Process Technology for High Speed InP Based Heterojunction Bipolar Transistors

Transcription

1 Proce Technology for High Speed InP Baed Heterojunction Bipolar Tranitor Der Fakultät für Ingenieurwienchaften der Univerität Duiburg-Een zur Erlangung de akademichen Grade eine Doktor() Ingenieur genehmigte Diertation von Serkan Topaloglu au Caramba Referent : Prof. Dr. rer. nat. F. J. Tegude Korreferent : Prof. Dr. rer. nat. D. Jaeger Tag der Mündlichen Prüfung:

2 Lit of Symbol Introduction Theory Heterojunction Bipolar Tranitor Structure of an HBT dc propertie of HBT RF Propertie of HBT Parameter extraction for HBT Different Material Sytem ued for InP baed HBT Vertical Deign of HBT Lateral Deign of HBT Fabrication of HBT Main Step of HBT Proceing Lithography Etching Formation of Contact Proceing Step of an SHBT Improvement in HBT Proceing Evolution of the HBT Deign Orientation of the Emitter on the Wafer Optimiation of Underetching for Emitter Directly Contacted Emitter Variou Layout Contact Optimiation Contact optimiation for InGaA-InP SHBT Contact optimiation for GaASb layer...68

3 4 HBT with the Optimied Layout and Proceing Meaurement Reult for GaASb DHBT Improvement of Maximum Current Denity Improvement of Current Denity for DHBT Improvement of Current Denity for SHBT Emitter Size Effect on HBT Performance Inductively Coupled Plama (ICP-RIE) Etching for HBT Application Baic of Plama Etching ICP-RIE Sytem Optimiation of the Proce Parameter for ICP-RIE etching Effect of Temperature Effect of Ga Mixture Effect of RF Power Optimied ICP-RIE Proce Parameter for HBT Application Removal of NiCl x reidue from the urface Optimiation of Hybrid Etching SHBT proceed with the optimied Hybrid etching proce Meaurement Reult for the Submicron HBT Influence of the Emitter Etching on HBT Uniformity Concluion Appendix APPENDIX A. Scattering Parameter relationhip APPENDIX B. The Tranferred Subtrate proce APPENDIX C. The Early Proce protocol APPENDIX D. The Early Mak Set (HBT97) APPENDIX E. The developed mak Set (HBT03) APPENDIX F. The layer tructure of the HBT APPENDIX G. The Optimied Proce Protocol APPENDIX H. The ICP-RIE Sytem APPENDIX I. The E-Beam layout

4 Reference

5 Lit of Symbol A A C A E B B max BV CEo C bc C dep C fb C jc C je χ D E C E v D nb D pe ε e * E F E g,bae E g,emitter φ B φ CB φ Cl2 φ Cl2+N2 φ M φ Μ f max degree of iotropy collector area emitter area dc Current gain maximum dc Current gain open-bae Breakdown Voltage bae-collector capacitance bae-collector depletion capacitance per unit area feedback capacitance bae-collector junction capacitance bae-emitter junction capacitance electron affinity of the emiconductor high field diffuion contant conduction band dicontinuity valence band dicontinuity minority electron diffuion coefficient (in bae) minority hole diffuion coefficient (in emitter) dielectric contant highly energetic electron fermi energy (thermal equilibrium) energy bandgap of bae energy bandgap of emitter energy barrier height bae-collector junction potential Cl 2 flow total ga flow work function of the metal work function of metal maximum ocillation frequency 4

6 φ N2 N 2 flow φ S f T I B * I B I B,bulk I B,cont I B,cr I B,urf I C I SC I SE * J B J B,bulk J B,urf J C,max J Kirk k K B,urf L B L D L E L pad L T work function of emiconductor cut-off frequency bae current back injection current bulk recombination current interface recombination current pace-charge recombination current urface recombination current collector current collector aturation current emitter aturation current back injection current denity bulk recombination current denity urface recombination current denity maximum collector current denity Kirk current denity Boltzmann' contant urface recombination current divided by perimeter bae length amount of preading emitter length length of the TLM pad tranfer length µ n electron mobility µ p hole mobility n 0 N B n B n C N c N C N E n i N v electron denity in thermal equilibrium bae doping denity bae ideality factor collector ideality factor effective denity of tate in the conduction band collector doping denity emitter doping denity intrinic carrier denity effective denity of tate in the valence band 5

7 p p 0 P E P ICP ρ R b,cont R b,pread R bb R bb xc bc R cont R E R gap R jc R je R L R h,bae R TOT R v S S c T T B τ b T C T dep T E τ e τ c V 2 V BC V BE V bi V CE V CE,offet v at preure hole denity in thermal equilibrium emitter Perimeter ICP power net charge denity bae contact reitance preading reitance bae reitance general time contant contact reitance emitter reitance bae-emitter gap reitance collector junction reitance emitter junction reitance lateral etch rate bae heet reitance Total reitance vertical etch rate electivity pecific contact reitance temperature in Kelvin bae thickne bae tranit time collector thickne bae-collector depletion region thickne emitter thickne emitter charging time pace-charge tranit time bae-collector bia depleting the collector layer bae collector voltage bae emitter voltage built-in potential collector emitter voltage offet Voltage aturation velocity 6

8 V trun-on W B W dep W E W EB W pad turn-on Voltage bae width depletion region width emitter width bae-emitter pacing width of the TLM pad 7

9 1 Introduction In1904, Fleming invented vacuum tube diode and thi wa the tarting point of Electronic Engineering. By the beginning of 1907, De Foret introduced a third electrode into the vacuum tube diode, and created a device called vacuum tube triode, which could generate ocillation and amplify radio ignal. With thi invention, live radio broadcating became poible and radio indutry wa founded. Thu, Lee De Foret i known a the father of radio [1]. Figure 1.1 Fleming firt vacuum tube diode [2] Until 1950, vacuum tube device were ued in electronic ytem. In 1947, William Shockley, John Bardeen and Walter Brattain have invented the firt olid-tate tranitor [3]. Thi invention wa one of the mot important miletone in electronic engineering [4]. E C Ge Plate B Figure 1.2 Firt Tranitor invented at Bell Lab in 1947 [5] 8

10 Up to now, a rapid progre and lot of improvement have been done. The firt tranitor wa realied on a germanium (Ge) plate. In comparion to the vacuum tube triode, olid-tate tranitor were offering reliability, a longer lifetime, le power conumption and maller device dimenion, which wa an important apect for integrated circuit. Jack Kilby ha ued thi integration apect and ha invented the firt integrated circuit [6]. At the ame time, by the beginning of 1950, Shockley and Kroemer have introduced a concept, which wa named a heterojunction concept, overcoming the contrain of homojunction tranitor [7, 8, 9]. But it ha taken nearly 20 year to realie thee junction. By the introduction of modern epitaxial crytal growth technique, it became poible to produce high quality junction produced with different material [10]. Thee erie of invention have reulted in a rapid improvement of electronic circuit and ytem. By the advance in high frequency communication ytem, and particularly Internet, wide bandwidth and high-peed tranitor became key device for the circuit. Epecially, optical fibre can tranport data at high rate. Therefore, high-peed electronic i neceary for all kind of data proceing. One limitation i the ultra high frequency modulation of the light for data tranport. Thi require high frequency and high voltage of operation. Second, high linearity of amplification i neceary for analog-to-digital converion (ADC). A technology roadmap i hown in table 1.1 to preent the high data rate potential of optoelectronic communication ytem. Table 1.1. Optoelectronic Communication Technology Roadmap [11] Year Long Haul Bitrate 10 Gbit/ 40 Gbit/ 80/ 160 Gbit/ 400 Gbit/ (?) (TDM) Indium Phophide baed Heterojunction Bipolar Tranitor (InP HBT) have the potential to provide high peed and high voltage for optoelectronic communication IC. Moreover, ince their energy band gap correpond to the 1.3 and 1.55 µm wavelength, which are the wavelength providing minimum optical lo in fibre, InP HBT are the bet choice for optical communication circuit [12]. 9

11 The objective of thi work i to demontrate HBT to be ued for InP Electronic for +80 Gbit/ (InP-Elektronik für +80 Gbit/) project. Thi project i funded by German Federal Minitry of Education and Reearch (BMBF). Here, high peed, reproducible, reliable and high yield HBT are required. Technique have been invetigated to decreae the paraitic capacitance and reitance leading to high peed HBT. Wet chemical and dry etching method have been optimied and compared to how that etching ha an important influence on reproducibility and uniformity. An ICP-RIE (Inductively Coupled Plama Reactive Ion Etching) proce with Cl 2 /N 2 chemitry ha been developed for the proceing of ubmicron emitter and maller device dimenion in a reliable way. The preent thei i tructured a follow: Chapter 2 - Theory The detail about the baic device operation are given. The phyical background and equation governing HBT operation are explained. The tructure of a conventional InP HBT i given. The dc (direct current) and RF (radio frequency) propertie of HBT are dicued and equation are given to calculate the performance of HBT. Simulation are performed for different material ytem to how pro and con, while dicuing the poible bandgap engineering. Detail are given about the epitaxial growth and about the lateral deign of HBT. Different approache are dicued to reduce the paraitic capacitance. Chapter 3 - Fabrication of HBT Main tep and the evolution of the optimied HBT fabrication proce are preented. The experiment and reearch, which have been performed to improve the performance and reliability, are preented. Chapter 4 HBT with the Optimied Layout and Proceing In thi chapter, HBT proceed with the optimied proceing procedure and the new deign are preented. GaASb DHBT meaurement reult are ued for the comparion of the previou deign and proceing with the optimied one. The Kirk effect i invetigated experimentally. The influence of the emitter width and length on HBT performance are elaborated. 10

12 Chapter 5 Inductively Coupled Plama- Reactive Ion Etching for HBT Application In thi chapter, baic about the plama proceing are given. A hybrid etching proce ha been developed to be ued for HBT application. SHBT proceed with the optimied hybrid etching proce have been invetigated. The influence of emitter etching on HBT uniformity i preented. The firt reult for ubmicron HBT proceed with ICP-RIE with Cl 2 /N 2 chemitry are preented. Chapter 6 Concluion and Outlook Thi chapter ummarie the improvement in proceing and the reult of the complete thei. With the outlook, potential for future application are pointed out to guide for the upcoming work. 11

13 2 Theory 2.1 Heterojunction Bipolar Tranitor Epecially, in the lat two decade, by the advance in epitaxial growth technique, heterojunction bipolar tranitor (HBT) became key device for optoelectronic circuit. In comparion to their homojunction counter part, the ue of wide band gap heterojunction emitter allow the deign of HBT with very high bae doping [13]. Thi provide a lower bae reitance and bae collector capacitance, and enable a better RF performance. In the following chapter, the tructure of an InP-HBT and the detail about device operation will be given Structure of an HBT In comparion to the Bipolar Junction Tranitor (BJT), the HBT contain a heterojunction, where two different material with different band gap are ued. In figure 2.1, the conventional layer tructure for a ingle heterojunction InP bipolar tranitor (InP-SHBT) i hown. E B n++ InGaA n InP p++ InGaA n- InGaA n++ InGaA emi inulating InP Subtrate C Emitter cap Emitter Bae Collector Sub-collector Figure 2.1 Layer tructure of a conventional InP/InGaA SHBT. A depicted in figure 2.1, an HBT conit of three main layer, which are emitter, bae and collector like in BJT. Thee three layer form two pn junction connected in a back-to-back configuration. In thi example, the HBT i npn type, where emitter and collector are doped n-type, in contrary the bae i placed between them, 12

14 I ( ma) C with p type doping. There are alo pnp tranitor. A the electron mobility i higher than the hole mobility for all emiconductor material, a given npn tranitor tend to be fater than an equivalent pnp type. Therefore, npn tranitor are preferred for circuit application and will be conidered here dc propertie of HBT C V > 0 BC V = 0 BC V < 0 BC B I C Saturationregion active region I B (µa) I B E VCE, offet cutoff region 0 V CE (V) Figure 2.2 The common emitter output characteritic of a typical npn InP-HBT. Under the normal operating condition (active region), the bae-emitter diode i forward biaed, V BE > 0 V, and the bae-collector diode i revere biaed, V BC < 0 V. In the aturation region both diode are forward biaed, V BC, V BE > 0 V. In the cut-off region, both diode are revere biaed, V BC, V BE < 0 V. The bae-emitter diode i revere biaed, V BE < 0 V and the bae-collector diode i forward biaed, V BC > 0 V, in the revere region. In the common emitter output characteritic hown in Figure 2.2, the output current I C, i plotted a a function of V CE for different input current I B. By dividing the output current I C into the input current I B at a certain bia point, dc current gain, B, can be identified. Here, one can alo extract the offet voltage (V CE,offet ), the turn-on voltage (V turn-on ) and the breakdown voltage, BV CEo. The dc current gain of an HBT i given in equation 2.1 [14]. 13

15 I I C * B N ETE DnB Ev = exp( ) equation 2.1 N T D kt B B pe Here, N and emitter and bae thickne; E N B are the emitter and bae doping level; T E and T B are the D pe and D nb are the minority hole diffuion coefficient in the emitter and the minority electron diffuion coefficient in the bae, repectively. i valence band dicontinuity at bae and emitter heterointerface [14]. The bae current * i denoted a I B referring to the back-injection current. By the help of thi valence band dicontinuity, the diffuion of the hole from the bae to emitter i reduced. Thi will keep the bae current lower leading to higher dc current gain. In addition to the backinjection current, the bae current i compoed of 4 main component. Thee are: the urface recombination current I B,urf, the interface recombination current I B,cont, the bulk recombination current in the bae region I B,bulk, the pace-charge recombination current in the bae-emitter depletion region I B,cr. I B,urf i defined a the recombination of the minority carrier injected from emitter with the bae majority carrier at the urface. Thi i proportional to the emitter periphery. Thi component i much more important for GaA urface where the urface recombination velocity i rather high. Epecially for mall device having large perimeter area ratio, thi component become dominant. Thi effect i alo named a emitter ize effect. The dc current gain decreae with the decreaing emitter area. I B,cont become important when the bae-emitter contact pacing i low. In thi cae, the minority carrier flow alo laterally to the bae contact and recombine with the majority carrier and increae the I B,cont. If the bae-emitter contact pacing i large, the minority carrier concentration reaching the bae contact i nearly zero. The recombination of the carrier in the depletion region reult in increae of I B,cr [14]. The offet voltage i caued by the different value for the turn-on voltage of baeemitter and bae-collector diode. The value of the offet voltage i determined by the ratio of bae-emitter and bae-collector junction area, high-level injection effect and erie reitance [15]. The offet voltage can be expreed a: Ev nkt AC nkt I SC V CE, offet = RE I B + ln( ) + ln( ) equation 2.2 q A q I E SE 14

16 Here, R E i the emitter reitance, T i the temperature, n i ideality factor, A C and AE are the collector and emitter area, repectively. I SC and I SE are collector and emitter aturation current, repectively. The turn-on voltage i defined a the bae-emitter voltage required to achieve a certain collector current [16]. It i given a: kt Eg, bae kt 2 V BE, turn on = ln( Rh, bae ) + ln( q µ N c N v Dn / J C ) equation 2.3 q q q The open bae breakdown voltage, BV Ceo, define the maximum allowed collectoremitter voltage. When the collector-emitter voltage i higher than BV Ceo, even though no current i applied to the input, there will be current flowing at the output. Thi breakdown effect i caued by the impact ioniation in the bae-collector diode and the punch through effect [17] I II III I C I B I B, I C (ma) 1E-3 1E-4 1E V BE (V) Figure 2.3 The typical Gummel Plot for an HBT (V CB = 0V) The Gummel Plot i the imultaneou plot of I C and I B a a function of V BE when V CB = 0 V a hown in Figure 2.3. The bae and collector ideality factor (n B and n C ) and leakage current for both diode can be extracted from thi plot. Region I i the nonlinear region, where leakage current and low voltage effect dominate. Exponential behaviour can be oberved in region II. In the ideal cae, the dc current gain i contant 15

17 and ideality factor can be extracted in thi region. High-current and erie-reitance effect are dominant in region III. For thee meaurement, the dc meaurement et-up hown in figure 2.4 i ued. The ample i placed on the Karl Sü PA 150 Prober. By the help of the controller, ample i aligned to contact the meaurement needle. Via cable, meaurement needle are connected to the emiconductor parameter analyer. Semiconductor parameter analyer i a tand-alone intrument capable of complete dc characteriation of emiconductor device. It timulate voltage and current enitive device, meaure the reulting current and voltage repone, and diplay the reult in a uer electable format on CRT diplay [18]. Here, the meaurement PC collect the data from the analyer and ave them. Karl Sü Controller Meaurement PC HP 4145B Semiconductor Parameter Analyer Karl Sü PA 150 Prober Figure 2.4 The dc meaurement et-up ued in thi work For the operation of an HBT, a key parameter i the thickne of the bae. Here, one hould conider two cae. Cae 1, when the bae thickne i relatively thick. Cae 2, when the bae thickne i le than the diffuion length (L n ). 16

18 Depletion Region Depletion Region carrier concentration Emitter N + Bae p ++ Collector n - p o exp(qv BE /kt) p o Depletion Region n o exp(qv BE /kt) n o Depletion Region p o thickne Figure 2.5 The minority carrier concentration for an npn tranitor with relatively thick bae layer In cae 1, ince all the electron emitted by emitter recombine with the hole in the bae, there are no electron reaching to the bae-collector junction. A a reult, the collector current i limited to the aturation current (I SC ). In figure 2.5, the ditribution of the carrier concentration belonging to cae 1 i hown. p o and n o are repreenting the equilibrium hole and electron concentration in emitter and in bae, repectively. The firt ha mall value equal n 2 i /N B and the latter i n 2 i /N E, where n i i the intrinic carrier concentration. carrier concentration Emitter N + Bae p ++ n o exp(qv BE /kt) Forward injection current Collector n - p o exp(qv BE /kt) Backward-Injection Current p o n o p o thickne Figure 2.6 The minority carrier concentration for an npn tranitor with proper bae layer 17

19 In cae 2, the thickne of the bae i le than the diffuion length of the minority carrier, here electron. The electron emitted by the emitter, can reach the collector before they recombine with the hole in the bae, becaue of the thinner bae. Even though, the bae-collector voltage i et to zero (V CB = 0 V), by the help of the electric field in the bae-collector depletion region thee electron can be wept into the collector. Thi lead to current flow in the collector. Here, the number of electron i not the ame a the number of electron entering the bae. Thi degradation occur becaue of the unavoidable recombination in the bae. In figure 2.6, the ditribution of the carrier concentration belonging to cae 2, i hown RF Propertie of HBT There are two important figure of merit for the high frequency characterization of HBT. Thee are the cut off frequency f T, and the maximum ocillation frequency f max. The current gain cut off frequency i determined by the tranit time of the electron emitted by emitter. The current gain cut-off frequency i given a: 1 2π f T = τ + τ + τ e b c + τ c equation 2.4 Here, τ e i defined a the emitter charging time. kt τ e = ( C je + C jc ) equation 2.5 qi c C je i defined a the bae-emitter junction capacitance and C jc i defined a the baecollector junction capacitance. τ b i the bae tranit time. 2 TB τ b = equation 2.6 2D nb 18

20 τ c tand for pace-charge tranit time and defined a the time for the carrier to drift through the depletion region of the bae-collector junction. Tdep τ c = equation 2.7 2v at T dep i the bae-collector depletion region thickne and v at i the aturation velocity. τ c i the collector charging time given a: τ = ( R + R ) C equation 2.8 c E C jc Thi term i greatly influenced by the paraitic emitter reitance, R E. With the equation , the f T can be written a: 1 2π f T kt T 2 B dep = ( C je + C jc ) + + qi c 2DnB 2vat + ( RE + RC ) T C jc equation 2.9 The equation 2.9 can be interpreted that the cut off frequency can be influenced mainly by the vertical deign of HBT, or in other word motly with epitaxy. Another important figure of merit for high frequency performance i the maximum ocillation frequency f max. It i related with the charging and dicharging time of the capacitance. It i defined a the frequency where the power gain drop to unity [19]. f f T max equation π Rbb Cbc The parameter are depicted in Figure 2.7 uing the triple mea deign of a HBT. The bae reitance R bb, conit of contact reitance b cont, bae-emitter gap R, reitance Rgap and preading reitance underneath the emitter R b, pread. The equation 19

21 2.11 depict the component of bae reitance R bb. R h and S C repreent the pecific heet and contact reitance, repectively. W E Emitter V BE R e,cont R e V CE Bae W EB R je C je R b,cont R gap R b,pread α ο i e e -jωt C fb R jc C jc Collector R c, pread R c,cont Figure 2.7. The triple mea deign HBT layout howing the mall ignal parameter and dimenion ued in the equation. The haded region depict the pace charge region. R R R R bb b, cont gap = R = b, pread b, cont S R S h = ρ W + R EB S = ρ W E C gap / 2 /2L E L /12L + R E E b, pread equation 2.11 C fb, C jc and C je are the feedback, the intrinic bae-collector and the intrinic bae-emitter capacitance, repectively. Cbc i the total bae-collector capacitance, and it i proportional to the bae-collector junction area. The component of the bae-collector capacitance can be repreented a: C = C W L equation 2.12 fb jc dep dep C = C ( W + W ) L E EB B B B 20

22 Here, C dep i the bae-collector junction depletion capacitance per unit area and L B i the length of the bae contact. There are everal method in the literature to improve the f max, which are motly related with technology and layout. The main idea for all of thee method i to find an optimum way to reduce the bae-emitter gap and bae-collector area. They will lead to a reduced bae reitance and a reduced bae-collector junction area will offer a lower bae-collector capacitance and hence better RF performance [20, 21, 22, 23, 24, 25, 26] For the meaurement of f T and f max, the RF meaurement et-up hown in Figure 2.8 i ued. Thi ytem i capable of performing dc and RF meaurement. The ample, DUT (device under tet), i placed on wafer prober from Alei and contact are realied with Picoprobe microwave needle with 150 µm pitch (GSG: ground ignal ground). Before dc meaurement, the ytem mut be calibrated in order to eliminate the influence of the cable and meaurement needle to the reult. For the calibration, a calibration ubtrate i ued. Up to 50 GHz RF meaurement i poible with network analyer HP 8510C with erial yntheizer HP By the help of -parameter tet et HP 8517A, -parameter are meaured in the range of 45 MHz up to 50 GHz. HP 4142B parameter analyer i ued to adjut the biaing point. High frequency calibration i performed by the SOLT (hort, open, load, through) tructure on the calibration ubtrate. HP 8517A Tet-Set HP 8510C Network Analyer HF-Cable alei HP 8510C Buytem HP 83650A Syntheier IEEE-488 Buytem Meaurement PC HP 4142B Parameter Analyer Alei-Prober Figure 2.8 The RF Meaurement Set-Up ued in thi work Since they are eaier to meaure and work with at high frequencie than other kind of parameter, cattering parameter are preferable. Scattering parameter are commonly referred to a -parameter. They are the parameter et defined by travelling wave that 21

23 are cattered or reflected when an n-port network i inerted into the tranmiion line. Unlike other parameter et (like Z, h, y parameter), they are eaier to meaure, becaue they do not require ucceive opened or hort circuited input and output of the device. Thi ituation i much more critical for RF frequencie where lead capacitance and inductance make open and/or hort circuit difficult to achieve. -parameter are convertible to all other parameter et a hown in Appendix A [27]. The cut-off frequency i alo defined a the frequency, where the current gain drop to unity. In other word, it i given a the frequency, where h 21 drop to unity. The h 21 of an HBT i given a follow and can be extracted by the meaured - parameter [14]: h 21 = i i c 21 i 0 b (1 0 11)(1 + 22) + v = = 2 i = v ce = equation Parameter extraction for HBT In addition to the dc and RF meaurement, by uing meaured -parameter, the extrinic and intrinic parameter of HBT can be extracted. For thi purpoe, a T-model mall ignal equivalent circuit ha been ued a hown in Figure 2.9. The determination tart with identifying the extrinic element uing evolutionary optimiation trategie [28, 29]. The optimiation procedure identifie the bia dependence of the extrinic mall-ignal equivalent element. And then, the intrinic parameter are optimied by keeping the extrinic parameter contant. All the procedure briefly decribed above i performed by a oftware called evolhbt, which i developed in the department [29]. 22

24 C IO C fb Cjc B L B R B R bb i R cc R C L C C B int C int R jc α 0 i e e -jωτ C IN C je E int i e R je intrinic HBT C OUT R E L E E E Figure 2.9 T-model mall ignal equivalent circuit for an HBT ued for the parameter extraction 2.2 Different Material Sytem ued for InP baed HBT The HBT can be claified according to the ubtrate material ued. In general, there are 3 main ubtrate material ued. Thee are; Gallium Arenide (GaA), Silicon (Si) and Indium Phophide (InP). The mot mature one i GaA. A wide band gap emitter, AlGaA wa grown on GaA [30, 31]. Later on, ince it offer eaier proceing, indium gallium phophide (InGaP) wa ued a emitter material [32]. In Table 2.1, poible material ytem ued for III-V HBT are hown [33]. A depicted in table 2.1, if one of the junction i a heterojunction, then it i named a Single Heterojunction Bipolar Tranitor (SHBT). If both junction are heterojunction, then it i called Double Heterojunction Bipolar Tranitor (DHBT). 23

25 Table 2.1 HBT type with different material ytem Subtrate Emitter Bae Collector Type GaA SHBT AlGaA GaA AlGaA DHBT GaA GaA SHBT InGaP GaA InGaP DHBT InGaA SHBT InP InGaA InP DHBT InP InGaA SHBT InAlA InGaA InP DHBT InP GaASb InP DHBT InP ubtrate ha everal advantage over GaA. Firt of all, the pecific compoition of InGaA i lattice matched to the InP and InGaA ha about 1.5 time higher electron mobility than GaA. Moreover, ince the Γ-L and Γ-X valley eparation are greater for InP and InGaA in comparion to GaA. They offer higher velocity overhoot, which will reult in higher electron velocity and higher f T. Table 2.2 Material parameter for different heterojunction Heterojunction (Emitter/Bae) E g,emitter (ev) E g,bae (ev) E C (ev) E V (ev) InP/In 0.53 Ga 0.47 A In 0.52 Al 0.48 A/In 0.53 Ga 0.47 A Al 0.30 Ga 0.70 A/GaA In 0.49 Ga 0.51 P/GaA Smaller band gap InGaA reult in lower bae emitter turn-on voltage, which i an important apect for low power application. InGaA ha much lower urface recombination velocity (~1000 time lower) than GaA. Thi lead to the lower bae current due to the le recombination and therefore higher current gain. Epecially for high power and high current application, thermal conductivity hould be conidered. InP ha thermal conductivity of 0.68 W/cm, where GaA ha 0.45 W/cm [14]. The mot 24

26 important propertie of InP ubtrate, which make it attractive for optical communication circuit, i the compatibility with laer working at 1.3 and 1.55 µm wavelength, where the fibre optic loe are minimum. On the other hand, InGaA ha lower mobility and lower doping capability degrading the bae reitance. Moreover, InP ubtrate cot much more than GaA ubtrate. Now, different HBT type on InP ubtrate will be compared. There are two common heterojunction for InP HBT. Thee are InP/InGaA and InAlA/InGaA. From Table 2.2, valence and conduction band dicontinuitie how that, InP/InGaA heterojunction will offer a larger valence band tep than InAlA/InGaA. Thi will reduce the hole back injection from bae to emitter and therefore it will offer higher current gain. But on the other hand, ince InAlA doe not contain Phophor (P), thi make it eaier to be grown in MBE (molecular beam epitaxy) ytem [34]. In thi work, we will concentrate on the InP/InGaA ingle heterojunction bipolar tranitor (SHBT). To compare thee tructure, imulation have been performed uing SimWin 1.50 [35] E (ev) E B C n p n InP InGaA InGaA E F Ec (ev) Ev (ev) t (nm) Figure 2.10 The band line-up of bae, emitter and collector in InP/InGaA SHBT (no voltage applied) SHBT do not uffer from the conduction band barrier in bae-collector junction but ince the collector material ha low energy band gap, thi reult in lower breakdown voltage. Therefore, DHBT are propoed having wider band gap material in the collector a hown in Figure

27 E (ev) E B C n p n InP InGaA InP Ec (ev) Ev (ev) E F t (nm) Figure 2.11 The band line-up of bae, emitter and collector in InP/InGaA DHBT (no voltage applied) A it i depicted in Figure 2.11, there i blocking of the electron tranport at the bae-collector junction due to conduction band barrier. Thi will degrade the HBT performance [36]. The electron emitted by emitter will tunnel through the bae emitter junction. Since the collector i low doped, the depletion region i thick and thi may reult in electron to be reflected from bae-collector junction [37]. To overcome thi problem, quaternary material and pacer can be introduced between bae and collector. Thi will minimie the conduction band barrier in BC junction and lead to better HBT performance. In Figure 2.12, a compoite collector InP/InGaA DHBT, with one InGaA pacer and two InGaAP quaternary layer, i hown E (ev) E B C n p n InP InGaA InP Ec (ev) Ev (ev) E F t (nm) Figure 2.12 The band line-up of bae, emitter and collector in InP/InGaA DHBT with compoite collector tructure (no voltage applied) 26

28 Even though DHBT offer higher breakdown voltage, there are ome difficultie in growth and proceing of InGaAP layer. Epecially, in the lat decade, there i a relatively new DHBT tructure with GaASb bae [38, 39, 40]. It ha everal advantage over the InP/InGaA heterojunction. Firt of all, ince they have no conduction barrier in the bae-collector junction, they do not require any complicated compoite collector tructure. Thee make them eaier to grow and proce E (ev) E B C n p n InP GaASb InP Ec (ev) Ev (ev) E F t (nm) Figure 2.13 The band line-up of bae, emitter and collector of InP/GaASb DHBT (no voltage applied) Moreover, the valence band dicontinuity of InP/GaASb DHBT i much higher than InP/InGaA HBT, which make the hole back injection almot negligible. 2.3 Vertical Deign of HBT There are two well-known growth technique for InP HBT. Thee are Molecular Beam Epitaxy (MBE) and Metal Organic Vapour Phae Epitaxy (MOVPE). In MBE ytem, elemental Group III and V ource are ued. It i difficult to handle the elemental phophoru (P). Becaue of thi, gaeou ource like AH 3 and PH 3 are introduced to thee ytem. Thee modified ytem are called ga ource MBE (GSMBE). In MOVPE, all Group III ource are metal organic ource [41, 42, 43, 44]. The HBT ued in thi thei were grown with an AIX200 RF 200 low preure 27

29 MOVPE ytem with fully non-gaeou ource. Further detail about the growth and doping are given in [41]. In Table 2.3, a typical SHBT layer tructure i hown. Table 2.3 Typical SHBT layer tructure ued in thi thei Emitter-cap n ++ InGaA 1x10 19 cm nm Emitter contact n ++ InP 1x10 19 cm -3 50nm Emitter n + InP 2x10 17 cm -3 50nm Bae p ++ InGaA 2x10 19 cm -3 50nm Collector n - InGaA nid 600nm Stop Etch n - InP 8x10 17 cm -3 10nm Subcollector n ++ InGaA 1x10 19 cm nm Buffer InP nid 50nm SI. InP Subtrate 1,5 1 Ec (ev) Ev (ev) 0,5 E (ev) 0-0,5 E-cap B C ub-c + E-cont E F -1-1, t (nm) Figure 2.14 The band line-up of complete layer tructure of an InP/InGaA SHBT given in Table 2.3 (no voltage applied) The InGaA emitter cap erve a a contact layer for the emitter contact, to achieve low value ohmic emitter contact. The doping denity of the emitter cap i in the 28

30 range of 1x10 19 cm -3. The thickne i choen a 100 nm to have a lower emitter reitance and an optimum underetching. Since wet chemical etching i ued intenively, thicker emitter will reult in higher underetching leading to higher emitter and bae reitance. On the other hand, ince it may caue hort circuit with bae contact for elf aligned HBT, it i critical to have too thin emitter cap. Here, there i another layer ued to reduce the contact reitance. 50 nm of highly doped (1x10 19 cm -3 ) InP emitter contact layer i andwiched between emitter cap and emitter layer. Wide bandgap InP i choen a emitter layer. The doping denity i around 2x10 17 cm -3. The doping denity i in thi range to keep the bae-emitter capacitance low. But, on the other hand, there i a lower limit for the doping, which hould provide enough electron for the current. The bae layer i one of the mot critical layer in HBT. A mentioned in chapter 2.1.2, thi layer define the current gain of the tranitor. The thickne hould be choen a thin a poible to prevent exceive recombination and lo of electron trying to reach the collector. Moreover, the thinner bae reult in horter bae tranit time (equation 2.6) leading to higher cut off frequency. On the other hand, the heet reitance i inverely proportional to the thickne. To overcome thi problem, the bae hould be highly doped to provide lower heet reitance. Here, in thi work, for SHBT, the bae i 50 nm thick with 3x10 19 cm -3 p-type doping. The collector layer i motly determining the maximum current denity and the breakdown voltage of the HBT. Here, for tandard SHBT, 600 nm thickne i preferred for two major reaon: One i to achieve ufficient breakdown voltage. The other i to have enough underetching for the collector to reduce the bae-collector paraitic capacitance and to improve the RF performance. On the other hand, thi thick collector will reult in a lower cut-off frequency. Another important apect for the collector i the doping influencing the maximum current denity. Thi maximum current denity i mainly pecified by the Kirk Effect [17, 45, 46]. When no current i flowing through the HBT, I C =0, the electric field profile and charge ditribution in the bae-collector junction will be a depicted in Figure 2.15 a. At thi cae, the hole in the bae layer will diffue toward the collector and the electron in the collector will diffue toward the bae layer. Thee carrier will recombine and there will be poitively ionied donor and negatively ionied acceptor on collector and bae ide, repectively. The electric field caued by thee ion will prevent the further diffuion. 29

31 When the collector current increae and reache to critical value ( I > qn v ), where the electron denity entering the bae-collector depletion equal to the donor denity, then the pace charge region in the collector ide i neutralized. The electric field i then caued by a dipole layer a hown in figure 2.15b. Further increae of current will caue bae depletion layer move toward the collector. Thi will lead to an increae in the bae width. C C at E (electric field) t C E (electric field) E (electric field) t C t C Bae collector ub collector Bae collector ub collector Bae collector ub collector t (thickne) t (thickne) t (thickne) Charge Charge Charge pilled-over charge qn C qn C qn C J C /ν at t (thickne) t (thickne) t (thickne) J C /ν at J C /ν at a) b) c) Figure 2.15 The charge ditribution and the electric field profile in baecollector junction of HBT. a) low current cae. b) I C i high enough to make n = N C. c) I C i o high that n > N C. A a reult electric field profile change and charge are pilled-over to the collector region Since there i a larger ditance for carrier to penetrate, thi will degrade the current gain and alo increae the bae tranit time and degrade RF performance. The current denity, where thi effect et in i defined a J Kirk. It i depicted a in Equation V + φ J Kirk = (1 + ) qnc vat equation 2.14 V CB CB 2 + φcb 30

32 Here, φ CB can be given a follow: E g kt N φ C CB = + ln( ) equation 2.15 q n 2 i J C =0 where; φ CB : Bae-collector junction potential V 2 N C : Applied bae-collector bia that totally deplete collector layer when : Collector doping v at : Saturation velocity n i E g k T : intrinic doping denity : Energy band gap of the material : Boltzmann contant (8.62x10-5 ev/k) : Temperature in Kelvin By uing equation 2.14 and 2.15, J Kirk for the tructure in Table 2.3, i calculated a 1 ma/µm 2. In thi calculation, φ CB i found to be 0.45 V. In the calculation of baecollector junction potential, E g,ingaa i accepted a 0.76 ev. V 2 i calculated to be around 21 V and aturation velocity for InGaA i taken a 7x10 6 cm/ [43, 14]. For mall area device there will be alo delay of the Kirk Effect. At high injection level, the carrier flow outward when they enter the collector, becaue of a lateral concentration gradient exiting in the depletion region. If the effect of lateral electric field i neglected, the amount of the preading i approximately: L D = Dτ. equation 2.16 where, L D i the amount of preading, D i the high field diffuion contant, τ i the collector tranit time. The tranit time i determined by the collector thickne (T C ) and the average drift velocity, υ. Thi mean that L D i determined by the layer tructure and i independent from the lateral deign of HBT. For our SHBT, L D i calculated to be 0.4 µm. Thi value i much more important when the emitter width get maller. Epecially for ubmicron HBT, the hift in the maximum current denity can be 31

33 explained in thi manner [47]. Thi effect i elaborated in Chapter 5 with ubmicron emitter. The 10 nm InP top etch layer i ued to top directly on the InGaA ubcollector during etching. The etching rate of InP i much lower than in InGaA with H 3 PO 4 baed etchant. In other word, InGaA i electively etched with thi chemitry. Thi layer enhance the tability and reproducibility of the proce technology. The 300 nm ubcollector erve a contact layer for the collector contact. Like in emitter cap layer, it i highly doped to achieve ohmic contact. Since thi layer i n- type and highly doped with a heet reitance of 8 Ω/q, it i poible to realie low reitance about 2-3 Ohm, which i an important apect for circuit. Since the epitaxial quality may degrade by higher doping level for InGaA, it i not preferable to dope the ubcollector with doping denitie higher than 1x10 19 cm -3 A thin InP buffer layer i ued to have better tarting condition for the HBT layer growth. 2.4 Lateral Deign of HBT A already explained in Chapter 2.1.3, two main paraitic affecting the RF performance are: bae reitance and bae-collector capacitance. Here, we will focu on technique ued to reduce the bae reitance and/or the bae-collector capacitance. L E W B L B W E E B R B, cont W EB R gap R B, pread C C fb C jc W C Figure 2.16 Schematic of HBT howing the dimenion influencing the RF performance. 32

34 A depicted in equation 2.11, there are two major parameter influencing the bae reitance. Thee are bae-emitter contact pacing (W EB ) and the emitter width (W E ). The bae-emitter contact pacing can be minimied by elf-aligned bae contact, which will reult in decreae in R gap. For the reduction of the bae-collector capacitance, one hould reduce the bae-collector junction area. One of the mot common and mature technique i to laterally etch the collector and, a a reult, to reduce the repective area and capacitance [48, 49]. Thi method i effective epecially for DHBT. But, ince in SHBT, the bae and collector material are ame and thi prevent elective etching, it i difficult to reduce the paraitic capacitance without affecting the bae reitance. While the collector i laterally etched, the bae will alo be underetched. If thi underetching i greater than the tranfer length of the contact, thi will lead to higher bae reitance, which will degrade the RF performance. To prevent thi problem, additional olution have already been propoed, like hown in Figure 2.17 [50]. With thi technique, the bae layer i protected by SiN from all ide during the etching of collector layer. So, even in SHBT, C bc i reduced without degrading the bae reitance. Figure 2.17 Schematic of the HBT underetched collector protected by SiN by Lee [50] Another old technique to reduce the C bc i the ion implantation in the external bae contact area [51]. There are alo ome growth related olution to reduce the bae-collector capacitance. Thee are regrown emitter and bae. In regrown emitter [52], active baeemitter junction area i defined with an inulator and the emitter i grown after thi tep. The advantage of thi procedure are: firtly emitter-bae junction area can be reduced 33

35 regardle of the ize of the contact hole. Secondly, emitter contact reitance can alo be reduced by uing larger metallization. [52] Figure 2.18 Schematic of the HBT with regrown emitter propoed by Tanoue Another technique i to regrow the bae [53]. With thi technique, bae layer underneath the bae contact i regrown with highly doped material. Figure 2.19 Schematic of the HBT with regrown bae contact propoed by Shimawaki [53] The main advantage i, bae can be thin without affecting the bae contact reitance and current gain. Thinner bae will lead to lower bae tranit time and higher cut off frequency. There are other imilar regrown method decribed in literature [54]. One of the latet growth technique i uing buried collector metal [55]. In thi technique, prior to the HBT growth, tungten metal wire are defined on the InP ubtrate. After that, layer tructure i grown and thee tungten metal wire tay buried underneath the 34

36 emitter area and eliminating the extrinic bae-collector reitance. Thee technique uffer from complicated epitaxial growth. One popular method i the tranferred ubtrate HBT. Uing thi technique, maximum ocillation frequency, f max, above 800 GHz i already preented [56]. The main idea i to proce the collector contact from the backide and achieving reduced paraitic capacitance. Baically, emitter and bae contact are proceed from the front ide and then the wafer i tranferred to a carrier ubtrate (motly Si). After the tranferring tep, InP material i etched away till collector. The collector contact i realied from the backide of the wafer and thi contact area define the bae-collector capacitance [57, 58, 59, 60, 61]. Thi technique i alo realied during thi work [62]. The detail about the developed proce and related reult can be found in Appendix B. 35

37 3 Fabrication of HBT The fabrication of HBT tart with the epitaxial growth of the deired HBT layer tructure on emi inulating InP ubtrate. In thi chapter, the proceing detail of the HBT proce will be dicued. The critical tep of proceing and the experiment performed to optimie the proceing will be preented. Moreover, different layout and their influence on the HBT performance will be dicued. 3.1 Main Step of HBT Proceing The epitaxial growth and the proceing of the HBT are performed in clean room environment. The tandard HBT i proceed in a triple mea deign. During the proceing of an HBT, 7 different mak layer are ued. 3 etching tep are performed to form the mea tructure and 4 metallization tep for the contact Lithography Thi i one of the mot critical tep for HBT proceing. Baically, it i defined a the tep replicating the pattern on the mak to the wafer. There are two main group of lithography technique. Thee are electron beam lithography (EBL) and optical lithography. During thi work, for prototyping purpoe EBL and for the proceing of HBT optical lithography are preferred. With EBL, it i poible to realie tructure of 100 nm width with a tolerance cloe to 0. But on the other hand, epecially for larger tructure, in comparion to the optical lithography, EBL conume much more time to write the pattern. Becaue of thi, EBL i not preferred a main lithography technique, here. For the evaluation of the new layout and new technique, to ave time and cot, EBL i ued only for prototyping purpoe. For optical lithography, there are 3 main component. Thee are mak, photoreit and mak aligner. The optical mak are quartz mak with chromium (Cr) urface layer on. The tructure to be replicated are tranferred to the mak. The chromium layer (opaque layer) i ued to block UV light (λ 1 = 365 nm, λ 2 = 405 nm), which i ued to expoe the tructure. The quartz i tranparent. Photoreit are photoenitive material. They are liquid depoited uing pin coater, to form thin film. By changing the pinning time and peed of the pin coater, the thickne of the 36

38 film can be adjuted according to the demand. There are two main type of photoreit. Thee are negative and poitive reit. The expoed negative photoreit i inoluble in the developer. In contrary, the poitive photoreit i oluble in developer when it i expoed to UV light. Negative reit offer lower reolution but are much more enitive than the poitive one. In our tandard proce, we prefer uing poitive reit to form mea and contact tructure. We ue AR-P 5350 and AR-P 3740 from Allreit GmbH [63] a tandard photoreit for contact and mea, repectively. Epecially, to provide good contact and adheion of the photoreit, pecial effort have to be given. Before applying any photoreit, an oxide dip hould be done to remove any poible oxide film from the urface. In addition, the ample hould be dried before applying the photoreit. For our procee, a Karl Sü MA6 mak aligner i ued. With thi ytem, it i poible to form tructure with 0.7 µm width, with a tolerance of ± 0.1 µm. Here, the mialignment tolerance i about ± 0.25 µm. To achieve reliable and reproducible tructure, 1 µm i choen a minimum width. To obtain better reolution and real dimenion cloe to nominal dimenion, vacuum contact i utilized, even though it degrade the durability of the mak Etching Baically, etching can be decribed a the removal of material in the depth of the wafer. There are two main technique: Wet chemical and dry etching. In wet chemical etching, motly acid are ued to remove the material from the urface. Baically, acid interact with the material; form new byproduct and thee byproduct are removed from the urface. According to the application, there may be different demand for etching, like higher etch rate, uniformity, an/iotropy, electivity, le damage to the urface. Epecially for HBT application, low etch rate, uniformity, electivity, aniotropy and le damage to the urface are main demand. Uniformity i defined a the etch rate deviation acro the wafer. A high etch rate uniformity i neceary to have reliable device with imilar dc and RF propertie. Selectivity i defined a the ratio of the etch rate for different material. Etch rateof Material1 Selectivit y = S = equation 3.1 Etch rateof Undeired Material 2 37

39 For example, to be able to top etching on really thin bae layer, emitter mea etching hould be elective to bae etching. An/iotropy i the etch rate difference for different crytallographic direction. The degree of iotropy A, i depicted a: where R L i the lateral etch rate and A R L = 1 equation 3.2 Rv R v i the vertical etch rate. When A = 0 then the etching i perfectly iotropic and when A = 1 it i called a perfectly aniotropic. Mak A=0.6 A=0.2 A=1 Material A=0 Figure 3.1 Schematic of underetching of the mak for wet chemical etching. The electivity of material to the mak i aumed to be infinite. (S= ) There are two type of etching procee. One i the diffuion controlled, where the diffuion of the active pecie to the urface and the removal of the oluble product occur. The other i: reaction limited, where chemical reaction at the emiconductor urface take place. Both diffuion and reaction limited etching i temperature dependent. Diffuion limited etching i more enitive in term of agitation, where the reaction-limited etching i crytallographic orientation dependent. For III-V emiconductor, e.g. for InP, ince the reaction will be different for In and P, thi will lead to aniotropy [43]. Diluted phophoric acid (H 3 PO 4 ) baed etchant i ued for InGaA material. Hydrochloric acid (HCl) i ued to etch InP material. Here both etchant are highly elective for InGaA and InP, repectively. 38

40 3.1.3 Formation of Contact Metallic thin film are required to form device contact, paive element, and conductive wire. For the formation of the contact, a Leybold LH560 evaporator i ued in thi work. Thi i a high vacuum ytem ued to depoit different contact material on the ample. An electron beam (e-beam) evaporator exit in the ytem to depoit Nickel (Ni), Germanium (Ge), Platinum (Pt), Titanium (Ti). For the depoition of Gold (Au), thermal evaporator i choen. The main requirement for a ucceful contact material are: high conductivity, high-reolution patterning, reitance to corroion and mechanical tre. For different contact and application different material compoition ha been choen offering better contact reitance and tability. The doping denity of the layer underneath the contact and the diffuion propertie of the contact material to the layer play important role for the determination of the contact reitance. To achieve table contact with proper profile, lithography parameter hould be optimied. A already mentioned in chapter 3.1.1, AR-P 5350 photoreit i preferred for the contact. The profile of the expoed photoreit i the key point. The profile of the reit with the optimied proce i hown in figure 3.2. Figure 3.2 The photoreit profile for the AR-P 1 µm5350 after developing [64]. After achieving thi profile, metal can be evaporated a hown in Figure 3.3. Metal Metal Photoreit Photoreit Wafer Figure 3.3 The photoreit and depoited metal jut before lift off tep. 39