Multidomain Ellipsometry For Thin Film Process Control

From a metrology point of view, parameterization means that accurate measurements at afew selected wavelengths can give results that meet the requirements of productionapplications. To do this, a broad spectrum (white) light source is needed to increase thenumber of wavelengths beyond a selected few. When using a white light, however, theaccuracy of the measurement at individual wavelengths is actually decreased because thewavelength purity and light intensity at any one wavelength is low.

Fig. 2. Each angle has a different "blackout" period where D = 0; thus, using multiple AOIs allows measurement of a wide range of film thicknesses without order ambiguity.

Until recently, spectroscopic-like multiple wavelength ellipsometry in production toolshas been limited by the available spectral range and light intensity of conventional lightsources and optics and by maintenance problems associated with lamp intensity, includingshort lamp lifetime and alignment. Another approach used in multifunction ellipsometry ismeasuring at a few selected laser-generated wavelengths, performing parameterization andreducing the number of directly measured parameters. The result is improvement in processcharacterization power similar to that of research spectroscopic ellipsometry (SE), butwith significantly higher throughput, smaller spot size, higher repeatability and greatlysimplified modeling.

In another departure from research SE, multifunction ellipsometry em-ploys a rotatingcompensator (polarization modulator) on the incoming beam side rather than depending on arotating analyzer for measurement of the reflected beam. Unlike rotating analyzer designswhere only one sinusoidal function is determined, this configuration determines sin D as well as Cand cos D, ensuring that there is no ambiguityin phase measurement. In addition, a rotating compensator achieves higher resolution inregions where D approaches 0° and 180°.

In addition to capability, production ellipsometers need to demonstrate speed andreal-time data processing. The multidomain production system has demonstrated 4-wavelengthx 128 angle ellipsometric data collection at 5 sec per measurement site. While throughputvaries with the number of wavelengths and measurement sites, a typical five-point,multifunction analysis has demonstrated throughput of >80 wph and single wavelengthmeasurements at more than 100 wph.

Production applications

Fig. 3. Measurements at multiple laser wavelengths provide accurate thickness measurements of TiNx films up to 1200 Å thick.

Representative applications for this new multifunction ellipsometry tool includetitanium nitride (TiN_x) and polysilicon. All applications data presented inthis article were acquired with Rudolph's SpectraLASER.

TiN_x films are widely used in device production as barrier layers againstmetal migration. However, they are absorbing, making optical evaluation by conventionalellipsometry difficult. Combining multiple-laser wavelengths and simultaneous multiple AOIdata improves measurement accuracy for these films. For example, multiple AOI dataeliminate "blackout" thicknesses (Fig. 2) because the D measurement at different AOIs provides unambiguousthickness determination even when D at one AOIcrosses zero, a region of thickness insensitivity.

While TiN_x films thicker than 600 Å are opaque at 633 nm, they can beprobed to greater than 1200 Å at a wavelength of 458 nm (Fig. 3). Not only does themultilaser, multifunction capability allow analysis of thicker films, it providesadditional data on optical constants, which allows better control of file composition.

To eliminate order ambiguity on absorbing films such as polysilicon, multiple AOI datawork best when combined with multiple wavelength data. Using the 905 nm IR wavelength canremove ambiguity from film thickness determination in polysilicon and amorphous silicon (a-Si) films.

Polysilicon

Numerous applications exist for doped-poly, thick and amorphous silicon, poly on thinoxide and hemispherical grain (HSG) poly4. In these applications, the 905 nm laserwavelength extends measurement capabilities for both poly and a-Si films because of the deeper penetration at the 905 nm IR wavelength(Table 1).

HSG poly is a film of interest for advanced memory devices because its roughnessincreases its surface area, enabling storage of twice the electrical charge of non-HSfilms of similar thicknesses. This capability allows HSG poly to be used for changestorage capacitors.

While ellipsometric data acquisition for HSG films is straightforward, the accuracy ofthe measurement depends on the modeling method employed. The rough layer in HSG mayconsist of a mixture of void plus polysilicon nodules and underlying polysilicon, which isitself a mixture of crystalline and a -Si.Using a multiwavelength measurement model that accounts for both the rough layer and theunderlying polysilicon gives repeatable thickness measurements withprecisionsof<.5 Å, a good fit to the model. There is a strong linear correlation of theroughness layer model to atomic force microscopy R_max (peak-to-valley) values(Fig. 4).

Polysilicon is a difficult material to characterize by optical means because itsproperties depend strongly on deposition and anneal conditions, which can produce anonuniform composite of crystalline types. One way to accurately evaluate these films isby fitting measured data with the effective medium approximation (EMA) method. EMAmodeling determines both the film thickness and the amount of crystallization in the film.The basic assumption is that the effective index is proportional to the indices of a-Si and crystalline silicon weighted by thepercentage of each in the film.

The abundant data collected by multidomain ellipsometry enable a good fit to a filmmodel as well as an estimate of the degree of fit. By iteratively tightening the degree offit, this technique can obtain a good film model and provide n, k and film thicknessmeasurements. Well-fitted film constants are important to ensure accuracy between actualand calculated film thicknesses. For instance, an index error of only 0.092 in a 4000 Åpoly film can generate a film thickness error of 86 Å.

For poly evaluation, the volume fraction (V_f), the percentage of a-Si and single crystal silicon present in thepolysilicon is a useful parameter. In IC production, process changes and drift will affectthe degree of crystallinity of the films, and these changes need to be detected by thetransparent-film metrology tool. A multifunction system and EMA modeling can effectivelymonitor the V_f of a poly process.

Anti-reflective coatings 

Fig. 4. Roughness layers of HSG films produced at 580°C, 575°C and 570°C are measured by AFM and SpectraLASER.

Applications for anti-reflective (AR) coatings and back AR coatings requirereflectometry measurements. Current multidomain ellipsometers can offer the addedcapability of a UV reflectometer. Device manufacturers employ AR coatings in thephotolithography process to reduce the negative effects of standing waves on criticaldimension pattern resolution. In order to tailor film thicknesses for optimalanti-reflection performance, the optical indices and extinction coefficients of the ARcoating materials must be known.

Semiconductor manufacturers are interested in the optical properties of these AR filmsat their actual exposure wavelengths, for example, 365 nm, 248 nm and 193 nm. This can beachieved using the reflectometer capability in the multifunction transparent filmmetrology tool. The light source for this added capability is a 4000 hr deuterium lampthat provides stable output over the UV range from 190 nm to above 450 nm.

One set of film structures of current interest to device manufacturers includes verythin nitride/oxide, oxide/nitride/oxide, oxide/poly and similar composites. A typical filmmight consist of a 40 Å layer of nitride on 10 Å of oxide. The ellipsometric parameters D and Ccan be somewhat indeterminate under these conditions, so that an ellipsometer might seethe film as a single layer. The addition of DUV reflectance data can provide a thirdindependent data point in determining the composite structure.

Tool matching

Fig. 5. Despite a slow 0.3 Å organic growth, two multidomain tools provided thickness measurements that are matched to an average of 0.01 Å.

Tool accuracy, stability and the ability to accurately match multiple metrology systemsacross a fab are important concerns for the modern production environment. Measurementstability and dependable tool-to-tool matching allows a manufacturer to make identicalproduct regardless of which metrology tool was used to control the process and in whichfab the product was made. Determining tool stability is very difficult. Excellent toolstability and zero drift are critical for good long-term, tool-to-tool matching. Sincethere are no inherently stable thin film thickness standards available, the best way totest tool stability is through tool-to-tool matching.

In one recent tool-matching test, two identical multidomain film metrology systemsanalyzed four wafers: two with initial 30 Å and two with initial 60 Å oxides. Five-pointmeasurements were taken twice a day for 30 days. Summary data shown in Table 2 indicatethat the two tools matched to better than 0.1 Å (1 s),and each was stable to better than 0.065 Å (1 s)during this time frame. Over the same 30-day period, the data showed an increase in themeasured film thicknesses of ~0.3 Å per month (Fig. 5). This is a well-known adsorptiongrowth, typical of thin films, and is thought to be due to adsorption of molecularairborne contaminants.²

Reference wafers are traditionally used to calibrate metrology tools. This approach,however, could lead to false thickness readings because of the adsorption growth, which inturn can lead to adjustments of the metrology tool to compensate for the change inthickness.

As a result, a film deposition chamber controlled by this metrology tool wouldcorrespondingly be adjusted, producing progressively thicker films over time. Multidomainellipsomet ers do not require internal reference wafers; they produce repeatable resultsand remain closely matched.

The future of production ellipsometry

The semiconductor industry has begun the transition to 300 mm substrates. Estimates ofthe future value of a fully processed, 25-wafer cassette of 300 mm wafers exceed $1million, and processed 300 mm wafers are expected to have a value added cost of $40,000 to$50,000 each. In this environment, the significance of process yield is obvious. The roleof film metrology in process control is critical and implicit in the guidance beingsupplied to equipment developers by the device industry's 300 mm planning consortia.

Transparent film metrology applications seem to expand as advances are made ininstrumentation capability. Anticipated 300 mm and advanced 200 mm applications forsubquarter-micron circuits will involve rapid film mapping, improved repeatability andsuperior long-term, tool-to-tool matching for controllingthin(<35 Å) gate processes,and CVD and PVD film control, including poly, thin metal films, AR coatings, thickpolyimides, dielectrics and passivating layers. In oxidation and diffusion, measurementswill be required on ultrathin films, oxynitrides and advanced OPO and ONO structures.

In etch, there are applications calling for small-spot analyses, measurement of roughsurfaces and multiparameter film measurements with pattern recognition at optimum waferthroughput. In interconnects and CMP, unambiguous thick film measurements must be made ina pattern recognition environment, metal dielectric sandwiches must be evaluated andcomplex substrates must be characterized. Lithography applications are ever present, withmetrology demands now extending to small-spot UV measurements and parameter determinationfor AR coatings and back AR coatings at wavelengths out to 193 nm.

These production instruments achieve accuracies rivaling those of the best researchgrade ellipsometers, yet operate at a speed demanded by the manufacturing environment.This transformation in technology from the manual, fixed-angle, single-wavelengthproduction instruments of 20 years ago has cemented ellipsometry's position as a keymetrology method for advanced semiconductor manufacturing. 

References

1. G.E. Jellison, "Thin Solid Films," 290-291, 40-45 (1996).

2. Rudolph Technologies Inc., "Characterizing the Stability of avery thin oxide film," Technical Report (1998).