Status of fully integrated GaAs particle detectors

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Nuclear Physics

B (Proc. Suppl.) 78 (1999) 511-515

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Status of fully Integrated GaAs Particle Detectors W. Braunschweig, J. Breibach, Th. Kubicki, K. Liibelsmeyer, Th. M ~ i n g , C. Rente, Ch. RSper, A. Siemes ~ ~I.Physikalisches Institut, R W T H Aachen, Sommerfeldstr.28, 52056 Aachen GaAs strip detectors are of interest because of their radiation hardness at room temperature and the high absorption coefficient of GaAs for x-rays. The detectors currently under development will be used in the VLQexperiment at the H1 experiment at the HERA collider. This will be the first high energy physics experiment where GaAs detectors will be used. The detectors have a sensitive area of 5 × 4 cm with a pitch of 62 p m. Due to the high density of channels the biasing resistors and coupling capacitors are integrated. For the resistors a resistive layer made of Cermet is used. The properties of the first fully integrated strip detector are presented.

1.

Motivation

Detectors m a d e of GaAs have been shown to be radiation hard against irradiation with highly energetic hadrons [1]. They could be operated at room t e m p e r a t u r e without any cooling even after a total fluence of 1 × 1015n/cm 2. This and their radiation hardness against irradiation with 7-photons up to a dose of 100Mrad make them an interesting choice for use in high energy physics experiments where no cooling is possible. The first experiment where a strip detector made of GaAs will be used, is the VLQ experiment at the H1 detector at the H E R A collider [2]. Each strip detector will cover a sensitive area of 5 x 4 cm with a pitch of 62~um. Due to the high density of channels the biasing resistors and coupling capacitors have to be integrated onto the detector chip. It has been already shown t h a t this could be done using capacitors with silicon nitride as dielectric and punch through resistors [3]. The latter have been shown to exhibit a high amount of noise thus worsening the signal to noise ratio. Therefore the actual device is equipped with resistors made of "Cermet", which is a resistive layer containing a mixture of a m e t a l and an insulator. Another i m p o r t a n t application for such a device is its use for the detection of x-rays. Compared to silicon G a A s has a much higher absorption coefficient for x-rays because of its high atomic n u m b e r (Za~ = 31, ZAs = 33). Therefore a strip detector m a d e of GaAs should combine a 0920-5632/99/$ - see front matter © 1999 Elsevier Science B.V.

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high spatial resolution with a high efficiency for the detection of x-rays. 2. S e t u p

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Figure 1. Schematic cross section of the strip detector with integrated biasing resistors and coupling capacitors

Fig. 1 shows the schematic cross section of the detector. It consists basically of semiinsulating GaAs which was grown by a LEC process [4]. The detector itself is formed by Schottky contacts on the top side. A system of 3 metal layers ( T i / P t / A u ) with a total thickness of 400 n m is used for that purpose. This metallisation is strucAll rights reserved.

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W. Braut~chweig et aL /Nuclear Physics B (Proc. Suppl.) 78 (1999) 511-515

tured by means of a standard photolithographic process in order to obtain strips with a width of 31# m and a length of up to 5 cm. The strips are covered by a layer of silicon nitride which serves as dielectric for the coupling capacitors and as passivation for the Schottky contacts. The coupling capacitors are then formed by the Schottky contacts and the second metallization on top of the dielectric. The biasing resistors which connect the guard ring with the Schottky contacts are made of strips of a Cermet layer with a thickness of 100nm. Each strip has a width of 10#m and a length of 2000pm. With the Cermet layer having a sheet resistance of 2kgt, this gives a resistance of 400kQ. The backside contact is an ohmic contact with a standard type metallisation of N i / A u G e / N i / A u . In order to improve the breakdown behaviour of the detector silicon ions are implanted into the GaAs layer beneath the metallisation. Experiments have shown that the contact does not necessarily have to be structured if the wafer is cut and not sawn. The main processing steps are carried out in the following order : • Schottky contact metallisation • Deposition of the silicon nitride layer by means of a PECVD process • Deposition of the second metallisation by evaporation • Etching of the silicon nitride by reactive ion etching (RIE) • Deposition of the Cermet layer • Etching of the Cermet by ion beam etching (IBE) • Ion implantation of the backside • Deposition of the backside metallization Regarding the order of the processing steps, two conditions had to be observed. The amount of pin holes within the silicon nitride has to be minimized; therefore the second metallisation must be deposited immediately after the deposition of

the silicon nitride. Furthermore, the backside contact should not be exposed to temperatures higher than 150C ° and for that reason it must be deposited by evaporation at the end of the process. 3. B i a s i n g R e s i s t o r s

The main requirements for the biasing resistors are listed below. • R > 1M~

• noise : i-Y= • yield>95% • radiation hard There exist four technologies which could be used to this purpose. Strip detectors based on silicon are often equipped with polysilicon resistors. This is a well known technique, but it has the disadvantage that it requires process temperatures in excess of 300°C. " F O X F E T " structures are also quite often used in silicon detectors. Because of the Fermi level pinning on the GaAs surface this solution is not possible for GaAs detectors. The punch through effect can also be used for the formation of a resistor, as has been already done in GaAs strip detectors [3]. Unfortunately the noise contribution of that type of resistor cannot be neglected and causes a degradation of the signal to noise ratio. The fourth technology is to use a resistive layer containing a mixture of a metal and an insulator (Cermet = CERamic and METal). This material is often used for the production of thin film resistors. It has been shown that it can be used in semiconductor technology as well [5]. The main difference between applications in thin film and semiconductor technology results from the substrate themselves. The substrates used in thin film technology are typically ceramics like A1203 with surface roughness of the order of l#m. In contrast to that, the surface of the semiconductor detectors is mirror polished. Since the sheet thickness of the cermet layer is of the order of 100 nm, a surface roughness of the order of lttm will probably have an influence on the layer properties.

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In order to examine the properties of the Cermet layer, we investigated samples from two different manufacturers in terms of ohmic behaviour, yield, noise, step coverage and radiation hardness. The layers were produced by means of a sputter process onto a silicon nitride substrate. In order to check whether a conducting strip covering a step with a height of 500 nm could be achieved, mesas were etched into the substrate with a plasma process. All the test samples showed an ohmic behaviour and sufficient step coverage. Within the frequency range from 10 Hz up to 1 MHz the noise current density was nearly independent of the frequency and agreed well with the theoretical expectation of 4 k T / R . The radiation hardness was tested by irradiation with neutrons at the ISIS source at the Rutherford Appleton Laboratory(RAL) [8]. The energy of the neutrons was 1MeV and the total fluence was 4.27 × 1014n/cm 2. No significant change of the parameters was observed. Fig. 2 shows the current-voltage characteristic of the Cermet strips. The behaviour is clearly ohmic with a sufficiently large resistance

of 270kQ. Investigations of the noise have shown that the noise current density agrees well with the theoretical expectation of 4 k T / R . Fig. 3 shows the noise current density of the resistor as a function of the frequency within a range of 10 Hz up to 1 MHz. The noise contribution of the resistors at a shaping time of 40ns is 300e-, which is neglible when compared to the noise contribution of the preamplifier. 4. C o u p l i n g C a p a c i t o r s

The most important requirements for the capacitors are listed below. • C > 20pf/cm • I < lnA

• Vbreakdown > 40V (single sided read out) • yield>95% • radiation hard Detailed investigations [6] have shown that silicon nitride manufactured with plasma enhanced

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Figure 5. Current voltage characteristic of a coupling capacitor

5. D e t e c t o r r e s u l t s chemical vapour deposition (PECVD) in a downstream process is able to fulfill all these requirements [7]. The radiation hardness was investigated by irradiating samples with 1 MeV neutrons up to a total fluence of 2.6 x 1014n/cm ~ at ISIS(RAL). Fig. 4 show the distribution of the change of the capacitance before and after irradiation. No significant change could be observed. A typical current voltage characteristic of a capacitor of the strip detector is shown in Fig. 5. The breakdown voltage, which is defined as the voltage at which a current of 1 nA is exceeded, is higher than 180 V and is sufficient for single sided read out. Taking the dielectric thickness of 500 nm into account, we obtain a breakdown field of 3.6 × 106V/cm. The distribution capacitance is shown in Fig. 4. The mean value of 92 pF agrees well with the theoretical expectation of 109 pf. The standard deviation of 2% shows that the homogeneity of the layer thickness is better than 10 rim.

Fig. 6 shows the dark current of the whole detector as a function of the applied voltage. The shape of the curve and the saturation value agrees well with the results obtained with pad detectors [6]. It is found that more than 90% of the Cermet resistors have contact to the strips and that the Schottky contacts were not damaged during processing. Thus the detector should work well as a particle detector. In Fig. 7 a spectrum for minimum ionising particles recorded with the strip detector is shown. The most probable value of the signal is 12500 electrons. This corresponds to a charge collection efficiency of 34.4%, which agrees well with the values obtained with pads [6]. 6. C o n c l u s i o n We have developed a process for the manufacturing of fully integrated GaAs micro strip detectors. The first prototypes with a size of 1 x 3cm 2 and a pitch of 50pm worked well giving a MIP sig-

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hal of 12500 electrons. The resistors were made of Cermet with a yield of more than 90% and displayed ohmic behaviour with a resistance of 270k~. The coupling capacitors were made of silicon nitride with a thickness of 500 nm and showed a breakdown voltage of more than 180 V. The yield was about 85%.

trometer Covering Very Small Momentum Transfers, (1996) D. Albertz et al., Advanced Technology Steps in the Fabriaction of GaAs Micro Strip Detectors, Conference Proceedings, Como (1996) Freiberger Compound Materials GmbH, Freiberg/Sachsen, Germany R. Irsiegler, PhD Thesis, University of Freiburg (1997) Ch. Roeper, Untersuchungen an integrierten Kapazitten und Ladewiderstnden fr GaAs Streifendetektoren, Diploma Thesis, Aachen, (1998) Ferdinand Braun Institut fiir HSchstfrequenztechnik, Berlin, Germany M. Edwards et al., The radiation facility at RAL, Proc. Large Hadron Collider Workshop, Aachen, Germany, CERN Report 90-10 (1990) Vol.III p.584

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7. A c k n o w l e d g e m e n t

6. We would like to thank Prof. Liith form the research centre Jfilich for giving us the opportunity to perform some of the processing steps at the institute for thin film and ion technology. Furthermore we would like to thank Prof. Veyhl and T. Seydlitz from the HEI-Tech Gmbh for their support concerning the Cermet deposition. REFERENCES 1. W . J . Xiao, Investigation of Radiation Hardness of SI GaAs Detectors for their Application in the Tracking System at the LHC Experiments, PhD. Thesis, Aachen, (1998) 2. Technical Proposal to build a Special Spec-

7. 8.