the entire emitter mesa in a specimen that has been life tested to the point where b dropped by 90%. There are small defects at the sub-collector–collector interface. There is also minor disorder, up to 100 nm thick, under the emitter and base contacts. Otherwise there are no anomalous features.
Fig. 6. Schematic diagram of an InP HBT, showing electron current flow that is allowed when the emitter etch is incomplete, leaving part of the spacer layer between the base and the base metal. This path may be blocked by disorder under the base metal, early in the life test, causing the observed increase of b.
face. It is not known whether they occurred during the initial epitaxial growth or during the aging. They are not expected to have any effect on device performance. The figure encompasses almost the entire width of the emitter
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mesa, and no disorder or extended defects are visible anywhere near the emitter–base or base–collector interfaces. Another specimen from the life test was also inspected: the one represented by open triangles in Figs. 2 and 3, with b degradation at 400 h of about 30%. This specimen did not display the defects at the collector– sub-collector interface. Otherwise the features of the images were the same in all respects. Fig. 9 shows detail of the base metal contact from Fig. 8. There is disorder under the metal, extending up to 100 nm into the semiconductor. It includes extended defects such as stacking faults and partial voids. These may result from some interaction between the metal and the semiconductor. But there are no well-defined phases with dark appearance, so we rule out any extensive reaction of Au or Pt with the semiconductor. The voids may arise from volume change due to local phase formation near the interface. Note that the disorder and voiding may effectively eliminate the small layer of spacer material that was inadvertently left below the base contacts. This could cut off the lateral flow of electrons from the emitter; with the result that b increases. This may well be the mechanism for the b increase observed early in the life tests. (But of course with this severely-aged specimen, one cannot tell if there was significant disorder under the base metal after the first few hours of life test.) We also note that although the base–collector interface is not visible, the disorder and voiding under the base contact almost certainly extend through the base layer and into the collector. This may lead to base–collector leakage, which indeed has been measured in our life tests, although with magnitude that is not significant relative to the normal operational currents [2,4]. Fig. 10 shows detail of the emitter metal contact from Fig. 8. This is the worst-case disorder of all that was observed along the entire widths of the mesas in the two specimens that were sectioned. But it is still extremely
Fig. 9. HRTEM close-up image of the region under the base contact in the specimen that had been aged such that b dropped by 90%.
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Fig. 10. HRTEM close-up image of the region under the emitter contact. The specimen is the same as that used for Figs. 8 and 9. The base–emitter junction is slightly below the superlattice, faintly visible at the bottom of the picture. No disorder or anomalous features of any kind are observed anywhere near the junction.
benign. There is disorder under the emitter metal, extending up to 100 nm in depth in some locations. Since the depth to the base–emitter junction is greater than 250 nm, this is no risk of it affecting the device operation. In fact the disorder remains in the nþ GaInAs contact layers––the emitter, spacer and base layers beneath them are completely featureless in all our HRTEM images. The disorder includes extended defects such as stacking faults, and probably results from interaction between the metal and the semiconductor. But we again observe no well-defined phases with dark appearance, and therefore rule out any significant reaction of Au or Pt with the semiconductor. The disorder may be responsible for the moderate increase in emitter resistance observed late in our life tests [8], and may explain part of the increase of Vbe shown in Fig. 3. But the disorder under the emitter contacts does not appear to explain the large drops in the b’s, or the drops in the Gummel plots at all voltages. An EDX image was taken of the disordered region under the emitter metal (see Fig. 11), and it showed no significant signals from Au or Pt. Note that the Cu peaks arise from the support grid while the Fe peaks arise from the iron pole piece in the microscope. Thus we find no evidence for any mechanisms involving migration of metal from the ohmic contacts, or extended crystal defects that might limit the reliability of this technology. And we rule out low-activation energy mechanisms since none was observed in extremelylong-duration low-temperature life tests reported earlier [4,5]. But no mechanism is observed for the eventual decline of b in the life tests, after the equivalent of 107 h of operation under normal conditions. We, therefore, speculate that the decline of b arises by diffusion of Be atoms from the base layer, through the superlattice and
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Wafer 564-054, device 5348-76, carrier s/n 005 EDX, disordered region under contact Cu
Counts
100
As 50
Ga
As Ga
We acknowledge Dr. Roger J. Graham and Dr. Fred Shaapur of NanoTEM, Inc. for the sample preparation, high-resolution transmission electron microscopy work, and help with interpretation. We also acknowledge the staffs of both the BSS Technology Qualification Laboratory and HRL Clean Room Operations for their contributions to this work.
In
In Cu Fe In Ti Ti Al Au 0
Acknowledgements
5
Fe Au Cu Ga Energy (keV)
As
10
Fig. 11. Energy-dispersive X-ray analysis of the disordered region under the emitter contact. The specimen is the same one from which Figs. 8–10 were taken. Peaks from Pt and Au are negligible, indicating insignificant migration of these elements from the contact, into the semiconductor.
into the emitter. Since Be atoms are known to migrate interstitially, this requires no defects in the host lattice. Also Be has a very low atomic number so the migration would not be expected to leave any signature that is detectable by HRTEM. This mechanism, which is known as Recombination-Enhanced Impurity Diffusion (REID) would also explain the increase of Vbe that is observed at about the same time (see Fig. 3). Finally we note that early reductions of base current have also been attributed to REID, by annihilation of recombination centers [11]. So this may be an alternative explanation of the increases of beta observed in the first hours of the life test.
5. Conclusion HRTEM, together with FIB-preparation of sample foils, has allowed us to see direct physical evidence of the changes induced in InP HBTs during life testing. We found no evidence of anomalies of any type near the transistor junctions. But small amounts of disorder under the base and emitter contacts can explain most of the observations in our life tests. The exception is the eventual major degradation of b, observed late in the life tests, at times equivalent to 107 h of normal operation: this may occur by migration of Be dopant atoms, which does not create any structures that are visible with HRTEM.
References [1] Henderson TS. The GaAs heterojunction bipolar transistor: an electron device with optical device reliability. Microelectron Reliab 1996;36:1879–86. [2] Kiziloglu K, Thomas III S, Paine BM, Williams Jr F, Fields CH. Investigation of process related reliability for InP-based heterojunction bipolar transistors (HBTs). Electrochem Soc Proc 1999;4:95–102. [3] Kiziloglu K, Thomas III S, Williams Jr F, Paine BM. Reliability and failure criteria for AlInAs/GaInAs/InP HBTs. In: Proceedings of the Indium Phosphide and Related Materials Conference. 2000. p. 294–297. [4] Paine BM, Thomas III S, Delaney MJ. Low-temperature, high-current life tests on InP-based HBT’s. In: Proceedings of the International Reliability Physics Symposium. 2001. p. 206–213. [5] Paine BM, Thomas III S, Delaney MJ. Very-Low-temperature life tests on InP-based HBT’s––An update. In: Proceedings of the GaAs Reliability Workshop. 2002. p. 119–136. [6] Magistrali F, Sala D, Turner J, Valli P, Vanzi M. Experimental evidence of Ti/Pt/Au–GaAs interaction obtained by means of TEM technique. In: Proceedings of the GaAs IC Symposium. 1991. p. 199–202. [7] Takahashi T, Sasa S, Kawano A, Iwai I, Jujii T. Highreliability of InGaP/GaAs HBTs fabricated by self-aligned process. In: Technical Digest of the International Electron Devices Meeting. 1994. p. 191–194. [8] Thomas III S, Fields CH, Madhav M. RF modeling approach to determining end-of-life reliability for InPbased HBTs. Microelectron Reliab 2001;41:1129–35. [9] Giannuzzi LA, Stevie FA. A review of focused ion beam milling techniques for TEM specimen preparation. Micron 1999;30:197–204. [10] Hussain T, Shi BQ, Nguyen C, Madhav M, Sokolich M. A numerical analysis to study the effects of process related variations in the extrinsic base design on dc current gain of InAlAs/InGaAs/InP DHBTs. In: Proceedings of the Indium Phosphide and Related Materials Conference. 2001. p. 216–219. [11] Zanotti Fregonara C, Salviati G, Borgarino M, Lazzarini L, Fantini F. Low-temperature spectrally-resolved cathodoluminescence study of degradation in opto-electronic and microelectronics devices. Micron 2000;31:269–75.