Indian Journal of Pure & Applied Physics Vo l. 40, August 2002, pp.543-55I

Design of microprocessor controlled RT A system for processing of ion implanted semiconductor m~terials

, \

. I I . I ' I 2 · ·1 M M B e lekar , A M Narsale , K V S ukha tankar , B M ro ra & Y Pl Air

I epartment of Physics, Uni versity of Mumbai, Vidyanagari , Mumbai 400 098)

2Tata Insti tute of Fundamental Research, Homi Bhabha Road, Mumbai 400 005

JDepart ment o f Physics, Had hramout Uni versity o f Science & Technology, Hadhramout, Yemen

Received 26 December 20 I; revised 8 March 2002; accepted 30 May 2002

Rapid thermal annealing (RTA) is one of the important techniques used fo r removal o f radiati on induced defects in ion implantcd semi conductor materials. A complete stand-a lone microprocessor cont ro lled RTA system has been designed and fabricated. It uses a 12 kW halogen lamp bank for rapid radi ati ve heating of the sample and provides good temperatu re ramp-up rate o f > 120 °C/s up to a temperature of 700 °C. A gas line assembly has been provided to carry out the anneal ing in hydrogen, nit rogen, argon and oxygen gas ambient. The system temperature is programmable in the step of I °C each up to the maximum attainable temperature of 1080 0C. The soak ti me can be programmed from a mi nimum of I s up to a maxi mu m of 15 mi n per set temperature. The system has been used for anneali ng of single crystal GaAs substrates impl anted wi th 70 MeV 56Fe ions with a dose of I x 10 14 ions/cm2

, in the temperature range 100-600 0C. T he implanted samples have been investi gated by o ptical transmission measurement s over photon energy range 0 . 1- 1.4 eV, after each anneali ng stage. T he mid-gap defect states are annealed out more rapid ly than the near-band edge defect states duri ng anneali ng up to 350 °C whereas , the near-band edge defect states are annealed out more rap id ly than the mid-gap defect

states du ri ng annealing between 350-600 0C.

1 Introduction

Ion implantati on is an important technique in the fabrication of semiconductor dev ices, particul arl y in GaAs l

. These devices are sensiti ve to radi ation­induced defects that can be removed by subsequent annea ling process . Annealing is, thus, an important process in the fabricat ion of semiconductor dev ices. For example, the annealing process empl oyed in fa bricati on of integrated circuits, should result in minimum diffusion of dopants and at the same time, prov ide excellent removal of radi ati on damage in the semiconductor. As the conventi ona l furnace annealing has limitations in ful filling of these requirements, a lternate annealing processes using lasers, e lectron beams, lamps, res istance heaters and ion beams are being increas ingly used by the dev ice engineers' . Amongst the variou s annea ling processes mentioned above, rapid thermal annea ling (RTA) based on e ither infrared or visible light seems to be most promi singJ

.

The commerc ially availab le RTA systems are very expensive and are ba. ica ll y des igned for commercial app li cations . RTA systems for research work developed in some Iaboratories~ have low

power, poor re liability, complex operating procedure, etc . Hence, it was decided to des ign and fabricate a comparati ve ly low-cost microprocessor controlled stand-alone RT A system having higher power, hi gher temperature ramp-up rates, excellent re liability, user-fri endly operati on and many additional features, suitable for research work of the authors. In thi s paper, the authors present the constructi onal and operati onal detai Is of our RT A system and its application in annealing of 70 MeV 5

6Fe-implanted GaA s supstrate to study the e ffect of annealing on near and mid-IR transmi ssion characteri stics of the implanted substrate. The schematic diagram of the complete RT A system is shown in F ig. I . It consists of a RT A chamber assembly and the e lectronic contro l unit.

2 RTA Chamber Assembly

The RT A chamber is a quartz reactor tube connected to a gas line. It houses a quartz push rod that ho lds a very thin (0.5 mm thick) graph ite sampl e holder. A thin Cr-A I thermocouple is anchored in to the graph ite holder fo r temperature measurement. Twelve halogen lamps, each of 1000 W power, are mounted around the qu artz reactor

544 INDIAN J PURE & APPL PHYS, VOL 40, AUGUST 2002

I ~ If-------,. ~ Cr-A1~

i~~I===~--~I-- I Thennoco~1

~II II I--MAOINS-;-~ .S.·. RTA El -:-:-:-:.:.:.:-:-::: :

, , , , POV'.ER :::: :.: . :-:.: .. ... .

r-~----~~-+------------~ - . - . - - .. . . - . - - - - - .

AT N2 02 H2

D 00

Fig. I - Schcmatic diagram of the complete RTA systcm

tube, for radiative heating of the sample. A pair of high refl ec tivity anodized aluminum reflectors is used to focus the radiant power on to the sample. The lamps and the quartz reactor tube are mounted on a structure made of stainless steel plates. Thick copper pl ates along with copper tubing are incorporated in the structure to provide water­cooling arrangement that avoids excess ive heating of the mechanical assembly. Fig. 2 shows photograph of the RT A chambe r assembly. A gas line assembly is provided to carry out annealing in hydrogen, nitrogen, oxygen and argon gas ambient. The flow rate for each gas can be controlled individually with the help of the control va lves and fl ow meters provided on the front pane l.

3 Electronic Control Unit

The control circuit is designed to contro l a bank o f 12 halogen lamps with total power of 12 kW . All the 12 halogen-lamps are operated from 3-phase ma ins power supply, with four lamps operating on each phase. The control unit preci se ly contro ls the ac power de li ve red to these lamps so as to achieve the des ired temperature . The bl ock diagram of the

comple te control uni t is as sho wn in Fig. 3. It consists of following sections : a microcomputer, power control sect ion, tempe rature monito ring sect ion and graphical display sec ti on.

3.1 Microcomputer

The heart of the co ntrolling unit is a commercia ll y avai labl e singl e hoard mi crocomputer designed around the microprocessor 8085 , hav ing 32 Kb of onboard memory space . The onboard memory space is modifi ed by us by replacing one of the RAM chips by an 8 Kb ROM Ie in order to permanently store the compl e te so ftware (4.7 Kb) developed for the contro l o f the RTA sys tem. The onboard PTe 8253 is used as rea l-time c lock to generate a pulse train o f 5 ms durati on that is used to inte rrupt the microprocessor, to pe rform the tasks like, read ing the temperature and do ing proport ional and integral (PI) correction, ca lcul ations, e tc. The sample temperature and the soak-ti me d uri ng th e annea ling cyc le arc d isp layed on the 7-segment display console th rough the keyboard and d isplay controlle r Ie 8279 . The onboard PPI ~255 is used to inrerface both a 12-b it A DC used for te mperature

BELEKAR ef al.:DESIGN OF MICROPROCESSOR CONTROLLED RTA SYSTEM 545

monitoring and a 12-bit DAC used for graphical di splay of the temperature profile on an osci Iloscope.

. j j . M-- -" ilf

Fig. 2 - Photograph of the RTA chamber assembly

3.2 Power control section

The power control circuit is designed and assembled on a card connected externally to the microprocessor buses through a 50-pin FRC connector. This circuit controls the 3-phase power supplied to the 12 kW halogen lamp bank, so as to achieve the desired temperature profile. It consists of frequency synchronizing unit, zero crossing detector unit, phase angle control unit and isolation and power switches unit.

Frequency synchronizing unit - This unit synchronizes operation of the phase angle control unit with frequency of the ac mains supply. It is basically a frequency multiplier circuit consisting of a Phase lock loop (PLL) chip LM 565 and a 12-bit ripple counter formed by using three 7493 ICs . The mains frequency is thus multiplied by a factor of 21 2 (=4096) and the resulting signal of frequency 204.8 kHz, is used as clock by the phase angle control unit.

Zero crossing detector unit (ZeD) - The unit consists of three identical ZCD circuits, one for each phase. Each ZCD circuit consists of a Schmitt trigger, an inverting amplifier connected to two passive differentiator circuits and a summing amplifier and is implemented by using a quad op­amp IC LM 324. The ZCD provides sharp TTL compatible pulses of 0.1 ms pulse width, at zero

PI P2 P3 N

Z.C.D.-l

Z.C.D.-2

Z.C.D.-3

PLL SYNC CIRCUIT

Fig. :> - Rinck diagram of the electroni c contro l unit for thc RTA systcm

546 INDIAN J PURE & APPL PHYS, VOL 40, AUGUST 2002

crossing points of the mains ac voltage waveform. These pulses are used by the phase angle control unit to trigger its delay counte rs.

Phase angle control unit - This unit generates delayed firing pulses for the power triacs, within half cycles of the ac mains voltage. The software controlled PTC 8253 programmed to operate in mode 5, is used for this purpose. It has three independent 16 bit down counters, each of which is used for phase angle control in each phase. The clock inputs of all the three counte rs are driven by output signal of frequency 204.8 kHz, from the frequency synchronizing unit. The gate input of each of the three counters is edge-trigge red with 0.1 ms duration pul ses from the ZCD unit for corresponding phase of main voltage. This results into the generation of de layed active low pul ses of 4.8 fl sec pul se width at the output of the counters. Thi s delay time is dete rmined by the 16-bit count , programmed by the microprocessor, into the respec ti ve counters. These de layed output pul ses from each o f the counte rs are fed to a 555 timer based mono-stabl e multi -vibrator c ircu it , which generates acti ve hi gh pul ses of 0.4 ms durati on, which are suitable fo r driving the opto- iso lator ICs.

Isolation alld power triac unit - It consists of three identical c ircuits, one for each phase. The pul ses from mono-stable multi-vibrator are fed to opto- iso lators MOC 3020 (11 -13), the outpu t o f which drives the gate c ircuit of the tri acs BTA 41/600B (T I-T 3) used as power switch. T hi s ensures good e lectrical iso lation between the low voltage microcomputer sect ion and hi gh vo ltage power sec tio n and pro vides the necessary no ise immunity and safety. The undesirable e ffec ts like fa lse tri ggering of triacs etc. due to the e lectromagnetic interference (EMI) are e liminated by uSIng snubber c ircui ts consisti ng of an induc tance co il for each tri ac.

3.3 Temperature monitoring section

T his compri ses o f a thermocouple coupled to an instrumentat ion ampli fier and a 12-bit ADC card .

Therll1ocollple alld illstrumentatioJl oll1pliflcr­A thin Cr-A I thermocouple (K-Type) has been used as temperature sensor due to its good linear response

up to a temperature of 1200°C. The. thin the"rmocou ple wires ensure fast response, whI ch IS

necessary for the re li abl e sens ing of the te mperature

in the RT A system. A high precIsIon instrumentation amplifier with appropriate gain , constructed by using IC LM 324, is used to amplify the thermocouple output. The output of the instrumentation amplifier is in turn fed to a 12-bit ADC.

12-bil ADC - A high speed 12-bi r success ive approximation type ADC (AD 574), with conversion time of 25 fl sec, is used to convert the analog output voltage of the instrumentation amplifier to 12-bit digital equivale nt signal. It is inte rfaced with the mi croprocessor through PPJ 8255 .

3.4 Graphical display section

It consists of a 12-bit DAC 1222 and an osc illoscope. The DAC is inte rfaced with microprocessor through ports A and C of the PPJ 8255. The output o f DAC unit is fed to Y -input of the CRO. The CRO is operated in Y-t mode for graphical di spl ay of the temperature profi le.

4 Software Details

A user-fr iend ly, menu-driven software is developed in assembl y language for an easy operati on o f the contro l system. It prov ides four opening menus viz LAMP TEST ?, TEMP DlSP ?, T EMP GRAPH? and RT A ? which are d isp layed sequenti a ll y on the 7-segment d isplay conso le .

LAMP TEST? - Th is provide ~ the user a simple met hod for the testing of hal ogen lamps. W hen thi s option is selected , all the lamps are swi tc hed ON with minimum power, so as to make their filaments just red-hot. This facilit y a ll ows the use r to ascertain that all the 12 lamps are working sat isfactori ly pr ior to the commencement of the RT A annea l ing cyc le.

TEM P DISP ? - The temperature o f the furnace is displayed continuous ly on 7-segment d isplay conso le.

TEM!' GRAPH ?-Thi s facility allows the user to display the temperature profi Ie of the RT A cyc le on the CRO screen with temperature plotted along Y-ax is. Duri ng RT A cycle the te mperature read ings are recorded digitally in the onboard memory at an interval of 100 ms . This stored data is fed sequentially through the 12-bit DAC to the Y-input of a CRO to display the temperature profile on it s screen. The calibration of X and Y axes of" the eRO

BELEKAR et al.: DESIGN OF MICROPROCESSOR CONTROLLED RTA SYSTEM 547

screen has been set by the software as 15 s/div for

time base setting of I ms/div and 250 DC/div for Y­input attenuator setting of I V /div , respective ly.

INITIALJSATlON OF VARIABLES

RECEIVE DESIRED PROCESS PARAMETERS FROM KEYBOARD

y

APPLY FULL 12 kW POW'ER

DISPLAY: 'SOAK TIME' AND PRESENT FURNACE TEMP.'

N

Fi g 4 - Flowchart or RTA main program

RT A ? - This opti on enables the user to proceed for the RT A cycle of the sample. The program for this opt ion compri ses of two program modu les vi z. RT A main program and RST 7.5 inte rrupt se rvice routine. T he flowchart for the RT A main program module is shown in Fig. 4. In thI s program module, the system sequentIal ly dI spl ays

appropriate messages and prompts the user to ente r the corresponding parameters like numbe r of set points, the number of set temperatures and corresponding soak-time inte rva ls . The present software can accept up to 10 sets of set temperature

N

CALCULA lE AND APPLY niE 'PI CORREcnON' TO

niE OUTPUT POWER

RET

Fig. 5 - Flowchart of the RST 7.5 interrupt routine

and soak time. The set te mperature in princ iple is programmable up to 2000 DC at a step o f I DC. However, at present our RT A syste m can reach a max imum te mperature of 1080 DC. The soak-ti me for each set po int can be set to a maximum of 15 min at a step o f I s. The syste m the n app lies full 12 kW power to the ha logen lamps, so as to achieve max imum te mpe rature ramp up rate and continuous ly di spl ays the present furnace te mperature and the soak time. At the end of the RT A cycle, al l the lamps are sw itc hed OFF and a message END is di splayed at the outpUI consol e. The system software a lso provides the fac ility of a panic key. In case of an e mergency s itu ati on, the press ing of any key on the keyboard of the mic r0processor k it aborts the entire RTA cyc le

548 INDIAN J PURE & APPL PHYS, VOL 40, AUGUST 2002

12)Q

1CDO

C> 8:X)

<>

UJ 0::

= 6)()

~

~ UJ CL

4X>

~ UJ ~

aJO

0

o 2 4 6 8 10 12 14 TIME (sec:)

Fig. 6 - Temperature-time profile of the RTA system

operation premature ly by switching off the lamp power.

The microprocessor is interrupted from the RT A main program after every 5 ms to execute the RST 7.5 interrupt service routine. The corresponding flowchart is shown in the Fig. 5. ]n this routi ne, the microprocessor mainly reads the furnace temperature, obtains average of 16 temperature readings at the end of 80 ms and stores it in the battery backed RAM for future use. Proporti onal and integra l (PI) al gorithm is used to contro l the lamps power for the necessary te mperature control. These P] calculations are done in the next 20 ms period and, thu s, the PI correcti on to the output power is appl ied once every 100 ms. Thi s rate is fa st enough to contro l the fast ri sing temperature to a des ired le ve l. The proportional correction to the output powe r provides fast control of the furnace temperature but leaves a finite res idual error in the te mperature setting. The integral correction continuously adjusts itse lf till this error IS

e liminated , setting the actual te mperature of the furnace to be ex actl y equal to the des ired set

temperatu re va lue.

S System Calibration and Performance

The system has been th oroughl y tes ted by operat ing it repea tedly at va ri ous annealing tempera tures and soak times, in nitrogen, argon and hydrogen gas ambien t. The RT A system was found to ope rate with exce llent re liab ility and

repeatability. The temperature contro l was found to be within an accuracy of ± I dc. The temperature calibration of the present RTA syste m was carried out. using the me lting point temperatures of hi gh punty (99.999 %) Au and Al metals, as follows. First, a small piece (- a few mg) of Au metal foil was placed on the graphite susceptor. The temperature of the RT A system was set to the me lting point of gold (= I 064 0C) and the soak-time was set for lO s. After the RT A cycle was complete, it was found that , the gold fo il had me lted and turned into a gold ball. Similar procedure was re peated fo r Al metal by setti ng the RTA temperature to the me lting point of aluminium (=660 DC) . Fig. 6 shows the measured te mperature­time profile of the system for nitrogen gas ambient. It shows that , the dynamic temperature ramp-up rate of the system is 125 DC/s up to 700DC, 83 DC/s in the temperature range 700-900 DC and 40DC/s at the temperatures 900-1000 DC.

Fig. 7 shows a photograph of the te mperature profile of a typical RTA cyc le with three set temperatures , as di splayed on the CRO screen. During thi s RTA cyc le, the furnace te mperature rises rap id ly to the first set te mperature of 400 DC. where it is he ld constant for 40 s. It then rises further to 600 DC and remains stab le for a soak-ti me of 20 s. The temperature finall y ri ses to 900 DC and it is maintained constant at that va lue for 5 s. At the end of the RTA cyc le, the sampl e cools natura lly and the sample te mperature fa ll s to room

BELEKAR et ai.: DESIGN OF MICROPROCESSOR CONTROLLED RTA SYSTEM 549

temperature. During soak-time, the te mperature

remains stable with in ± I DC o f the set temperature value . It should be noted here that , in order to obta in

Fig. 7 - Photograph of temperature profile of an anneal ing cycle displayed on eRO screen

a full sc reen di splay of the tempe ratu re profil e, X and Y cali brati on settin gs are di ffe rent than those menti oned ea rlier in prev ious secti on. T hi s RTA system was then used to study the e ffect of

An neal j ng Temp_ ( ° C) 5

As imp!. 100 2 00

4 250 300 350 400

3 450 500 >< 550

iD 600 V i rg i n

2

1

0

annea ling on near and mid IR transmt ss ton charac te ri stics of GaAs substrates imp lanted with 70 M eV S('Fe ions, as described be low.

6 Experimental Details

Sing le c rysta l Cr doped semi -in sulating (SI) GaAs substrates of < I 00> ori entati o n, hav ing 400

/lm thickness and po lished on both sides , were care full y c leaned in o rga nic so lvents (TCE, acetone, methano l)4. It was then implanted at room temperature w ith S('Fe ions at an energy o f 70 MeV,

to a dose of Ix l0 14 ions/cm\ using 16 U D Pe ll etron accelerator at Nuc lear Sc ience Centre, New De lhi . During impl antati on the ' ('Fe beam was de-foc used in order to cover the entire sampl e surface . The

sample was ori ented at an ang le of 7D wi th respect to the beam ax ts, so as to minim ize the c hanneling effec t.

The impl anted sampl e was the n isochronally annea led in the present RT A system at d iffe rcnt temperatures from 100-600 DC in hi gh puri ty nitrogen ambient for 10 min. O ptica l transm ission measurements of the samp le were carri ed out at room te mperature, in near and mid-IR range afte r eac h annea ling stage . T he near-IR transmission measurements in the wave length range 800-2000 nm were done us ing CA RR Y 17D spectroph otomete r whil c the mid-IR meas ure ments

. . . ..

-: ............... ~ ~ --~ •• ... + . ~

, •• +++

~

-- ,

0.0 02 0 .4 0.6 0 .8 1.0 1.2 1.4 1.6 P HOT ON ENERGY (eV)

Fig. ~ - Dependence of Ox versu s photon energy plots on annealing temperature for the sample implanted

to the dose I x 10 1•1 ions/cm2

550 INDIAN J PURE & APPL PHYS , VOL 40, AUGUST 2002

in the range 2000-13000 nm were carried out by using FTIR spectrophotometer (lasco 610) .

7 Results and Discussion

The transmiss ion spectra of the vlrglll and implanted samples annealed at different temperatures were measured at room temperature in the photon energy range of o. I to 1.4 e V . Optical density (ax) values of the samples were calculated from measured spectra by using the express ion5:

II I" = [( I -R)2 exp(-ax)]/[I-R2 exp(-2ax)] ... (1)

where, R is the refl ectivity, a. the absorption coefficient of the materia l, x the thickness of the absorbing layer and II I" is the transmittance . The va lue" of the re fl ectivity R is taken to be 0 .3. Fig. 8 shows ax versus photon energy curve for the sample

implanted to the dose of I x 10 14 ions/cm2, annealed at

di fferent temperatures in the present RT A system.

It is seen that , the optical density (ax) gradually decreases over the entire photon energy range 0 . 1-1.4 eV with increase in annealing temperature, indicating an overall reduction in concentration of the defect states caused by the ion implantation. The

ax values for as implanted sample and the sample annealed to 100 °C, remain almost same over the entire photon energy range . Thi s indicates that, no s ignificant annealing of defects occurs at 100 0c. At

the annealing temperature of 350 °C, the a.x value decreases from 3 .1 to 0 .6 at 0 .7 eV whereas, it decreases from 5 .3 to 4.6 at 1.35 e V as compared to the as- implanted sample. Thi s shows that, annealing of the sample at 350 °C results into a rapid recovery of deep-lying defect states, as compared to the near­band edge defect states. Further annealing of the sa mple to 600 °C results into decrease in the ax va lue by 0 .6 at 0 .7 eV and by 4.4 at 1.35 eV as compared to the sample annealed at 350 °C. This indi ca tes a rapid decrease in the concentrat ion of near band edge defect states as compared to the concentration of deep-lying defec t states in this annealing temperature range. At annealing temperature of 600 °C, the ax values of the impl anted and the vi rg in samples are very c lose to each othe r over the entire photon energy range. Thi s shows that, the defect states generated due to the implantation of ·\(,Fe ions in GaAs substrates have been almost removed after the anneali ng treatment at 600 °C. Similar results have been reported for 2XSi impl antati on at 100 MeV in GaAs substrates(,.

8 Conclusion

A low cost, microprocessor controlled stand­alone RTA system useful for research work, has been successfully designed, developed and fabricated in our department. The system provides a maximum temperature ramp-up rate of 125 °C/s up to 700 °C . The maximum temperature achievable is 1080 °C but with a lower ramp-up rate. The user­friendly, menu-driven software allows an easy usage of thi s RT A system. The system accepts as many as 10 set temperatures and corresponding soak-times. The temperature stability is within ± I °C of the set temperature value. Soak-times can be set-up to maximum of 15 min in the step of I s. Once programmed, the system ope rates automatically to achieve the desired temperature profile . The fac ility of panic key allows the user to shut down the system in stantly in case of an emergency . The lamp testing facility provides s imple method for the user to detect any faulty lamp. The graphical display on CRO gives detail s of the sample tempe rature profile in the RT A furnace.

The RTA system was used for annealing s ingle crystal GaAs substrates implanted with 5"Fe ions at an energy of 70 MeV to a dose of I x I Ol ~ ions/cm2 in the temperature range of 100-600 0C. Optical densities have been calculated by measuring the IR transmi ss ion spectra in photon energy range 0 . 1-1.4 eV, after each annealing stage. Radi ation-induced defect concentration is found to decrease graduall y over entire photon energy range with increase in annealing temperature. The re is no s ignifi cant annea l ing of defects up to 100 °C. Mid-gap defect states are annealed out more rapidly than the near band edge defect states during annealing up to 350 °C whereas the near band edge defect states are annealed out more rapidly than the mid-gap defect states in the annealing temperature range of 350-600 °C. Thus. it is seen that annealing of the sample at 600 °C results into almost complete reducti on of the radiation induced damage.

Acknowledgement

The authors grate full y acknowledge the financial support for this work fro m Nuclear Science Centre, New Delhi , unde r its UFUP program (Proj ect no. UFUP 230 I). They also thank Dr S K Dubey, Dr A 0 Yadav, Shri KG Bhole and Ms Bhagyashri Rao of Physics Department, Mumbai University and Dr 0 Kanjil al and scientific

BELEKAR et al.:DESIGN OF MICROPROCESSOR CONTROLLED RTA SYSTEM 55 1

staff of Nuclear Science Centre, New Delhi , for the implantation work. Two of the authors, (MMB and KVS) are thankful to University Grants Commission, Government of India, New Delhi , for the award of teacher fellowships under its Faculty Development Program.

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