Large-area shower implanter for thin-film transistors

Y. wu J.H. Montgomery A. Refsum S.J.N. Mitchell B.M. Armstrong H.S. Gamble

Indexing term.: Ion implantation, Polysilicon technology, Active matrix

Abstract: Solid-state diffusion and conventional ion implantation are not suitable for source and drain regions formation of polysilicon thin-film transistors on glass substrates. A 30 cm diameter large-area low-energy ion shower implanter with RIPE ion source and double-grid extraction system was developed as a possible low-cost solu- tion. The ion beam current density for hydrogen plasma was 100 pA/cm2 for 3 keV implant energy, 300 W RF power, 140 gauss magnetic field and 3 x mbar pressure. The uniformity of beam current density over the central 20 cm diameter was & 3.5%. Phosphorus implantation has been performed using a 15% PH, in H, gas mixture. Implantation at 3 keV for 5 min. results in an integrated dose of 2.48 x 10L6cm-2, with the concentration peak at a depth of 8.3 nm. Planar and mesa diodes fabricated on p-type silicon sub- strates have yielded fine rectifier characteristics. The shower implanter is thus suitable for TFTs source and drain region formation.

1 Introduction

For the fabrication of polycrystalline silicon thin-film transistors on low-cost glass substrates it is necessary to keep all processing temperatures below 650°C. This means that solid-state diffusion is not suitable for the doping of source and drain regions and either metal sili- cide or ion implantation is required. The conventional ion implantation with mass separation and focused beam scanning is expensive for large-area substrates. The source and drain doping for TFTs is not very critical and satisfactory transistor action would be obtained for implant doses in the range loL5 to loL6 ion/cm2. Implant energies of 2 to 5 keV should be suflicient to obtain good ohmic contact. Thus a low-energy ion shower implanter, without mass separation and beam scanning, was devel- oped as a possible low-cost solution. The essentials of a shower ion implantation system are a low-pressure

5 IEE, 1994 Paper 98266 (E3). first received 28th April and in revised form 12th August 1993 The authors are with Northern Ireland Semiconductor Research Centre, the Department of Electrical and Electronic Engineering The Queen’s University of Belfast, Ashby Building, Stranmillis Road, Belfast, BT9 5AH. United Kingdom

IEE Proc.-Circuits Deuices Syst., Vol. I l l , N o . I , February I994

plasma, an extraction grid and an accelerating voltage of a few kV. The options for a low-pressure, high-density plasma are either a microwave ECR source or an RF inductively coupled plasma. The object of this work was to produce an implanter suitable for implanting A4/A5 glass substrates. ECR sources are very expensive and so it was decided to use an inductively coupled plasma. To achieve a high-density plasma at low pressure the reson- ant inductively coupled plasma excitation (RIPE) tech- nique was adopted.

2 Shower implanter

The configuration of the large-area ion shower implanter is shown in Fig. 1. The upper section of the shower

g a s inlet RF input-- antenna

Pyrex c h m n b e r z

L + screen grid

,substrate

magnetic coil

-

, - f t ?

lower chamber HV OC power supply to vacuum pump

Configuration ofion shower implanter Fig. 1

implanter consists of a 30 cm diameter RIPE ion source, a double-grid broad-beam extraction system separates the ion source and the lower chamber which contains the sample support. The implanter is pumped by an oil diffu- sion pump and backed by a rotary pump.

2.1 RIPE ion source The effective production of dense and uniform large- volume plasma from an R F resonance has been reported by R.W. Boswell [l] in 1970. The technique is known as resonant inductive plasma excitation (RIPE) and is based on the ‘helicon’ resonance. In the RIPE ion source, the energy absorption by wave damping is due to Landau damping of the helicon wave that has an electric field component parallel to the magnetic field [2]. The primary electrons are produced by trapping and acceler- ation of electrons in a helicon wave, which can quickly

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accelerate electrons to the optimum energy and reaccel- erate to that energy after each ionisation event owing to the phase velocity of the right magnitude. The helicon- wave discharge has been shown to have unusually high ionisation efficiency, and does not require internal elec- trodes or large sheath voltages. It avoids large magnets, complex field coils and microwave system, and is simpler and cheaper than ECR ion source.

For the RIPE ion source, a 30cm diameter Pyrex cylinder with external antenna was used. A 13.56 MHz RF power is supplied to the antenna through an imped- ance matching network and SWR R F power meter. To produce the helicon-mode resonance, an axial static mag- netic field is formed by a solenoid surrounding the chamber. A series of detailed characterisation tests were carried out for optimisation of the RIPE ion source. It was found that a dense plasma was obtained if a novel four-loop antenna was used. The plasma density was determined using a double Langmuir probe. The double Langmuir probe is generally preferred when static mag- netic fields are present and for RF discharges where a reference electrode is usually not available. The plasma density measured as a function of nitrogen pressure for an RF power of 300 W and an axial magnetic field of 140 gauss is presented in Fig. 2. The plasma density

1E . l l j 1

1 I

1E-05 1E-04 1E-03 1E-02 nitrogen pressure, mbar

Relationship between plasma density and nitrogen pressure for Fig. 2 3W W RF power and 140 gauss magneticfield

exceeds 1.5 x 1010cm-3 over the pressure range 8 x 10-4mbar to 7 x lO-'mbar. The density falls at lower pressure, however it remains above about 1 x 10'0cm3 for the pressures as low as of 4 x mbar. The plasma density radial uniformity improves as the magnetic field is increased.

2.2 Extraction system The ions are extracted by a double-grid extraction system. Each grid contains numerous small holes in a hexagonal pattern. The electric potential on either grid can be vaned independently. The plasma potential is con- trolled by the voltage that applied to the screen grid. A plasma sheath forms at the boundary with the holes in the screen grid. Positive ions which reach the plasma sheath are accelerated by the electric field between the sheath potential and the holes in the negatively biased accelerator grid. The accelerator grid is biased below the ground potential. This means that the ions are first accel- erated, then decelerated to reach the final beam velocity. A small amount of deceleration is required to prevent beam electrons from backstreaming through the acceler- ator system. A 10 kV DC power supply was designed and

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built for the supply to the extraction system. During implantation the screen grid is held at + 1 to + 6 kV and the accelerator grid is held around -1/10 of the screen voltage. A water-cooled substrate support which is held at ground potential is provided in the lower chamber. The extracted ion beam current was measured in a Faraday cup with a secondary electron repeller grid at

For the extraction tests a source gas pressure of 3 x mbar was used to ensure good insulation of the high-voltage components within the chamber. The extracted ion beam current density was found to increase almost linearly with magnetic field and RF power. The typical extracted ion beam current density for a nitrogen plasma produced by 300 W RF power, 140 gauss mag- netic field at a pressure of 3 x mbar and 3 keV screen voltage, was around 40 pA/cm2. With increased magnetic field of 200 gauss and RF power of 450 W, the extracted ion beam current density for a hydrogen plasma at the same pressure and screen voltage was 100 pA/cm2.

The uniformity of the ion beam current density was measured by moving the position of the Faraday cup and found to be & 3.5% over the central 20 cm diameter. The uniformity was further confirmed by using an argon plasma of 3 x mbar pressure, 200 gauss magnetic field, 350 W RF power and a screen voltage of 1 kV to sputter-etch an oxidised silicon wafer for 20 min implant- ing time and 90" incident angle. The oxide layer thickness was measured before and after sputter etching. The oxide layer etch depth against position is shown in Fig. 3,

-40 V.

I chomber wall 0)'

0 5 10 15 20 25 3 distance olong chamber diameter,cm

Uni/ormity of ion beam sputter-etch depth for SiO, layer Fig. 3

which confirms that the ion beam current density is uniform over a 20 cm diameter.

3 Phosphorus ion implantation

A 15% PH,-in-H, gas mixture was used for plasma dis- charge. Hydrogenation is needed for passivating the grain boundaries of polysilicon and so it is no disadvan- tage to implant with H + as well as P+. Undoped 200 nm thick polycrystalline silicon layers on oxidised substrates and 42 to 57 R cm p-type single-crystal silicon wafers were used for characterisation of phosphorus ion implan- tation.

SIMS analyses were carried out on implanted layer in p-type single-crystal silicon substrates to determine the implant doses and profiles. Implantation were performed for 5min under 450 W R F power, 1.5 x to

IEE Proc.-Circuits Deoices Syst., Vol. 141, No. I , February I994

3 x mbar pressure and 200 gauss magnetic field, for three implant energies of 2, 3 and 5 keV. The resultant impurity profile of 3 keV is given in Fig. 4, the integrated

'"7

0

depth,nm

SIMS-tested depth profile ofphosphorus atoms in p-type silicon Fig. 4 wafer for 3 keV 5 min implant

dose is 2.48 x t016cm-2 and the depth of the implant peak concentration is 8.3 nm. It can be seen that the con- centration exceeds 1 x 1019cm-3 down to a depth of 77 nm. For 2 and 5 keV energy, the depth of the implant peak concentration is 5.2 and 14 nm, respectively. Fig. 5

41 I 2 1 , , , , , , ' I , I

1 2 3 4 5 6 implanting energy, keV

Fig. 5 Peak position of phosphorus concentration against implant energy W-W calculated

shows the peak position of phosphorus concentration versus implanting energy and compared with the values calculated. The measured values for phosphorus are deeper than those calculated using Linhard-Scharff- Schiott range theory. This agrees with the published results on low-energy ion shower doping [3]. Fig. 6 shows the effect of 1 h anneal temperature on the sheet resistance of a phosphorus-implanted polysilicon layer. The results in the range from room temperature to 600°C may be explained by the annealing of the implanting

IEE Proc.-Circuits Devices Syst., Vol. 141, No. I , Fehruary 1994

damage resulting in a decrease in electron concentration [4]. The sheet resistance shows an abrupt decrease at about 650°C and falls to 400 Q/square after 800°C; this

1E.07

1E.06

lE+O4

1 E *03

0 200 400 600 0 anneal temperature. 'C

Sheet resistance against anneal temperature Fig. 6

suggests that recrystallisation of the implanted damage region is occurring. In general, the activation rate is about 50% at 600°C anneal.

During implantation, sputtering of the substrate will occur, the sputter-etch depth increases with implant duration. The very shallow etch depth is difficult to measure for silicon. However, the sputtering rate was measured for Si02 film under 2 to 5 keV phosphine implanting and same 3 x mbar pressure, 450 W RF power, 200 gauss magnetic field, 5 min implanting time, and 90" incident angle. According to published results, the sputter rate of SiO, layer is slightly higher than silicon for 1 keV and 90" incident implant [SI. Fig. 7

13,

shows the effect of implant energy on sputtering etch depth. Under the same 2 to 5 keV energy and 5 min implant the etch depth is less than implant depth.

Planar and mesa plane diodes were made on 5- 10 em p-type single-crystal silicon substrates by 3 keV phosphorus implantation and 600°C 1 h anneals. Both have fine rectifier properties. The structure of the planar diodes is shown in Fig. 8. Since the implanted layer is very shallow, the metal contact was made to the sur- rounding guard ring which was formed by solid-source phosphorus diffusion at 1ooo"C. The depth of phosphorus-diffused guard ring is 0.31 pm and surface

25

concentration is z 1 x loz1 ~ m - ~ , The area of the guard ring is about 1/3 of the total diode area. The I-V charac- teristics of 3 keV phosphorus-implanted planar diodes

50

2 > ‘;I 1 ,J, ,

4, m 0

0 0 0 2Vldiv *O 1OOmVldiv 700

reverse forward implanted N+layer guard ring

p-twe silicon

planar d iode cross-section junction area. 2 5 O x 2 5 0 p

Fig. 8 a Diode with guard ring and 3 keV phosphorus implant b Diode consisting or guard ring only

Structure ofplanar diode and I-V characteristics

with guard ring, and only guard-ring diode without implant are also shown in Fig. 8. At 500 mV, the forward current of the diode with the central area implanted is about three times higher than the forward current of the diode consisting of only the guard ring. This verifies that the phosphorus-implanted layer gave p n junction char- acteristic in p-type silicon. The fine rectifier characteristic indicates that activation rate of 600°C 1 h annealing is

enough for p n junction and TFTs contact-layer forma- tion.

4 Conclusion

A large-area low-energy ion shower implanter using a 30 cm diameter RIPE ion source has been developed for low-cost fabrication of TFTs. The ion beam current density uniformity is &3.5% over the central 20cm diameter. With magnetic field of 200 gauss and R F power of 450 W, the extracted ion beam current density at 3 x mbar hydrogen pressure and 3 keV screen voltage is 100 pA/cm - ’. For 3 keV and 5 min phosphorus implanting, SIMS-tested integrated dose and projected range are 2.48 x 10l6 cm-2 and 8.3 nm, respectively. The thickness of the silicon layer with a phosphorus concen- tration of > 1 x 1019 cm-3 was 77 nm. The fine rectifier characteristics of implanted diodes for 600°C 1 h anneal- ing show that this shower implanter is suitable for TFTs contact layer formation.

5 References

1 BOSWELL, R.W.: ‘Plasma production using a standard helicon wave’, Phys. Lett., 1970,33A, (7). pp. 457-458

2 CHEN, F.F.: ‘Experiments on helicon plasma sources’, J . Vac. Sci. Technol., 1992, A10, (4), pp. 1389-1401

3 MASUMO, K., KUNIGITA, M.. TAKAFUJI, S., NAKAMURA, N., IWASAKI, A., and YUKI, M . : ‘Low-temperature polysilicon thin film transistors by non-mass-separated ion flux doping technique’, J p n . J. Appl. Phys., 1990,29, (12), pp. L2377FL2379

4 SETO, J.Y.W.: ‘Annealing characteristics of boron- and phosphorus- implanted polycrystalline silicon’, J . Appl. Phys, 1976, 47, (12). pp. 5167-5170

5 BRODIE, I., and MURAY, J.J.: ‘The physics of microfabrication’ (Plenum Press, 1983)

26 IEE Proc.-Circuits Devices Syst., Vol. 141, No. I . February 1994