T M A Bui

H Le Trong

L Presmanes

A Barnabé

C Bonningue

P Tailhades
email: tailhade@chimie.ups-tlse.fr

Thin films of Co 1.7 Fe 1.3 O 4 prepared by radio frequency sputteringthe first step towards their spinodal decomposition

Pure thin films of Co 1.7 Fe 1.3 O 4 spinel iron cobaltites were prepared for the first time by radio frequency sputtering. Such films are made of small crystallites of about 20 to 30 nm in diameter. Because Co 1.7 Fe 1.3 O 4 films have a composition located in the miscibility gap of Fe 3 O 4 -Co 3 O 4 , they can be submitted to spinodal transformation below about 900 °C. This transformation was also confirmed at 600 °C by X-ray diffraction and transmission electron microscopy studies. It was demonstrated however that this spinodal transformation occurs after only a few hours at low temperature. Indeed, after annealing in air at 300 to 450 °C for a few hours, the spinodal transformation leading to two-phase spinels, one rich in iron and the other rich in cobalt, was clearly revealed by Raman spectroscopy and electrical measurements.

Introduction

Inside a miscibility gap of a phase diagram, a homogeneous solid solution is not stable. It is progressively broken down into two different phases either by nucleation and growth or spinodal decomposition. 1 The first process can occur in the whole composition range of the miscibility gap and generally leads to random microstructures. The second one, which only operates in a limited area of the gap, can induce periodic microstructures at a submicronic scale. Such microstructures are particularly interesting when they contain at least a magnetic ordered phase. Giant magnetoresistance was thus observed in ferromagnetic alloys obtained from spinodal decomposition. [2][3][4] Other collective properties could be obtained and are used to make, for instance, magnonic crystals, the magnetic counterpart of photonic crystals. 5,6 The study of spinodal decomposition of magnetically ordered phases could then be a key step in the quest for materials with original properties, which are likely to find new technological applications.

In the CoFe 2 O 4 -Co 3 O 4 phase diagram, [7][8][9][10][11] there is a miscibility gap in which spinodal decomposition can lead to regular alternation of ordered magnetic phases made of iron-rich and cobalt-rich spinel oxides. However, this was mainly observed in powders. 9,13 Two studies were about the preparation of thin films of iron cobaltites in this range of composition and the related spinodal decomposition. 11,12 Such films were obtained by the sol-gel process after annealing carried out above 800 °C. In the context of potential future applications, it is however necessary to be able to prepare thin films and to induce in them spinodal decomposition at low temperatures. This work was then devoted to the preparation of thin films of iron cobaltites by radio frequency sputtering and to the study of the thermal behaviour of such films at moderate temperatures (<500 °C).

CoFe 2 O 4 -Co 3 O 4 phase diagrams have already been established experimentally [7][8][9][10][11] or from thermodynamic calculations. 14 These phase diagrams are very similar even if there are some discrepancies in the position and the area of the miscibility gap in which two spinel phases co-exist at the thermodynamic equilibrium. The global composition Co 1.7 Fe 1.3 O 4 is however close to the centreline of this region and to that of the more limited area, in which the spinodal decomposition appears. The composition Co 1.7 Fe 1.3 O 4 was then chosen for this work.

Experimental

Film preparation

Pure Co 1.7 Fe 1.3 O 4 was prepared by heating at 600 °C for 2 hours a mixed oxalic precursor precipitated in a hydro-alcoholic medium. After adding polyvinylic alcohol, the resulting powder was pressed inside a matrix 100 mm in diameter. The uniaxial pressure applied was 75 MPa. The green disc obtained was then slowly heated to eliminate the organic binder and then sintered at 900 °C for 12 hours. The relative density of the ceramic target thus prepared was close to 60%.

Thin films were prepared by radio frequency (RF) sputtering of the previous ceramic target. The sputtering gas was pure argon and the apparatus was an Alcatel A450 equipped with a radio frequency-generator (13.56 MHz) device as well as a pumping system (a mechanical pump coupled with a turbo molecular pump) which reaches residual pressures down to 10 -5 Pa, a gas flow controller, a water cooled target holder and two water cooled sample holders. The films were deposited on silicon or glass substrates. Their thickness was 300 nm.

A residual vacuum close to 5 × 10 -5 Pa was reached in the sputtering chamber before introducing the argon deposition gas. The target was sputtered for 20 hours at the beginning. The target was also sputtered for 10 minutes before each film deposition on the substrate. The sputtering conditions (RF power and argon pressure) have been optimized to obtain pure Co 1.7 Fe 1.3 O 4 spinel films.

Characterization

Structural characterization of films was performed at room temperature by grazing angle X-ray diffraction (α = 1°) using a Siemens D5000 diffractometer equipped with a Bruker Sol-X detector. In situ high-temperature XRD data were collected in the Bragg-Brentano θ-θ geometry using a Bruker D8 Advanced diffractometer equipped with an Anton Paar HTK 1200N heating chamber and a Bruker LynxEye position sensitive detector. XRD patterns were recorded under air every 50 °C from 50 to 900 °C with a data collection time of 20 min per scan and a 30 °C min -1 heating rate. The films used for these experiments were 300 nm thick and deposited on a pure silicon substrate because this material can withstand up to 900 °C despite low oxidation of the surface not covered by the cobaltite. The copper Kα X-ray radiation (K α1 = 0.15405 nm and K α2 = 0.15443 nm) was used for both diffractometers.

Imaging and elemental analyses were performed using a JEOL 2100F field emission gun transmission electron microscope (FE-TEM) operating at 200 kV and equipped with an X-ray energy dispersive spectroscopy (X-EDS) system. X-EDS chemical composition mapping was also performed in scanning TEM (STEM) mode. For the cross-section TEM and STEM observations, the thin films were deposited on a silicon substrate, cut into thin slices normal to the interfaces which were glued together between spacers and finally ion milled to perforation.

Raman spectra were collected under ambient conditions using a LabRAM HR 800 Jobin Yvon spectrometer with a 633 cm -1 Ar + ion laser. Spectra acquisition was carried out for 400 seconds using a ×100 objective lens and 600 g mm -1 grating. During the measurement, the resulting laser power at the surface of the sample was adjusted to 0.7 mW. Examination of multiple spots showed that the samples were homogeneous. The substrate chosen was a quartz microscope slide able to be heated up to 600 °C without damage. Unlike silicon, this substrate has no Raman signal in the spectral range analysed.

The magnetic properties were measured in the plane of the films using a vibrating sample magnetometer Versalab Free from Quantum Design. The maximal applied field for the measurements was 30 kOe. The magnetization of the samples was corrected for substrate contribution.

Film thicknesses were measured using a Dektak 3030ST mechanical profilometer. Atomic force microscopy (AFM) was carried out using a Veeco Dimension 3000 atomic force microscope. The microstructure of the samples was also investigated by scanning electron microscopy using a JEOL JSM 6700F apparatus.

Results and discussion

As-deposited films

The nature of the phases obtained after RF sputtering depends on the preparation conditions. The RF power density has a strong influence, as revealed in Fig. 1. For a RF power close to 40 W and an argon pressure of 0.5 Pa, the film is made of mixed cobalt-iron monoxide. This phase seems pure because all of the diffraction peaks can be ascribed to the XRD pattern of cobalt-iron monoxide. Moreover, the magnetization (M) versus magnetic field (H) curves do not display hysteresis that could reveal an extra ferrimagnetic spinel phase. When the RF power density is decreased, the X-ray diffraction pattern of a spinel phase replaces the previous pattern (Fig. 1). Some changes in the (220) peak intensity are observed for samples containing a little monoxide. These changes cannot be attributed to changes in cationic distributions because the very small differences in the electronic densities of the Co 2+ , Co 3+ and Fe 3+ ions cannot produce such large variations in the XRD peak intensity. The reason for these changes seems rather related to crystallite growth in a preferred direction.

The M = f(H) curves are clearly hysteresis curves for these samples as shown in Fig. 2 for the film deposited at 20 W. The coercive force is very high at 150 K (close to 12 kOe) but decreases down to 3.6 kOe at room temperature. This behaviour, due to high magnetocrystalline anisotropy strongly dependent on temperature, is similar to that observed by Takahashi and Fine. 15 Slightly lower magnetization at 30 kOe is observed for the film prepared at 30 W. This magnetization is only 72.8 emu cm -3 , which is close to 82.6 and 86.4 emu cm -3 for the films prepared at 20 and 10 W, respectively. This could be explained by the presence of a small quantity of monoxide, which does not contribute as strongly to the magnetization of the film as the ferrimagnetic spinel phase.

Some experiments carried out at argon pressures increasing from 0.5 to 1.5 Pa show also that this modification of sputtering conditions leads to a decrease in magnetization measured at 30 kOe. Moreover, at higher argon pressures, the X-ray diffraction patterns reveal the formation of monoxide in addition to the spinel phase.

When the argon pressure and the RF power density were fixed at 0.5 Pa and 20 W, respectively, the films obtained display the X-ray diffraction pattern of a pure spinel phase and have high magnetization. They also display the same Raman spectrum as that of a pure Co 1.7 Fe 1.3 O 4 spinel powder (Fig. 3). Similar results were obtained at 0.5 Pa and 10 W but the deposition rate is lower in this case. The sputtering conditions selected for the preparation of the samples studied in the following section, dedicated to the study of the thermal stability of the films, were then 0.5 Pa and 20 W. The microstructure of such films is made of well crystallized small columns of about 20 to 30 nm in diameter (Fig. 4). The Ra roughness of the films, measured by atomic force microscopy, is close to 2 nm.

Annealed films

As was pointed out in the Introduction, the Co x Fe 3-x O 4 solid solutions acquired so far, at about 900 to 950 °C and then quenched to room temperature, are not stable in the miscibility gap (from RT to about 875 °C). The Co 1.7 Fe 1.3 O 4 RF-sputtered films of this work were not treated at such temperatures because the sputtering process of oxides generally leads to temperatures lower than 200 °C on the substrate surface. However, due to thermodynamic reasons, they also have to be transformed in the miscibility gap even if different parameters, such as mechanical stress and crystallite size, could influence the mechanism and the speed of transformation as observed for samples studied so far. Their thermal stability inside the miscibility gap was then carefully studied, mainly at quite low temperatures compatible with the use of ordinary glass substrates.

The first experiments were carried out in an X-ray diffractometer equipped with a heating chamber. Fig. 5 shows that the peaks corresponding to the spinel structure are not clearly modified by these heating conditions apart from a slight shift of the Bragg position towards low 2 theta angles due to the thermal expansion. However, broadening of peaks is observed from the powder diffraction patterns recorded at 600 °C and higher temperatures. Some other diffractograms were recorded at 600 °C for different durations (Fig. 6). Above about 5 hours of hold time, some peaks started to split (i.e. (440) for instance). This split indicates that the initial single phased film is transformed into two spinel phases. An iron-rich phase and another phase rich in cobalt, with crystalline parameters respectively greater and smaller than the initial phase, grew progressively according to the characteristic process of the spinodal decomposition. 11 After cooling to room temperature, the sample remained two-phased. The structural changes of Co 1.7 Fe 1.3 O 4 films were also studied by Raman spectroscopy. The Co 1.7 Fe 1.3 O 4 film deposited on the quartz was heated in a conventional furnace at 600 °C for increasing durations. Raman spectra revealed the main peaks related to the spinel oxides, already identified in previous studies. [16][17][18] They are also changing with the duration of the heat treatment (Fig. 7). The main change, especially above 6 hours, is the very strong growth of the F 2g (1) and A 1g peaks. The emergence of low-intensity F 2g (2) and F 2g (3) peaks must also be reported. X-ray diffraction results show that the spinodal decomposition is only observed after more than 5 hours of treatment. This is in good accordance with XRD analysis results which show that the spinodal decomposition is only observed after more than 5 hours of treatment.

To understand such changes, it is interesting to have a look at the Raman spectra of cobalt-rich (Co 2.46 Fe O 4 ) and iron-rich (Co 1.22 Fe 1.78 O 4 ) spinel powders as reference samples (Fig. 7).

These powders have chemical compositions very close to those (Co 2.69 Fe 0.31 O 4 and Co 1.16 Fe 1.84 O 4 ) obtained after complete transformation of the Co 1.7 Fe 1.3 O 4 cobaltite. 19 In other words, they have very similar compositions to those which limit the miscibility gap at low temperatures. The Raman spectra of these powders are quite different. Very well defined peaks of F 2g (1) and A 1g and, to a lesser extent, F 2g (1) are observed for the Co-rich phase. These can be ascribed to a high content of Co 2+ ions in tetrahedral sites (F 2g (1) peak) and to a high content of Co 3+ ions in octahedral sites (A 1g peak). According to ref. 19 the structural formula of the Co-rich phase is indeed:

Co 1 2+ [Co 1.46 3+ Fe 0.54 3+ ]O 4 2- (1)
(cations outside and inside the brackets are located in the tetrahedral and octahedral sites, respectively). By contrast, the small F 2g (2) peak is characteristic of the iron-rich phase.

It is then clear that the superposition of such spectra leads to a spectrum similar to that observed after a long heat treatment of the Co 1.7 Fe 1.3 O 4 film. The growth of the F 2g (1) and A 1g peaks and the emergence of low-intensity F 2g (2) and F 2g (3) peaks after heat treatments are so simply explained by the formation of cobalt-rich and iron-rich phases.

According to previous results, 19 the transformation of Co 1.7 Fe 1.3 O 4 powders by spinodal decomposition or nucleation and growth does not lead to a strong difference in the average number of oxygen anions surrounding each cation in the spinel lattices of the material. The real change is that the cations are located in two separated phases in such a manner that some cations are "concentrated" in a sub-lattice in a given spinel phase. For instance, the formation of a cobalt-rich phase, resulting from the spinodal transformation, concentrates 100% of Co 2+ ions and 75% of Co 3+ ions in its tetrahedral and octahedral sites, respectively (see formula (1)). The contribution to the Raman spectra of such a phase, which is significantly different than the iron-rich phase in which the cobalt ions are "diluted" in the two sub-lattices, thus becomes clearly visible. Raman spectroscopy is then very sensitive to the formation of a new spinel phase very rich in cobalt ions.

No clear contrast between the two phases formed by spinodal transformation at 600 °C for 24 hours was highlighted. Local chemical analyses were also unable to reveal the phases individually as shown by the homogeneous distribution of the iron and cobalt species in the cross section STEM view (Fig. 8a). A complete segregation occurs however after a much longer annealing time. For instance, a sample heat treated in air at 600 °C for 96 hours clearly displays cobalt rich and iron rich regions of about 50 nm in diameter (Fig. 8b). The random distribution of the two phases in these regions along and perpendicular to the columns reveals however that the spinodal transformation is replaced by a nucleation and growth process for such a very long time of treatment at 600 °C.

In order to achieve spinodal decomposition at moderate temperature, annealing treatment at 450 °C was also carried out. The annealing of the films at 450 °C increases a little bit the average grain size. Indeed, atomic force microscopy reveals that the mean diameter of the grains changes from 30 to about 50 nm. After 24 hours of treatment at 450 °C, there is no clear change in the X-ray diffraction patterns. A careful examination of the patterns shows however a very small peak broadening. This is in contradiction to the increase of the size of the grains. Coarser grains would suggest an improvement of crystallization, revealed by narrower diffraction peaks. The peak broadening could then reveal the beginning of the spinodal decomposition.

Raman spectroscopy is however more informative (Fig. 9). After only 5 minutes of treatment at 450 °C, the F 2g (1) and A 1g peaks as well as the F 2g (3) peak grew significantly in a manner similar to that observed for films decomposed at 600 °C. This growth continued for longer annealing times. It seems reasonable then to conclude that the spinodal decomposition starts a few minutes after the Co 1.7 Fe 1.3 O 4 is heated at 450 °C, even if no clear change in the X-ray diffraction patterns is observed. The changes observed through Raman spectroscopy could be the first step of spinodal decomposition already described by Harrison and Putnis 20 which corresponds to the development of low-amplitude sinusoidal fluctuation of compositions. The gathering of cobalt ions in a small region could make its detection by Raman spectroscopy possible, but the gradual variation of the composition induced by this first step of the spinodal decomposition could make the characterization by X-ray diffraction difficult. Just a small peak broadening could be expected from this technique.

Treatments at temperatures even lower than 450 °C were also carried out. It was very surprising to see that after only 2 hours at 300 °C, the Raman spectra of Co 1.7 Fe 1.3 O 4 films begin to resemble the shape of decomposed cobaltites spectra (Fig. 10). This evolution is obviously more pronounced for treatments at higher temperatures. Fig. 11 clearly illustrates the initiation of decomposition for mild heating and its progress for more energetic treatments. The F 2g (1) peak intensity, normalized by that of the E g peak, which remains approximately constant along the treatments, grows regularly and significantly with the heating temperature and time.

Electron exchanges between Mn + and M (n+1)+ cations located in the same sub-lattice induce semi-conducting properties in spinel oxides. 21

according to ref 19. However, due to the high activation energy of the electronic exchange for the Co 2+ /Co 3+ couples, 22,[START_REF] Cox | Electronic structure and chemistry of solids[END_REF] the electrical resistivity of such a material is quite high at room temperature but decreases strongly at higher temperatures.

The electrical resistivities of the as-sputtered and heat treated Co 1.7 Fe 1.3 O 4 samples were then studied at 250 °C, to make the measurements possible in our experimental set-up. A quite high activation energy, close to 0.48 eV, was measured in agreement with the bibliographic data. 22,[START_REF] Cox | Electronic structure and chemistry of solids[END_REF] On the other hand, Fig. 12 shows that the resistivity of the films increases almost linearly with the time of annealing at 450 °C. This clearly demonstrates significant changes in the material at this moderate temperature of treatment. According to ref.

which shows that the number of Co 2+ /Co 3+ couples is much lower than that of the initial single phase Co 1.7 Fe 1.3 O 4 (see structural formula ( 2)). The spinodal decomposition of the latter necessarily leads to a progressive increase in resistivity. This is in agreement with the results of the electrical measurements shown in Fig. 12. The combination of the results coming from Raman spectroscopy and electrical measurements then seems to prove the beginning of the spinodal transformation, after heat treatments at very moderate temperatures for quite a short time of annealing. compatible with the use of cheap substrates such as conventional glass slides or silicon wafers.

Conclusion

Fig. 1 X

 1 Fig. 1 X-ray diffraction patterns of films deposited at a 0.5 Pa argon pressure at different RF powers.

Fig. 2

 2 Fig. 2 Hysteresis loops registered at 150 K and 300 K of films deposited at 0.5 Pa and 20 W.

Fig. 3

 3 Fig. 3 Raman spectra of films deposited at 0.5 Pa and 20 W.

Fig. 4

 4 Fig. 4 Microstructure of films deposited at 0.5 Pa and 20 W: the AFM planar view (4a) and the SEM (4b) and TEM (4c) cross section views.

Fig. 5

 5 Fig. 5 XRD patterns registered at different temperatures of films deposited at 0.5 Pa and 20 W.

Fig. 6

 6 Fig.6XRD patterns registered at 600 °C of films deposited at 0.5 Pa and 20 W on a silicon substrate.

Fig. 7

 7 Fig. 7 Raman spectra of films deposited at 0.5 Pa and 20 W and annealed at 600 °C for different annealing times compared with Raman spectra of Co 1.22 Fe 1.78 O 4 (iron rich spinel) and Co 2.46 Fe 0.54 O 4 (cobalt rich spinel).

Fig. 8

 8 Fig. 8 Cross section STEM micrographs and corresponding EDS maps obtained using iron (red) and cobalt (blue) Kα of the films annealed at 600 °C for 24 h (a) and 96 h (b).

Fig. 9

 9 Fig. 9 Raman spectra of films deposited at 0.5 Pa and 20 W and annealed several times at 450 °C.

Thin films of Co 1 . 7

 17 Fe 1.3 O 4 were prepared for the first time by RF sputtering on a substrate at room temperature. Pure as-deposited samples are made of small crystallites of about a few tens of nanometers. Because they have a composition inside the miscibility gap of the phase diagram of CoFe 2 O 4 -Co 3 O 4 , their thermal stability was studied. It was shown that RF sputtered Co 1.7 Fe 1.3 O 4 films are not stable at 600 °C as for cobaltites of a similar composition already prepared at high temperature. More original results were obtained however. It was demonstrated using mainly Raman spectroscopy and electrical measurements that Co 1.7 Fe 1.3 O 4 films can be partially transformed at quite low temperatures, ranging from 300 to 450 °C, and after quite a short time of annealing. This partial transformation is a first step of spinodal decomposition. From the present work it can then be concluded that the preparation of thin films structured by spinodal transformation could be obtained at temperatures

Fig. 10

 10 Fig. 10 Raman spectra of films deposited at 0.5 Pa and 20 W and annealed for 2 h at different temperatures.

Fig. 11

 11 Fig. 11 The evolution of the F 2g (1) Raman peak relative intensity of films deposited at 0.5 Pa and 20 W versus annealing temperature and time.

Fig. 12

 12 Fig. 12 Electrical measurements at 250 °C of films deposited at 0.5 Pa and 20 W and annealed at 450 °C for different durations.

  Co 1.7 Fe 1.3 O 4 is thus a semiconductor because its structural formula is close to

	Co 0.61	2+ Fe 0.39	3+ [Co 0.39	2+ Co 0.73	3+ Fe 0.88	3+ ]O 4	2-

Acknowledgements

Thi Mai Anh Bui wishes to thank the Vietnamese Government Scholarship (project 322) of the Vietnam Ministry of Education and Training and the support of Carnot Institute CIRIMAT for her PhD grant.

Notes and references


Matthew J Fraidakis

Claude Jardel

Stéphane Allouche

Isabelle Nelson

Karine Auré

Abdelhamid Slama

Isabelle Lemière

Jean Philippe Thenint

Jean Baptiste Hamon

Fabien Zagnoli

Phenotypic diversity associated with the MT-TV gene

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Table 2 :

 2 Spectrophotometric assays of OXPHOS in the muscle of the patients with the mrespiratory chain activities are expressed as nanomoles per minute and mg protein. They were performed on muscle homogenates for Patients 1, 2 and 3 and on isolated muscle mitochondria for Patient 4 with the exception of the two assays marked with ‡ that were performed on muscle homogenates. Each patient has been analyzed with different assay protocols, the results have therefore to be compared to their own set of control values expressed as mean±standard deviation (Patients 1, 2 and 3) or as normal range (Patient 4). Numbers marked with * are significantly abnormal. CI=respiratory complex I (rotenone sensitive NADH ubiquinone oxido-reductase) activity; CII= respiratory complex II (succinate ubiquinone oxido-reductase) activity; CIII= respiratory complex III (antimycin sensitive ubiquinol cytochrome c oxido-reductase) activity; CIV= respiratory complex IV (cytochrome c oxidase) activity; II+III=combined respiratory complex II and III (succinate cytochrome c oxido-reductase) activity; CS=citrate synthase, a Krebs cycle enzyme, activity.

Table 1 : Patients characteristics

 1

	Patient (Family)	1 (F1)	2 (F2)	3 (II-3 F3)	4 (III-1 F3)
	Age (onset/present)	25/50	18/25	3/42	NN/7
	First sign	Deafness	Suicide	Dystonia	Dev. delay
	Cognitive decline	+	+	+	+
	Cerebellar ataxia	+	-	-	+
	Psychiatric signs	-	+	-	-
	Deafness	+	-	-	-
	Pyramidal Syndrome	-	-	+	-
	Seizures, myoclonia	-	-	-	+
	Dystonia	-	-	+	-
	Muscle weakness	+	-	-	-
	MRI Cerebellar A	+	-	+	+
	Cortical A	-	+	-	-
	Brainstem A	+	-	-	-
	Striatum HS	-	+	+	-
	White matter HS	-	-	-	+
	EMG + NCV	Myogenic	Axonal	Axonal	NA
	Lactatemia	5.3 (45 y)	3.7 (23 y)	1.7 (39 y)	1.6 (3 y)
	Lactatorrachia	NA	High (MRS)	4.4 (39y)	N (MRS)
	Muscle Histology	SDH++/COX-	COX-	COX-	Low COX
	Age is indicated in years: first number is the patient's age at onset; second number is the
	patient's age at diagnosis or at death (Patient 1); + = present; -= absent; A=Atrophy; Axonal
	= axonal sensorimotor polyneuropathy; COX-=presence of scattered cytochrome c oxidase
	(COX) negative fibers; Dev. Delay Development Delay; EMG + NCV = electromyography

Table 3 : Combined respiratory chain defect was observed in the cultured skin fibroblasts and in cybrids with the m.1644G>A mutation

 3

		Patient 2	Patient 3	Patient 4	Control	Mutant	Control
						fibroblasts	cybrids	cybrids
	VO 2 (succ)	10.6	11.3	11.0	11.5±3.2	* 6.6	12.9±4.1
	VO 2 (pyr)	3.3		4.5	4.31	7.3±2.5	2.9 *	9.7±3.8
	ratio	3.11	*	2.51 *	2.55 *	1.63±0.55	2.27 *	1.50±0.63
	CII	31		36	34	26±8	49	43±19
	CIII	28		80	30	55±17	50	47±12
	CIV	24
	II+III	26		35	35	31±7	62	50±21
	CS	68		92	72	77±19	238 *	115±28
	CII/CS	0.46	0.39	0.47	0.37±0.10	0.21	0.39±0.17
	CIII/CS	0.41	0.87	0.42	0.79±0.25	0.21	0.41±0.11
	CIV/CS	0.35	*	0.41 *	0.58 *	1.11±0.22	0.05 *	0.63±0.19
	All values are expressed as expressed as nanomoles per minute and mg protein. Mutant
	cybrids had 100% m.1644G>A mutation. VO 2 (succ)]: respiration with succinate that enters
	the respiratory chain at complex II; VO 2 (pyr): respiration with pyruvate that enters the
	respiratory chain at complex I; ratio: ratio between the respiration rates on these two
	substrates; CII= respiratory complex II (succinate ubiquinone oxido-reductase) activity; CIII=
	respiratory complex III (antimycin sensitive ubiquinol cytochrome c oxido-reductase)
	activity; CIV= respiratory complex IV (cytochrome c oxidase) activity; II+III=combined
	respiratory complex II and III (succinate cytochrome c oxido-reductase) activity; CS=citrate
	synthase, a Krebs cycle enzyme, activity; control values are expressed as

* 38 * 42 * 83±16 13 * 71±18 * are significantly abnormal.

Table 4 : Homogeneous distribution of the m.1644G>A mutation in the patients'

 4 Proportion of the mutation evaluated with fluorescent PCR-restriction. Buccal C=DNA extracted from buccal mucosa; Urinary S=DNA extracted from the urinary sediment; Nasal C=DNA extracted from nasal mucosa; fibros=DNA extracted from cultured skin fibroblasts.

	different tissues
		muscle	blood	Buccal C Urinary S Nasal C	Fibros
	Patient1	92%
	Patient2	100%	100%				100%
	Patient 3 (II-3)	100%		98%	98%	100%	100%
	Patient 4 (III-1)	61%	63%	66%	74%		66%
	Pt3 mother (I-1)		64%	54%	65%	64%
	Pt3 sister (II-1)		65%	68%	79%
	Pt3 sister (II-5)		56%	52%	47%

(22±7) 4


This paper deals with the design of a compact Process, Voltage and Temperature (PVT) probe architecture, in 32nm CMOS technology. The sensor, hereafter named MultiProbe, is composed of 7 different ring oscillators, each one presenting a particular sensitivity to PVT variations. The architecture allows MultiProbes to be chained, so that a single controller is needed. Simulation results exhibit the non-linearity behavior of the ring oscillators under temperature and voltage variations as well as their particular behavior. Due to their small size, the Multiprobe blocks can be easily integrated within a complex digital SoC architecture.

I. INTRODUCTION

ING-oscillators are widely used as sensors to monitor effects of Temperature (T), Voltage (V), and Process (P) variations on the timings of CMOS logic gates [START_REF] Quénot | A temperature measurement cell for VLSI circuits[END_REF][START_REF] Bhushan | Ring Oscillators for CMOS Process Tuning and Variability Control[END_REF][START_REF] Nekili | Spatial characterization of process variation via MOS transistor time constants in VLSI and WSI[END_REF], and on critical paths. Former works have made use of such sensors frequency shifts under PVT variations to ensure a safe functional mode [START_REF] Wolpert | A low-power safety mode for variation tolerant systems-on-chip[END_REF], or to estimate the operating frequency as a function of V and T [START_REF] Quénot | A temperature measurement cell for VLSI circuits[END_REF]. First probes dedicated to process variation monitoring were implemented as static sensors to quantify, after fabrication, the process variations along the chip, from die-to-die, or from wafer-to-wafer [START_REF] Nekili | Spatial characterization of process variation via MOS transistor time constants in VLSI and WSI[END_REF], and then to estimate the maximum attainable performance of a chip after post silicon characterization.

Several works have proposed smart sensors to differentiate Temperature from Voltage effects and to provide dedicated sensors. Zero Temperature Coefficient (ZTC) property has been used in [START_REF] Prasad | A single circuit solution for voltage sensors[END_REF] to design a Voltage sensor. [START_REF] Abuhamdeh | Separating temperature effects from ring-oscillator reading to measure true IR-drop on chip[END_REF] proposes IR-drops measurements via the difference between two identical Ring-Oscillators (RO) with separate power supplies. ROs with temperature controlled frequency were reported in [START_REF] Datta | Low-power and robust on-ship thermal sensing using differential ring oscillators[END_REF] and [START_REF] Chen | A timeto-digital-converter-based CMOS smart temperature sensor[END_REF]. Both works rely on a chain of current-starved inverters whose propagation delay is controlled by a biasing voltage. In both approaches, temperature is deducted from the frequency difference of two ROs with different temperature dependencies. The three variables (P, V, and T) exhibit very different dynamic evolutions. [START_REF] Muhtaroglu | On-die droop detector for analog sensing of power supply voltage[END_REF] presents the wide range of Voltage drops dynamics (1kHz to 100GHz) while Temperature variations can be estimated around 1MHz and Process is usually considered constant during the life-time, except in case of wearout. The difficulty of designing this kind of sensors remains in the fact that the whole dynamics has to be sensed.

This paper aims at presenting a compact PVT sensor IP easy to be integrated into a complex digital SoC architecture. This IP, denoted by MultiProbe afterward, is based on several ROs multiplexed all together so as to meet the main constraints applying to all on-chip sensors: (1) a very low area, (2) a wide dynamic of frequency measurements, (3) an easy integration and (4) a capability to discriminate P, V and T impacts with a minimum set of measurements. Distributed over the SoC, MultiProbes IPs may provide dynamic PVT variation measurements to both global and local system controllers. Coupled with the observation of critical paths and with specific algorithms allowing P, V and T discrimination, these measurements are used in adaptive Voltage and Frequency scaling architectures to reach the optimum energetic point, in presence of PVT variations while meeting the required applicative performances.

In this paper, the design of the MultiProbe IP, as a set of 7 PVT probes based on 7 different ROs, is presented. All the constituting RO are sensitive to variations but in a slightly different way. For modularity purpose, all the distributed MultiProbes are chained so that a unique controller has to be implemented. Section II gives the MultiProbe architecture. The full architecture is first described, and then the ring oscillators are presented with an emphasis on the Dedicated Temperature sensor. Section III summarizes the MultiProbe control and section IV reports the behavior of 2 particular probes, results being obtained from post layout simulations.

II. MULTIPROBE ARCHITECTURE PROPOSAL

The MultiProbe is a low area on-chip macro-block embedding several Ring Oscillators (ROs) as PVT sensors. The MultiProbe Controller is integrated only once for an IP to be sensed whereas the MultiProbes are distributed close to the monitored circuit point. Using a daisy chain, several MultiProbes can be implemented for a single controller with common input/output signals.

A. Full architecture

The architecture proposed for the MultiProbe, (see Figure 1), includes 7 different ROs as sensors, an address decoder to select the running RO, an 8-to-1 multiplexer to output the selected RO frequency, and finally 32 bits register. This register is partitioned into 28 bits for counting, an overflow bit and the 3 bits to configure the RO selection. All parts of the MultiProbe are made of standard cells from 32nm technology libraries.

The register has a dual functionality depending on the selected mode: sampling or scanning. During the sampling A Fully Integrated 32 nm MultiProbe for Dynamic PVT Measurements within Complex Digital SoC

L. Vincent 1 , E. Beigné 1 , L. Alacoque 1 , S. Lesecq 1 , C. Bour 1 , Ph. Maurine 2 R
mode the counter computes the number of rising transition occurring at the output of the selected RO, while the "sample" pin is activated. The scanning mode, allows in one hand lifting the data contained in the register throughout "scan_out" and in the other introducing the 32 new bits, containing the new configuration, via the "scan_in" input. As a result of this communication protocol, each measure is necessarily followed by a scanning. The address decoder activates a single sensor according to the chosen configuration stored in the register. Thus, a single RO operates at a time. This operating mode avoids RO frequencies coupling, occurring when all RO run simultaneously and could bias the PVT variation monitoring.

B. Ring oscillators

A Ring Oscillator (RO) is an unstable closed-loop system. As shown in Figure 1, each RO is made of a chain of several stages and a 3-input NAND gate. This latter activates the sensor with the two inputs "sample" and "select". All stages of one RO are identical but they are different from one RO to another. Each delay cell acts as a buffer, thus the RO oscillates due to the inverter NAND function.

1) Probe dedicated to temperature sensing In this work, a temperature dependent RO based on current-starved inverters biased by a thermally dependent current generator is proposed.

a) Current-starved inverters

Current-starved inverters are inverters with additional current limiting transistors that can also be used as clockgating circuits when the RO is disabled. Figure 2 (left) shows the current-starved delay cell used in the proposed architecture. To save silicon area, the relatively large polarization transistors M1 and M6 are shared by a pair of inverters.

Because of the current limitation, during a transition, each inverter loads its output capacitance at constant current. This results in a slower, linear, output voltage swing.

As it has been shown in [START_REF] Datta | Low-power and robust on-ship thermal sensing using differential ring oscillators[END_REF], when the current is sufficiently low, the current limiting dominates the gate delay and the propagation time t d can be expressed as:

bias L H load d I V V C t / ) ( 2 - ≈
(1) where C load is the output capacitance of each stage, V H -V L is the output voltage swing and I bias is the biasing current imposed by transistors M1 and M6. When N delay cells are used in the loop, the RO frequency can be expressed as [START_REF] Datta | Low-power and robust on-ship thermal sensing using differential ring oscillators[END_REF]:

)] ( . . 4 [ / ) . . 2 /( 1 L H load bias d V V C N I t N f - ≈ =
(2) As can be seen from ( 2), the RO frequency is proportional to the biasing current. The biasing cell is responsible for the generation of a temperature dependent current on the basis of MOSFET thermally dependent parameters. In MOSFET, the mobility of carriers and the threshold voltage parameters dominate the temperature dependence. They can be modeled as:

) ( ) ( ) ( ) / )( ( ) ( 0 0 0 0 T T T V T V T T T T T T k - - = = - α µ µ (3)
with T, T 0 the circuit and reference temperatures, µ the mobility, V T the threshold voltage and k and α two positive technology-dependent parameters. Note that a unique biasing cell is used to bias all delaycells in the RO, as can be seen on Figure 2 (right). It is made of two diode-connected complementary MOSFETs. This structure ensures the saturation of both transistors as long as V DD > (V TP +V TN ). This current-matching structure constrains its both transistors operating with a lowest Vgs-Vth than others digital transistors (composing the others ROs). This magnifies Vth relative variations regarding others ROs and hence the impact of relative variations on the biasing current is increased. The cell was tuned to maximize temperature sensitivity and linearity of its output value around the nominal supply voltage value (V DD =1.0V).

c) Results

The resulting Ring Oscillator (RO) frequency of Figure 3 was extracted from electrical transient simulations at different temperatures for both the thermally-dependent and the standard Ring Oscillators. As shown, the RO exhibits a sensitivity of 14.1 MHz/K whereas standard inverters show a lower sensitivity, (0.7 MHz/K). This high sensitivity to temperature ensures a finer discrimination and discretization of temperature evolution.

2) Description of the 6 other probes

The 6 other ROs constituting the MultiProbe architecture are designed with standard cells. Each RO is made of a different number of stages, and the stage of each sensor is different in terms of gate type and/or layout (see Table 1 for a summary). They have been chosen because of their large sensitivity to PVT variations in any technology and because they do behave as any critical path of a circuit. The output frequency of each sensor is around 1GHz fitting the internal 32-bits counter frequency. An important advantage of chained MultiProbes architecture is that a single fully digital controller is needed to monitor several probes, limiting the silicon area and wires. The goal of the controller (Figure 5) is to handle MultiProbes, by sending and retrieving data to and from them, but it also controls the sampling procedure.

The Register interface "Regs Intf" communicates with the global on-chip communication network. The Serializer/Deserializer "Ser/Des" block respectively sends and receives data to and from the chained MultiProbes. It communicates with the Register interface to store results and to receive the next configuration (Cfg). The programmable sampling pulse generator aims to accurately control the pulse width during which the sensors are activated. A. Dedicated Temperature probe simulation This sensor has been specifically designed to be more sensitive to Temperature than the other ones. Figure 6 shows its characteristic surfaces. As shown, these surfaces are non linear. Each surface equation can be fitted, using a least squares method, into: a fifth order in Voltage and, a second order in Temperature polynomial (or posinom).

By nature, the polarization cell has a high dependence on V DD variations. A linear increase of V DD results in a quadratic augmentation of the biasing current, and thus of the RO frequency. Actually, near the nominal V DD (1V), the frequency dependence in Temperature is nearly linear, as forecasted in II.B.1.b).When V DD is higher, the biasing current gets large enough to stop limiting the inverter speed and the delay-cells behavior becomes similar to standard

T V P T V P V P T V P T V P V P T V P T V P V P T P T V P V P T P V P P F typ ⋅ ⋅ + ⋅ ⋅ + ⋅ + ⋅ ⋅ + ⋅ ⋅ + ⋅ + ⋅ ⋅ + ⋅ ⋅ + ⋅ + ⋅ + ⋅ ⋅ + ⋅ + ⋅ + ⋅ + = (4)
inverters. On the other-hand, when V DD < (V TP + V TN ), both polarization transistors are in sub-threshold region and the biasing current decreases and quickly approaches 0. As a result, the RO oscillates very slowly, as illustrated by the bended corner on the bottom right of Figure 6.

B. Inverter probe simulation

This RO is the simplest one in the MultiProbe and it may be considered as the reference sensor. With the same simulation scenario, each characteristic surface of this RO is obtained on Figure 7. As can be seen, the behavior is highly non linear in V and T. Fitting the surface equation of the inverter probe response with a least squares method leads to: which is a fourth order in Voltage and second in Temperature polynomial. Note that in [START_REF] Quénot | A temperature measurement cell for VLSI circuits[END_REF], the non-linearity was identified to an order 4. However this result was obtained considering a micrometric technology (2µm) and above the Temperature Inversion Point. In [10], the probe behavior is totally different from the present probe behavior, with non-linearity in 1/V².

The ZTC effect around 1.2V can also be observed, and so the best case corner moves to [-40°C, 1.5V] operating point.

Note that the characteristic surfaces of the 5 other ROs are similar to that of the inverter probe. Therefore, they can be approximated by similar polynomials.

V. CONCLUSION AND FUTURE WORK

The architecture and design of a compact digital PVT sensor, named MultiProbe, and its controller have been presented. The proposed structure, distributed over a complex digital SoC, allows a dynamic PVT measurement within the circuit. The probes are based on ring-oscillators sensitive to PVT variations that were chosen to allow the discrimination of P, V and T dynamic evolution during sensing. A single controller is implemented to access several chained MultiProbes, limiting drastically the silicon area and routing overheads compared to former approaches. RC simulations have confirmed the non-linear behavior of ringoscillators under Temperature and Voltage variations, in low voltage 32nm CMOS technology.

Implemented into an adaptive Voltage and Frequency scaling architecture, the fusion of information provided by all these probes will be performed in a future work to dynamically monitor the PVT variations. The main challenge is to adapt the control law of the DVFS policy to reach an optimum energetic functional point. Circuits, pp. 651-660, 2004.

[10] J.J.L.Franco, E. Boemo, E. Castillo, L. Parrilla "Ring oscillators as thermal sensors in FPGAS: Experiments in low voltage", Programmable Logic Conference (SPL), pp. 133-137, 2010.

Figure 1 :

 1 Figure 1: Scheme of the MultiProbe architecture proposed The principle of sensors based on RO is to measure their output frequency by counting the number of rising edges occurring during a sensing time window of chosen duration. Because their output frequency depends on the Process quality and the averaged value of V and T over the sensing time window, ROs may be used as PVT sensors.

Figure 2 :

 2 Figure 2: Delay cell (left), temperature dependent biasing (right) b) Thermally dependent biasing cell

Figure 3 :

 3 Figure 3 : RO frequency vs temperature (at Vdd = 1V); (blue diamond) thermally dependent (red square) standard-inverters RO.

  RO NameDescription of Stage Gate type Inverters 13 stages of 2 standard inverter cells Long wires 10 stages of 2 inverters linked together with long wires using several metal layers and vias. Latch 5 stages made of 1 latch standard cells Xor 8 stages made of 1 XOR standard cells. Ncap 6 stages of 2 inverters loaded with capacitors made of a NMOS transistor. Pcap 6 stages of 2 inverters loaded with capacitors made of a PMOS transistor.

Figure 4

 4 Figure 4 gives the layout of the long wire delay cell (right) to be compared with the similar short wire delay cell (left).

Figure 4 :

 4 Figure 4: delay cell layout of inverter (left) and long-wire inverter (right)

Figure 5 :

 5 Figure 5: Global scheme of the MultiProbe control

Figure 6 :

 6 Figure 6 : Characteristic surfaces for the Temperature Dedicated probe F=f(V,T) for the 3 Process cases SS, TT, FF upward.

Figure 7 :

 7 Figure 7 : Characteristic surfaces for the Inverter ring-oscillator.F=f(V,T) for the 3 Process cases SS, TT, FF upward

TABLE 1 :

 1 RING OSCILLATORS DESCRIPTION

Lionel Vincent, Edith Beigné, Laurent Alacoque, Suzanne Lesecq and Catherine Bour are with CEA-LETI MINATEC Campus, 17 rue des martyrs, 38054 Grenoble Cedex 9, France, {firstname.name}@cea.fr ²Philippe Maurine is with the Laboratoire d'Informatique, de Robotique et de Microélectronique de Montpellier (LIRMM), Université Montpellier

2, UMR 5506 -CC477, 161 rue Ada 34095 Montpellier, Cedex 5, France (e-mail: Philippe.Maurine@lirmm.fr).

ACKNOWLEDGMENT

This work has been done in the context of the Platform 2012 program, a joint project between STMicroElectronics, CEA and the UE (ENIAC Joint Undertaking) in the MODERN project (ENIAC-120003). Authors also wish to associate Sylvain Miermont to this work.