System of optical noncontact microtopography

Manuel F. M. Costa and Jose B. Almeida

In this paper we describe a method of noncontact optical microtopography based on discrete triangulation.We show that a light beam with an oblique incidence on a surface can be used to assess the distance of thelatter to a reference plane if the bright spot produced on the surface is imaged onto an array of detectorsthat tracks its lateral displacement. The light beam is swept over the surface so that large areas can bescanned. The authors have used their system with success for the topographic inspection of severalsurfaces, e.g., thin copper and silver films, polyethylene rough films and molds, graphite, machinedmetallic parts, and fabrics.

Key words: Microtopography, profilometry, triangulation, rough surfaces.

1. Introduction

Better knowledge of a product's physical characteris-tics is essential if quality control is to improve inindustrial companies. Several surface evaluation sys-tems have been developed. Among these, noncon-tact optical systems have attracted well-deservedattention because of their particularly good adapta-tion to specific applications.

Roughness measurements and microtopographicinspection of rough surfaces requiring measuringranges from some micrometers to a few millimeters,for roughness measurements from a few to hundredsof micrometers preserving a high lateral resolution,are subjects of growing concern, and several profilersand microtopographers have recently been reported.Methods and techniques (interferometric, scattering,triangulation, fringe projection, etc.) that have beenknown for a long time are being rediscovered anddeveloped to meet new requirements reliably andaccurately.1-'2 Specifically triangulation-based meth-ods are being used extensively in range sensing, inthree-dimensional (3-D) shape inspection, in topo-graphic evaluation, and even in the microinspectionof rough surfaces.2,3,6' 2 Although area methods,such as fringe projection and moir6, have proveduseful in many situations and the inherent problemsof heavy data processing seem no longer to be a majordrawback, the point-by-point inspection approach hasa clear advantage in simplicity and especially inflexibility.

The authors are with the Department of Physics, University ofMinho, p-4719 Braga Codex, Portugal.

Received 23 March 1992.0003-6935/93/25486004$06.00/0.© 1993 Optical Society of America.

In this paper we describe a noncontact-optical-microtopographyl system that is discrete triangula-tion based. It was developed first for the determina-tion of fabric parameters. It has been used withsuccess for measuring the surface roughness of ma-chined surfaces" and the thickness of silver 0 andcopper 3 thin films for the topographic inspection ofthe edge of silver films sputtered through differentmasks,' 0 for the surface inspection of polyethylenefilms and graphite samples, and for the noncontactmeasurement of fabric thickness and relief map-ping.9,10,12

2. Principles

The method is based on the triangulation procedureillustrated in Fig. 1. The surface being inspected isswept point by point by an oblique light beam, whichcreates a bright spot on it by diffuse reflection. Thelateral displacements are tracked by a microscopicsystem with the axis perpendicular to the surfaceplane.

Figure 2 illustrates the method: The intersectionof the light beam with the surface creates a brightspot at position X, the real sampled surface point,which corresponds to the P,, position on the sweepingprocess that is the intersection of the direction of thelight beam with the reference level. The lateral shiftof the bright spot 8,, is proportional to the normaldistance between the surface and the reference level:

Z = (/M)cot T,

where q is the angle of incidence of the light beam andM is the magnification of the microscopic optics.

In actual use the light beam is stationary and thesurface is moved laterally by equal increments in the

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Observation

Incidence

Surface

Fig. 1. Our triangulation geometry.

direction defined by the intersection of the plane ofincidence and the plane of the surface. Usually theincrements of the sweep A are known values, and onlythe lateral shifts of the bright spot 8,, must bemeasured and recorded for later processing.

Assuming that the position Po is the origin of thecoordinate system, we obtain the following:

P = nA, the sampling position.Xn= nA - n, the X coordinate of the nth real

sampled surface point.Zn = (8n/M)Cot TI, the surface's height at point Xn.

The profile of the surface, i.e., the line where thesurface intersects with the incidence plane, is the linelinking all the coordinate points:

Xn = nA - 8n,

Zn = (8n/M)cot Tl.

To obtain a 3-D map of the surface, we take severalprofiles separated by a distance CF normal to theincidence plane. Thus the complete set of coordi-nates is

Xn m = nA -8nm

Ynm = mqD,

Zn,m = (8n,m/M)ctg TI.

The whole system is calibrated by means of a smoothreference surface, which has precise displacementsalong its normal. At each vertical position of thesurface, the shift incurred by the bright spot isrecorded, permitting experimental determination ofthe conversion factor (ctg T)/M, which is used for

Incidence 8 nm

IQ I

X Pn,m n,m

Fig. 2. Basic principles. The intersection of a light beam with asurface creates a bright spot whose lateral position depends on thesurface height. The relation between height Zn and spot shift 8n isZ. = n ctg'q .

later measurements while the configuration staysunchanged. (It keeps the same incidence angle andmagnification.) For different configurations the con-version factors can be calculated easily or experimen-tally determined. Once the relationship between8

nm and Z,,,m has been established through a calibra-tion procedure, the system becomes relatively im-mune to geometric distortions and lens aberration.' 4

The accuracy of triangulation-based profilometry isdirectly affected by the accuracy at which X,, (theposition of the imaged spot) can be measured andtranslated into Z,, coordinates.14 The depth resolu-tion of our system is limited in essence by themodified Rayleigh limit7

With a minimum resolvable lateral shift of the spot,

bmin = /sin ao,

where sin a0 is the aperture of the reception micro-scopic optical system (Fig. 1) and X is the wavelengthof the incident beam.

The depth resolution will be

Zmin = (/sin ao0 )cotq.

The use of lasers as illuminating sources, which isrecommended because of their monochromaticity,stability, high intensity, low cost, and user friendlycharacteristics, nevertheless disallows higher resolu-tions on inspection of optically rough surfaces be-cause of the speckle effects. Speckle reduction tech-niques, such as spatial averaging,7"14"15 may allow abetter resolution limited by technical considerationssuch as mechanical noise and the physical and electri-cal characteristics of the observation system. Thecalibration procedure outlined above may resolveeven the diffraction limit of the imaging microscopicsystem.14

One problem common to all triangulation-basedinspection systems is the existence of shaded (hidden)areas that go uninspected on the surface or object tobe analyzed and the mutual reflections effect. Toovercome these difficulties, we performed two scan-nings of the sample with opposite angles (TI and -TI).

The measuring range will depend on the particularsystem's implementation, and ranges from severaltens of micrometers to some millimeters with a depthresolution from some tens of micrometers to severalmicrometers are possible. The area to be analyzedcan be as large as desired with positioning resolutionsin the submicrometer range permitted by precise X, Ypositioning stages.

3. Systems

Different implementations of this method are possible.We have built a system (Fig. 3) that is both simple andversatile, leading to possible applications to differentsituations: thickness measurement, roughness mea-surement, and relief mapping of samples, which aresmooth or optically rough, requiring different measur-ing ranges and resolutions.

The light source used is an He-Ne TEMOO laser

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Fig. 3. System: 1, He-Ne laser; 2, laser supporting structure; 3,incidence optical system; 4, sample support and positioning setup;5, reception optical system; 6, camera; 7, sample positioning anddata acquisition control system.

operating at 632.8 nm with 1 mW of power. Thelaser is mounted on a rotational stage that permits in-cidence angle changes. The incident optical systemcomprises a neutral-density filter for optical powercontrol and a lens system that focuses the beam ontoa diffraction-limited spot of reduced dimensions.

The reception optical system, which tracks theposition of the bright spot, consists of a microscopeobjective (1 Ox) that can be changed easily (5 x, 20 x)to suit the type of surface to be analyzed. Althoughan a priori knowledge, even if only qualitative andsubjective, of the characteristics of the surface reliefor sample's dimensions is useful for obtaining thebest results, it is not mandatory. A macroscope,which is a system of lenses characterized by a largedepth of field (> 6.5 mm), long working distance (32mm to infinity) with variable magnifications (2.8-10x or even 20x with loss of measuring range), canbe used easily in place of the microscope objectiveswhen the sample shows great unevenness and espe-cially when 3-D objects are to be inspected.

Tied to the reception optical system is an electronicimaging device that permits the automatic measure-ment of the spot's position. We used two systems:

A linear CCD array, with 2048 elements and a13-[Lm pitch operating at 15 MHz, located aligned onthe incidence plane. Currently it is used mainlywhen measuring smooth surfaces. The edges of thespot are defined by a certain threshold level, and theyrepresent the spot position9 at each sampling point.The process is quite simple, and high speeds arepossible. When speckle effects become important,limiting the overall resolution, speckle reduction tech-niques7"15 may be implemented.

As an alternative, a video camera can be used; thisis a CCIR standard with a Saticon tube. The imageplane of the camera is parallel to the reference (thesample's support) surface, and its line scan directionis perpendicular to the plane of incidence. Thus, themovement of the bright spot will be imaged byvertical movement on the video image. The videosignal output is integrated over each camera line.Thus a type of spatial averaging is easily imple-mented, and higher resolutions with rough surfaces

are obtained. The camera is used to measure thick-ness and whenever velocity and the number of sample-point requirements are not strict.

We chose to scan the sample surface by moving thesample under a stationary light beam. This solutionposes less alignment, and optical problems other thanthe reverse method, and larger areas can be scannedat high speeds with higher positioning accuracy andrepeatability. The motorized sample-positioningsetup consists of a X, Y precision linear stage movedby two-step motors that permit the sampling of pointson a rectangular array separated by distances assmall as 1 m. A high-precision rotational stageequipped with a step motor is used to help positionthe samples, to help in the 3-D inspection of smallirregular objects, and especially to permit an easychange of the light incidence to resolve the problem ofshaded areas and mutual reflections. (Usually oneuses high-incidence angles, depending on the surfacecharacteristics, to obtain higher conversion factors.)A vertical movement precision stage is controlled by acomputer through a reliable and accurate dc encoderwith high positioning repeatability and resolution(- 0.1 plm). It is used in the calibration procedureand also to keep the incidence and reception opticalsystems in focus over the sample. The utilization ofthis kind of focus-sensing technique, although at theexpense of processing speed (this option is to be usedwhenever the relief of the sample is required), per-mits us to achieve high dynamic ranges up to 1:25000with the present system configuration.

A personal computer controls the entire process:sample positioning, data acquisition, data processing,and results in the form of statistical parameters, suchas rms roughness and profile or 3-D relief plots.

4. Applications

As we mentioned in Section 1 the system was devel-oped for use in thickness measurements and topo-graphic inspection of fabric samples. Several kindsof fabric have been tested and, as far as we know, forthe first time without deforming the samples in themeasuring processes.

For example, Fig. 4 shows the relief map (with apositioning resolution of 2.5 pm and a height resolu-

Fig. 4. Microtopographic inspection of one thread of a fabric.

4862 APPLIED OPTICS / Vol. 32, No. 25 / 1 September 1993

tion of 1 ,um) of a 500 ,um x 300 [um area of thesurface of one thread of a linen fabric.

Other sample types have already been inspectedwith promising results,912 which prove the system'sflexibility. They include thickness measurements ofsilver and copper sputtered films, topographic inspec-tion of the edges of thin silver films produced bysputtering with a planar magnetron source throughdifferent masks, for which we tuned the system togive the best resolution of a few tenths of a microme-ter, roughness measurements of machined surfacesand polyethylene molds, and microtopographic inspec-tion of polyethylene films and graphite samples.

5. Conclusion

Microinspection of surfaces with optical systems basedon discrete triangulation is possible with good reliabil-ity and accuracy. The proposed configuration, de-spite its inherent limitations, presents advantagesover other methods, especially because of its versatil-ity and applicability to the inspection of rough, nonop-tical, surfaces.

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