System of optical noncontact microtopography

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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 the latter to a reference plane if the bright spot produced on the surface is imaged onto an array of detectors that tracks its lateral displacement. The light beam is swept over the surface so that large areas can be scanned. The authors have used their system with success for the topographic inspection of several surfaces, e.g., thin copper and silver films, polyethylene rough films and molds, graphite, machined metallic parts, and fabrics. Key words: Microtopography, profilometry, triangulation, rough surfaces.

1.

Introduction

Better knowledge of a product's physical characteristics is essential if quality control is to improve in industrial companies. Several surface evaluation systems have been developed. Among these, noncontact optical systems have attracted well-deserved attention because of their particularly good adaptation to specific applications. Roughness measurements and microtopographic inspection of rough surfaces requiring measuring ranges from some micrometers to a few millimeters, for roughness measurements from a few to hundreds of micrometers preserving a high lateral resolution, are subjects of growing concern, and several profilers and microtopographers have recently been reported. Methods and techniques (interferometric, scattering, triangulation, fringe projection, etc.) that have been known for a long time are being rediscovered and developed to meet new requirements reliably and accurately.1-'2 Specificallytriangulation-based methods are being used extensively in range sensing, in three-dimensional (3-D) shape inspection, in topographic evaluation, and even in the microinspection of rough surfaces.2,3,6' 2 Although area methods, such as fringe projection and moir6, have proved useful in many situations and the inherent problems of heavy data processing seem no longer to be a major drawback, the point-by-point inspection approach has a clear advantage in simplicity and especially in flexibility.

In this paper we describe a noncontact-opticalmicrotopographyl system that is discrete triangulation based. It was developed first for the determination of fabric parameters. It has been used with success for measuring the surface roughness of machined surfaces" and the thickness of silver 0 and copper 3 thin films for the topographic inspection of the edge of silver films sputtered through different masks,' 0 for the surface inspection of polyethylene films and graphite samples, and for the noncontact measurement of fabric thickness and relief mapping.9,10,12 2.

Principles

The method is based on the triangulation procedure illustrated in Fig. 1. The surface being inspected is swept point by point by an oblique light beam, which creates a bright spot on it by diffuse reflection. The lateral displacements are tracked by a microscopic system with the axis perpendicular to the surface plane. Figure 2 illustrates the method: The intersection of the light beam with the surface creates a bright spot at position X, the real sampled surface point, which corresponds to the P,, position on the sweeping process that is the intersection of the direction of the light beam with the reference level. The lateral shift of the bright spot 8,, is proportional to the normal distance between the surface and the reference level: Z

The authors are with the Department of Physics, University of Minho, p-4719 Braga Codex,Portugal. Received 23 March 1992.

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Optical Society of America. APPLIED OPTICS / Vol. 32, No. 25 / 1 September 1993

=

(/M)cot

T,

where q is the angle of incidence of the light beam and M is the magnification of the microscopic optics. In actual use the light beam is stationary and the surface is moved laterally by equal increments in the

Observation Incidence

Surface

Fig. 1. Our triangulation geometry.

direction defined by the intersection of the plane of incidence and the plane of the surface. Usually the increments of the sweep A are known values, and only the lateral shifts of the bright spot 8,, must be measured and recorded for later processing. Assuming that the position Po is the origin of the coordinate system, we obtain the following: P

=

Xn=

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

sampled surface point. TI, the surface's height at point Xn. Zn = (8n/M)Cot The profile of the surface, i.e., the line where the surface intersects with the incidence plane, is the line linking all the coordinate points: Xn = nA - 8n, Zn = (8n/M)cot

Tl.

To obtain a 3-D map of the surface, we take several profiles separated by a distance CFnormal to the incidence plane. Thus the complete set of coordinates is

nA -8nm

Xn m =

TI.

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

The whole system is calibrated by means of a smooth reference surface, which has precise displacements along its normal. At each vertical position of the surface, the shift incurred by the bright spot is recorded, permitting experimental determination of the conversion factor (ctg T)/M, which is used for 8

IQ

bmin= /sin ao, where sin a0 is the aperture of the reception microscopic optical system (Fig. 1) and Xis the wavelength of the incident beam. The depth resolution will be Zmin = (/sin

The use of lasers as illuminating sources, which is recommended because of their monochromaticity, stability, high intensity, low cost, and user friendly characteristics, nevertheless disallows higher resolutions on inspection of optically rough surfaces because of the speckle effects. Speckle reduction techniques, such as spatial averaging,7"14"15 may allow a better resolution limited by technical considerations such as mechanical noise and the physical and electrical characteristics of the observation system. The calibration procedure outlined above may resolve even the diffraction limit of the imaging microscopic

nm I

One problem common to all triangulation-based inspection systems is the existence of shaded (hidden) areas that go uninspected on the surface or object to be analyzed and the mutual reflections effect. To overcome these difficulties, we performed two scannings of the sample with opposite angles (TIand -TI). The measuring range will depend on the particular system's implementation, and ranges from several tens of micrometers to some millimeters with a depth resolution from some tens of micrometers to several micrometers are possible. The area to be analyzed can be as large as desired with positioning resolutions in the submicrometer range permitted by precise X, Y positioning stages. 3.

X Fig. 2.

Basic principles.

n,m

P

n,m

The intersection of a light beam with a

surface creates a bright spot whose lateral position depends on the surface height. . Z. = n ctg'q

ao0 )cotq.

system.14

Ynm = mqD,

Incidence

later measurements while the configuration stays unchanged. (It keeps the same incidence angle and magnification.) For different configurations the conversion factors can be calculated easily or experimentally determined. Once the relationship between 8 nm and Z,,,m has been established through a calibration procedure, the system becomes relatively immune to geometric distortions and lens aberration.' 4 The accuracy of triangulation-based profilometry is directly affected by the accuracy at which X,, (the position of the imaged spot) can be measured and translated into Z,, coordinates.14 The depth resolution of our system is limited in essence by the modified Rayleigh limit7 With a minimum resolvable lateral shift of the spot,

The relation between height Zn and spot shift 8n is

Systems

Different implementations of this method are possible. We have built a system (Fig. 3) that is both simple and versatile, leading to possible applications to different situations: thickness measurement, roughness measurement, and relief mapping of samples, which are smooth or optically rough, requiring different measuring ranges and resolutions. The light source used is an He-Ne TEMOOlaser 1 September 1993 / Vol. 32, No. 25 / APPLIED OPTICS

4861

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 and data acquisition control system.

operating at 632.8 nm with 1 mW of power. The laser is mounted on a rotational stage that permits incidence angle changes. The incident optical system comprises a neutral-density filter for optical power control and a lens system that focuses the beam onto a diffraction-limited spot of reduced dimensions. The reception optical system, which tracks the position of the bright spot, consists of a microscope objective (1Ox) that can be changed easily (5x, 20 x) to suit the type of surface to be analyzed. Although an a priori knowledge, even if only qualitative and subjective, of the characteristics of the surface relief or sample's dimensions is useful for obtaining the best results, it is not mandatory. A macroscope, which is a system of lenses characterized by a large depth of field (> 6.5 mm), long working distance (32 mm to infinity) with variable magnifications (2.810x or even 20x with loss of measuring range), can be used easily in place of the microscope objectives when the sample shows great unevenness and especially when 3-D objects are to be inspected. Tied to the reception optical system is an electronic imaging device that permits the automatic measurement of the spot's position. We used two systems: A linear CCD array, with 2048 elements and a 13-[Lmpitch operating at 15 MHz, located aligned on the incidence plane. Currently it is used mainly when measuring smooth surfaces. The edges of the spot are defined by a certain threshold level, and they represent the spot position9 at each sampling point. The process is quite simple, and high speeds are possible. When speckle effects become important, limiting the overall resolution, speckle reduction techniques7 "15 may be implemented. As an alternative, a video camera can be used; this is a CCIR standard with a Saticon tube. The image plane of the camera is parallel to the reference (the sample's support) surface, and its line scan direction is perpendicular to the plane of incidence. Thus, the movement of the bright spot will be imaged by vertical movement on the video image. The video signal output is integrated over each camera line. Thus a type of spatial averaging is easily implemented, and higher resolutions with rough surfaces 4862

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

are obtained. The camera is used to measure thickness and whenever velocity and the number of samplepoint requirements are not strict. We chose to scan the sample surface by moving the sample under a stationary light beam. This solution poses less alignment, and optical problems other than the reverse method, and larger areas can be scanned at high speeds with higher positioning accuracy and repeatability. The motorized sample-positioning setup consists of a X, Y precision linear stage moved by two-step motors that permit the sampling of points on a rectangular array separated by distances as small as 1 m. A high-precision rotational stage equipped with a step motor is used to help position the samples, to help in the 3-D inspection of small irregular objects, and especially to permit an easy change of the light incidence to resolve the problem of shaded areas and mutual reflections. (Usually one uses high-incidence angles, depending on the surface characteristics, to obtain higher conversion factors.) A vertical movement precision stage is controlled by a computer through a reliable and accurate dc encoder with high positioning repeatability and resolution (- 0.1 plm). It is used in the calibration procedure and also to keep the incidence and reception optical systems in focus over the sample. The utilization of this kind of focus-sensing technique, although at the expense of processing speed (this option is to be used whenever the relief of the sample is required), permits us to achieve high dynamic ranges up to 1:25000 with 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, such as rms roughness and profile or 3-D relief plots. 4.

Applications

As we mentioned in Section 1 the system was developed for use in thickness measurements and topographic inspection of fabric samples. Several kinds of fabric have been tested and, as far as we know, for the first time without deforming the samples in the measuring processes. For example, Fig. 4 shows the relief map (with a positioning resolution of 2.5 pm and a height resolu-

Fig. 4. Microtopographicinspection of one thread of a fabric.

tion of 1 ,um) of a 500 ,um x 300 [um area of the surface of one thread of a linen fabric. Other sample types have already been inspected with promising results,91 2 which prove the system's flexibility. They include thickness measurements of silver and copper sputtered films, topographic inspection of the edges of thin silver films produced by sputtering with a planar magnetron source through different masks, for which we tuned the system to give the best resolution of a few tenths of a micrometer, roughness measurements of machined surfaces and polyethylene molds, and microtopographic inspection of polyethylene films and graphite samples. 5.

Conclusion

Microinspection of surfaces with optical systems based on discrete triangulation is possible with goodreliability and accuracy. The proposed configuration, despite its inherent limitations, presents advantages over other methods, especially because of its versatility and applicability to the inspection of rough, nonoptical, surfaces. References

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