RESIDUAL STRESSES AT AN OXIDESILICON INTERFACEM. V. Whelan, A. H. Goemans, and L. M. C. Goossens Citation: Applied Physics Letters 10, 262 (1967); doi: 10.1063/1.1754802 View online: http://dx.doi.org/10.1063/1.1754802 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/10/10?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Photoemission spectroscopy study of the lanthanum lutetium oxide/silicon interface J. Chem. Phys. 138, 154709 (2013); 10.1063/1.4801324 The Impact Of Organic Contamination On The OxideSilicon Interface AIP Conf. Proc. 1395, 217 (2011); 10.1063/1.3657894 Electrical properties of buried oxide–silicon interface J. Appl. Phys. 80, 1605 (1996); 10.1063/1.362958 Residual stress at fluid interfaces with application to silicon oxidation Appl. Phys. Lett. 58, 2488 (1991); 10.1063/1.104852 Time resolved annealing of interface traps at the silicon oxidesilicon interface J. Appl. Phys. 64, 5013 (1988); 10.1063/1.342453

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Volume 10, Number 10 APPLIED PHYSICS LETTERS 15 May 1967

to the right branches of the curves in Fig. 1, causes electron tunneling from the valence band to the metal, cf. Fig. 2(b). The left branches, V < -1.1 V, in Fig. 1 represent electron tunneling from metal into the conduction band, cf. Fig. 2(c). The mini­mum of conductance at small negative voltage, cf. Fig. 1, is a result of two components, cf. Fig. 2(c): Tunneling of metal electrons into the empty part of the valence band and into the empty interface states from where they momentarily recombine with holes in the valence band. If the low conduct­ance in the region -1.1 V < V < 0, coinciding with the silicon band gap, is assumed to be proportional to the density of states N~s, Fig. 1 leads to the con­clusion that there is an increase of 1 to 2 orders of magnitude in interface state density, when changing from annealed-steam-grown, to steam-grown, to dry-oxygen-grown oxide layers. This is in qualitative agreement with an experimentally determined increase of interface states,3 from annealed-steam­grown to steam- and oxygen-grown oxide layers of large thickness (= 1000 A).

Measurements of similarly prepared N++ silicon samples, revealing no obvious influence of inter­face states on the I-V characteristics are in qualita­tive agreement with an N++ -type band model similar

to Fig. 2. The measured temperature dependence of the tunneling characteristics was found to be small as considered typical for tunneling.

Typical Schottky barrier characteristics4 were observed for very thin oxide layers or low-doped semiconductors, where the Fermi level of the metal is pinned directly to the surface of the semiconduc­tOr.5•6

A separation of the effects of recombination and tunneling of carriers into interface states appears possible by ac measurements. Extended measure­ments combined with theoretical analysis are in progress.

The author thanks S. M. Sze for suggesting the problem, A. Goetzberger for encouragement and helpful discussions, and J. McGlasson for assistance with provision of samples and performance of measurements.

1 A. Goetzberger,j. Electrochem. Soc. 113, 138 (1966). 2 A. S. Waxman, Ph.D. thesis, E. E. Dept., Princeton University,

November 1966. 3E. H. Nicollian and A. Goetzberger, Bell System Tech. j., to be

published. 'D. Kahng, Bell System Tech.]. 43, 215 (1964). "C. R. Crowell, H. B. Shore, and E. E. Labate,j. Appl. Phys.

36,3343 (1965). ·C. R. Crowell and S. M. Sze, Solid-State Electron. 9,1035 (1966).

RESIDUAL STRESSES AT AN OXIDE-SILICON INTERFACE

M. V. Whelan, A. H. Goemans, and L. M. C. Goossens Philips Research Laboratories

N. V. Philips' Gloeilampenfabrieken Eindhoven, The Netherlands

(Received 20 March 1967)

Residual stresses occurring at an oxide-silicon interface are studied by measuring the bending of an oxidized silicon slice which results when the oxide is removed from one face.

The stresses are independent of the orientation of the silicon surface, and over a wide range of the doping level of the silicon. Stresses increase with the speed at which the samples are cooled after the oxidation: An oxide grown in dry O2 causes larger stresses than one grown in wet O2, The stresses measured do not appear to be capable of causing considerable numbers of interface states.

Stresses are apparently present at an oxide-silicon interface since removal of the oxide from one face of a slice oxidized on both faces results in a bending of the sample. These stresses may be important in the formation of defects at the interface or may even cause a decrease of the band gap of the silicon at the surface. 1 This Letter presents results of meas­urements of the magnitude of these stresses ob­tained by measuring the bending of silicon slices which resulted upon the removal of the oxide from

262

one of the two oxidized faces: The bending was measured by an x-ray diffraction technique. The influence of various factors such as oxide type, orientation of silicon surface doping level of the silicon and speed of cooling after oxidation are studied, and a possible relationship between the stresses and interface states is discussed.

Silicon slices of various types were oxidized either in dry O2 (dew point temperature -40°C) or in wet O2 (d.p.t. 90°C) at 1200°C. Some of the latter

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Table I. Interfacial stresses measured at room temperature for (111), (11 0), and (100) oriented silicon slices oxidized in O 2 (dew point temperature 90°C) and for similar samples subjected to an additional p 205 diffusion step. The silicon slices were out from a 5 O-cm (Ill) pulled crystal: The oxide thickness was about 0.84 JLM and the oxide after the P205 diffusion step was about 1 JLM thick. The column Dir. indicates directions along the surface in which the bending of the slice was measured: This bending resulted after the removal of the oxide from one of the two oxidized faces of a slice; the columns <Tox,

and (TSi denote, respectively, the stress values in the oxide and silicon at the interface in dynes/cm2•

Orientation of (111)

Silicon Surface

Samples Dir. (Tax (TSi

Oxidation in O2 (110) 4.70 X 108 8.5 X 107

(d.p.t. 90°C) (211) 4.50 X 109 8.2 X 107

Oxidation in O 2 (110) 2.5 X 109 5.5 X 107

(d.p.t. 90°C) plus P20 5 diffusion step.

oxides were subjected also to a P20 5 diffusion. This latter step was carried out after the oxidation in a two-zone furnace, using P20 5 powder as a source: Deposition was at 920°C in N2 for 30 min with the source held at 250°C; the drive-in followed for I hr during which the source oven was switched off and the sample oven was at I 120°C. All oxides were of the order of 1 JLm thick.

The oxide was etched from one face and the residual bending of the slices was then measured.

The values of the stresses existing in the oxide at the interface corresponding to the values of bend­ing measured were calculated using a slightly modi­fied form of Eq. (2) of ref. 4. This equation enables the oxide stress to be calculated as follows:

where

(Tax = the stress in the oxide at the interface (dynes/cm2);

E Sl = Young's modulus for the silicon surface (dynes/cm2);

'VSi = Poisson's ratio for the silicon surface; ,R = radius of curvature of bending of the slice;

dSi = silicon slice thickness; dox = oxide thickness.

This formula is valid if the oxide is thin relative to the thickness of the silicon substrate, if the bend­ing is small, and for uniform stress in the oxide. The results verify this latter fact. The values of ESI and VSI used, for the various orientations of silicon and for the various directions on the sur­face in which the bending was measured, were ob­tained from the various curves in ref. 3.

(110) (100)

Dir. (Tax (TSi Dir. (Tax (TSi

< 110) 3.9 X 109 8.6 X 107 (110) 4.0 X 109 9 X 107

< 100) 3.9 X 109 8.5 X 107 (110) 3.8 X 109 9 X 107

(110) 2.6 X 109 6.9 X 109 (110) 2.4 X 109 6.6 X 107

Using Eq. (15) of ref. 4 the maximum stress in the silicon (TSi corresponding to the oxide stress was calculated:

(2)

The stresses in the oxide at the interface are uniform, independent of orientation of the silicon

Table II. Room-temperature interfacial stresses for ( 110) oriented, phosphorous-doped silicon slices oxidized at 1200°C in, respectively, wet O 2

(d.p.t. 90°C) and dry O 2 (d.p.t. -40°C). Rapidly cooled samples were drawn quickly from the oven after the oxidation while the slowly cooled sam­ples were left in the oven until room temperature was reached; cooling time was about 5 hr. The oxide thickness varied between 0.86 and 0.92 JLM. The stress was found to be independent of direc­tion of measurement of bending along the surface and the values quoted are for the (11 0) direction. The bending resulted after the oxide was removed from one of the two oxidized faces of a slice.

Samples

Oxidation in O2 (d.p.t. -40°C)

Slowly cooled Quickly cooled

Oxidation in O2 (d.p.t. 90°C)

Slowly cooled Quickly cooled

Oxide Stress (dynes/cm2)

2.7 X 109

3.7 X 109

l.6 X 109

2.8 X 109

Silicon Stress (dynes/cm2)

7.2 X 107

8.1 X 107

4.1 X 107

6.6 X 107

263

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Volume 10, Number 10 APPLIED PHYSICS LETTERS 15 May 1967

Table III. Stresses at a silicon oxide interface at room temperature for samples of varying values of bulk doping. The stresses were obtained by measuring the bending of (111) oriented slices along

the (200) reflex after removal of the oxide from one of the two oxidized faces of a slice. The oxide was about 0.92 JLM thick and grown in wet O2 (d.p.t. 90°C) at 1200°C.

Slice Resistivity Doping Level (O-cm) (cm-3)

1000 4 X 1012

5-10 8.5 X 10lL4 X 1014

0.37 1.61 X 1016

0.035 4.5 X 1017

0.0015 5.5 X 1019

surface, and of the doping level of the silicon over the range (4 X 1012 to 5.5 X 1019 cm-3) used, and they increase with the rate of cooling after oxida­tion. The values of the stresses are of the same order of magnitude as those measured by Jac­codine and Schlegel.4 These authors point out that the stresses are most probably caused by the dif­ference between the thermal expansion coefficients of the silicon and oxide, and that the maximum stresses in the silicon are unlikely to cause plastic deformation of it. The~e latter stresses also cause negligible change of the band gap of the silicon at the interface since a value of lOll dynes cm~2 (ref. 1) is required for this to be significant. Oxides made in wet O2 have less stress than those made in dry O2: This may be due to an increase of the thermal ex­pansion coefficient of the former oxide by OH group; such is the case with vitreous silica.5 The P20 5 diffusion step appears to cause a certain small reduction of stress but this may be due to the slow cooling after the drive-in step.

A very interesting aspect of this study is the pos­sibility of a connection between the stresses meas­ured and interface states. Large numbers (1012-1013 cm-2) of interface states are present at the interface after a dry oxidation6 or P20 5 diffusion7

step. The number of these states is strongly de­pendent on the orientation of the silicon surface6,8

while the residual stresses measured at the inter­face are not. Furthermore we have found that sam­ples oxidized in wet O2 (d.p.t. 90°C) contain only about lOll cm-2 interface states which is about a factor ten to one hundred less in number than measured using samples which had been subjected to a P20 5 diffusion step or oxidized in dry O2 (d.p.t. -40°C): The stresses between the samples

264

Maximum Stress in Oxide Stress the Silicon (dynes/cm2) (dynes/cm2)

2.8 X lOll 9.6 X 107

2.7 X lOll 9.7 X 107

2.0 X lOll 8.7 X 107

2.6 X 109 11.5 X 107

3.0 X 109 8.8 X 107

oxidized in wet and dry O 2 differ only by a factor of about 1.5 and the samples after the P20 5 dif­fusion step have even less stress at the interface than the samples oxidized in wet O2, The various preceding factors indicate that the stresses existing at an oxidized silicon interface are not capable of causing any significant number of interface states.

Some efforts were also made to relate induced stress and interface states density. The slope of a high-frequency C-V curve gives a reasonable indi­cation of interface state density in the mid-gap region of the silicon. C-V curves of a metal oxide­silicon diode were measured while pressure was applied to a metal contact on the oxide. No notice­able change occurred in the slope of the C-V curve up to the point at which the sample shattered. Another test consisted of again measuring the high frequency C-V curves of a MOS diode, made on a thin (100 JLM) silicon bar, while the bar was being bent. No noticeable change occurred in the slope of the C-V curves up to the point of breaking of the bar.

It is a pleasure to acknowledge the helpful criticism of E. Kooi, Prof. L. J. Tummers, and P. A. H. Hart.

1 K. Bulthuis, Philips Res. Repts. 20, 415 (1965). 2 R. Glung, R. A. Holmwood, and R. L. Rosenfeld, Rev. Sci.

Instr. 36, 7 (1965). 3].]. Wortman and R. A. Evans,]. Appl. Phys. 36, 153 (1965). 4R. ]. Jaccodine and W. A. Schlegel,.J. Appt. l'hys. 37, 2429

(1966). 5G. Hetherington and K. H. Jack, Phys. and Chemistry of Glasses

3, 129 (1962). ·P. V. Gray and D. M. Brown, Appl. Phys. Letters 8,31 (1966). 7M. V. Whelan, Philips Res. Repts. 20, 562 (1965). 8M. V. Whelan, to be published in Philips Res. Repts. (1967).

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