Design of microprocessor controlled RT A system for ...nopr. 40(8) 543-551...Design of microprocessor controlled RT A system for processing of ion implanted semiconductor m~terials

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  • 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 ation-induced 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 water-cooling 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 op-amp 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 e