Characterization of Bulk Metallic Contamination in Epitaxial Wafers

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Characterization of Bulk Metallic Contamination in Epitaxial Wafers

Sangwook Wee,^∗ Sungwook Lee, Byungseop Hong and Boyoung Lee R & D Center, LG Siltron, Gumi 730-724

(Received 19 November 2005)

Metal contamination is the dominant cause of device failure. In the case of an epitaxial (Epi) wafer, trace metal may well be in the bulk as well as on the surface. Generally, we can analyze it in the bulk using Surface Photo Voltage (SPV) or Micro-Photo Conductance Decay (µ-PCD), but the metal contamination in an epi wafer on a heavily doped substrate, for example p/p+, cannot be measured by those methods. We can analyze the metal contamination in the bulk using the photolu- minescence (PL) and local etching Inductively Coupled Plasma Mass Spectrometry (LE/ICP-MS).

The PL technique at room temperature is applied to evaluate the metallic contamination in the epi layer and the epi substrate, and the LE/ICP-MS can offer us the qualitative and quantitative results on infinitesimal metal contamination. We can also get the depth profile of the trace metal in epi wafer (p/p+) by using those methods. In this paper, we explain the effect on the device of infinitesimal metal on an epi substrate.

PACS numbers: 81.05.Cy, 61.72.Tt

Keywords: Epitaxial wafer, Metal contamination, Depth profile, PL, ICP-MS, Si, Cu, Ni, Fe

I. INTRODUCTION

Both crystal defects and metal contaminations are pre- sumed to be causes of device failure. With the reduction in the design rule in ultra-large-scale-integrated (ULSI) silicon technology, it is very important to eliminate those causes [1–3]. Silicon wafers with epitaxial grown films have been widely used in the semiconductor industry to manufacture bipolar transistors and advanced CMOS ULSI devices with improved electrical performance [4, 5]. The epitaxial layer can be made free of oxygen and carbon and with well-controlled dopant concentrations, and a perfect crystalline structure can be achieved. In addition, the epitaxial layer is made free from the crystal originated particles (COPs) that are common with non-epitaxial silicon wafers [6].

Metal contaminations, more than crystal defects can detrimentally affect the performance of microelectronic devices on the epitaxial (epi) wafers more [7]. Thus, it is of great importance to evaluate whether solubility or segregation-induced gettering mechanisms can describe the reaction pathways of a trace metal in silicon. A trace metal in a silicon wafer may well be in the bulk or on the surface. Both surface and bulk techniques are commonly used for monitoring metal contamination in wafer processing. Surface techniques, such as total reflection X- ray fluorescence spectroscopy (TXRF), time-of-fight secondary ion mass spectrometry (TOF-SIMS) and vapor- phase decomposition- Inductively Coupled Plasma Mass

∗E-mail: windy91@lgsiltron.co.kr; Fax: +82-54-470-6015

Spectrometry (VPD/ICP-MS) are generally suitable for the detection of surface-deposited contaminations. On the other hand, these techniques usually provide a lim- ited mapping capability only, and they are unsuitable in the case where contaminations are diffused into silicon, as may happen when contamination is introduced by using an epitaxial growth procedure, thermal treatment or device process. The bulk analysis methods, such as the lifetime technique, deep level transient spectroscopy (DLTS), or dynamic secondary ion mass spectrometry (D-SIMS), are suitable for the detection of bulk contamination [8,9]. The carrier lifetime is related to the metal contamination. A microwave-detected photoconductive decay (µ-PCD) and a surface photovoltage (SPV) are frequently used as methods to measure the lifetimes in silicon wafers without making any special test devices [10, 11]. They are, however, not applicable to lifetime measurements in p/p+ silicon epitaxial wafers because of the heavy boron doping in the substrates. The photo- luminescence (PL) technique can be used for non-contact characterization of silicon wafers and has been employed for a long time. In particular, analysis of the PL spectra for deep level assisted PL at liquid helium temperature was found to be very effective for defect identification [12, 13]. The room-temperature PL technique also has great practical advantages, such as high sensitivity, high spatial resolution, non-destructiveness, and no need for special sample preparation [14].

In this paper, we report a novel technique, the PL technique at room temperature, which has been designed to evaluate the metal contamination in the bulk of an epitaxial wafer. Also, additional sample preparations for -1160-

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Table 1. Scan modes and conditions to measure the PL intensity.

Sample Scan depth Laser wavelength Laser Power Remark

Surface 1 µm 532 nm 14.0 mV

PL Whole wafer

10 µm 823 nm 9.8 mV

AP PL Specimen 1 µm 532 nm 14.0 mV Depth profile

Fig. 1. Schematic diagrams of experimental procedure.

obtaining depth profiles by using both PL and ICP-MS are introduced. The D-SIMS technique is the most pop- ular method to measure trace metal in the bulk silicon.

However, in this study, it could barely detect infinitesimal metal contamination occurring during the wafering and epitaxial layer growth process. We also provide in- sight into the movement of metal impurities in an epi wafer. The origin of metal contamination is on the surface of substrate. Metal impurities are driven into the epi wafer during the epitaxial growth process without any additional diffusion process.

II. EXPERIMENTS

We investigated p/p+ epitaxial wafers. The substrate wafers were 200 mm in diameter, Czochralski-grown (CZ), and <100> oriented and had a thickness of 725 µm and a boron doping level of 1.35 × 10¹⁸ atom/cm³ (0.01 Ω·cm). A low temperature oxide (LTO) for additional backside getter was deposited. The initial oxygen concentration was specified to be 5.5 × 10¹⁷ atom/cm³ (11 ppma, New ASTM). Using the spin-coating method, these wafers were contaminated intentionally on the front surface with 4 metal elements (Fe, Ni,⁶³Cu,⁶⁵Cu). Each of the samples was spun dried with a solution contain- ing a single element solution of a specified concentration in 50, 100, or 500 ppb. The surface contaminations were calculated as 5 × 10¹⁰, 1 × 10¹¹, and 5 × 10¹¹atom/cm². After contamination, an epitaxial layer of 6 ± 0.5 µm was grown on the front side of the substrate wafer with

Fig. 2. Schematic diagrams of the local etching procedure.

a boron doping level of (1.0 ± 0.2) 10¹⁵atom/cm³(10 ∼ 14 Ω·cm). Without cleaning, we analyzed all samples for surface metal contamination by using VPD/ICP-MS.

Some of samples in the⁶⁵Cu contaminated group were analyzed for bulk metal contamination according to experimental scheme (Fig. 1). The room-temperature PL intensity was measured by using silicon photo enhanced recombination (SiPHER, Accent) [15]. The PL measure- ment conditions are given in Table 1. Angle polished PL (AP PL) measurements offered a depth profile of PL intensity from the metal contamination in the bulk layer.

(Angle polished: 11^◦51’) D-SIMS measurements were performed on a CAMECA IMS-6F spectrometer using 13 keV O⁺₂ primary ions, but, it did not detect any trace metals including⁶⁵Cu because of its high detection limit, so a local etching (LE) procedure, as shown in Fig. 2, was used. The sample prepared by using the LE procedure was analyzed using ICP-MS. The other samples (Fe, Ni and⁶³Cu) underwent the Breakdown voltage (BV) test.

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Fig. 3. Dependence of bulk metal contamination obtained by using LE/ICP-MS for different initial copper contamination levels (diamond shape). The ratio of measured Cu contamination to the initial Cu contamination concentration (square shape).

Fig. 4. Surface PL results for 1 and 10 µm depth scans vs.

the bulk metal contamination obtained by using LE/ICP-MS (⁶⁵Cu).

Phosphorous-doped poly Si of 100 nm in thickness was deposited and patterned for a gate electrode by using photolithography and a wet etch process. BV was measured on capacitors of 1 × 1 mm²at room temperature.

The range of BV is divided into four categories according to the electric field range –0 ∼ 6, 6 ∼ 8, 8 ∼ 10 and 10

∼ 13 MV/cm.

III. RESULT AND DISCUSION

1. Analysis of Metal Contamination in the Bulk layer

After epitaxial layer growth, we measured surface metal contaminations of all samples. ⁶⁵Cu contaminated samples were analyzed for bulk metal contamination by using D-SIMS, Surface PL, AP PL and LE/ICP-MS.

Depth profiles of ⁶⁵Cu, ⁶³Cu, Ni and Fe were observed

Fig. 5. Dependence of the normalized PL calculated by using a surface PL method (10 µm ) on the bulk metal contamination from the surface to 12 µm (LE/ICP-MS).

using D-SIMS and LE/ICP-MS. No metal was detected in the D-SIMS depth profile due to its high detection limit. The bulk metal contaminations measured by using LE/ICP-MS are shown in Fig. 3. The relationship between the measured contamination and the initial contamination is linear. The ratio of the measured contamination to the initial contamination was constant at 10 %.

These are the sums of four 6 µm steps from the surface to 24 µm.

Surface PL intensities for 1 and 10 µm depth scans were measured using SiPHER (Table 1). The results of the 1 and 10 µm depth scan are presented in Figs. 4 and 5. The regressions between the normalized PL intensity of the 10 µm depth scans and the sum of the bulk metal contamination from surface to 12 µm are presented in Fig. 5:

NPL(NormalizedPL) = (ReferencePL

−ContaminatedPL)/ReferencePL × 100. (1) The reference PL intensity is the PL intensity for the non contaminated sample (10 µm). The contaminated PL intensities are PL intensities for different copper contamination levels. The PL intensities (10 µm) express well the bulk copper concentration.

The AP PL was measured with SiPHER equipment (Table 1). The results of five 6 µm steps from the surface to 30 µm are expressed in Fig. 6. The bulk copper concentrations were measured using the LE/ICP-MS method for each 6 µm step from surface to 24 µm. Both Fig. 5 and Table 2 show the NPLs (1) of the AP PL intensities and the bulk copper concentrations at the same depths in of epitaxial wafer. Fig. 7 shows a regression plot of the NPL and impurity.

The results of the copper depth profile show that cop-

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Table 2. LE/ICP-MS and AP PL results.

Initial Copper ⁶⁵Cu-1 ⁶⁵Cu-2 ⁶⁵Cu-3 Ref.

(atom/cm²) 5 × 10¹⁰ 1 × 10¹¹ 5 × 10¹¹ None

LE/ICP-MS AP PL LE/ICP-MS AP PL LE/ICP-MS AP PL AP PL

Depth (um)

(atom/cm²) (mV) (atom/cm²) (mV) (atom/cm²) (mV) (mV)

6 5.43 × 10⁹ 2.10 4.53 × 10⁹ 2.24 1.81 × 10¹⁰ 2.13 2.41

12 ND 8.35 6.34 × 10⁹ 7.50 1.81 × 10¹⁰ 6.60 8.80

18 ND 4.76 9.05 × 10⁸ 4.50 1.09 × 10¹⁰ 3.90 5.03

24 ND 2.79 ND 2.58 1.09 × 10¹⁰ 2.34 2.91

Total 5.43 × 10⁹ 4.50 1.18 × 10¹⁰ 4.21 5.79 × 10¹⁰ 3.74 4.78

Table 3. Analysis methods for bulk infinitesimal metal contamination in the Epi wafer.

Bulk metal Classification layer Quality Quantity Remark

Surface PL Detect metals in 1 & 10 µm X O X

AP PL Detect metals in all depth O O X

LE/ICP-MS Detect metals in all depth 4 O O

D-SIMS Detect metals in all depth O O O D. L. : High

Fig. 6. Depth profile of normalized PL obtained by using the AP PL method and of the bulk metal contamination obtained by LE/ICP-MS (⁶⁵Cu). (open symbols: PL intensity, solid symbols: Bulk metal contamination).

per impurities can exist in the interface between the epitaxial layer and the substrate. The concentration from the surface to 24 µm is about 10 % of the initial contamination concentration, so we should assume that copper also exists in the bulk of heavy boron substrate or on the backside of wafer. Not only Fig. 3 but also Fig. 6 shows a good correlation between the PL intensity and the copper contamination.

Fig. 7. Dependence of the normalized PL calculated byus- ing the AP PL method on the bulk metal contamination (LE/ICP-MS).

2. Surface Metal vs BV failure rate

After epitaxial growth, the concentrations of surface metal impurities were measured using VPD/ICP-MS.

The results are shown in Fig. 8. The results for the Ni and Cu groups showed similar increasing trends. We assumed the metal impurity ascending from the surface of epitaxial substrate during the epitaxial growth process remained on the surface of epitaxial layer. Fe showed no relationship between the initial impurities on the substrate and the impurity remaining on the epitaxial layer

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Fig. 8. Surface analysis results for Fe, Ni, ⁶³Cu and⁶⁵Cu after an epitaxial growth process without cleaning vs. the initial contamination level.

Fig. 9. BV failure rate vs. initial contaminations of Fe, Ni and Cu.

(Fig. 8). When the Ni concentration on the surface of substrate was over 5 × 10¹¹ atom/cm², the BV failure rate was significantly increased. While in case of Cu and Fe, even when the concentration on the the surface of substrate was up to 5 × 10¹¹ atom/cm², the failure rate did not increase, as shown in Fig. 9.

IV. CONCLUSION

We investigated the infinitesimal metal contamination in an epitaxial wafer. The surface metal analysis was easier than the bulk metal analysis. We used two novel methods, AP PL and LE/ICP-MS. The PL technique at room temperature is applied to evaluate metallic contamination in the epitaxial layer and the substrate. The

LE/ICP-MS can offer qualitative and quantitative results for infinitesimal metal contamination. We could get a depth profile of the trace metal in an epitaxial wafer (p/p+) by using those two methods. The accuracy analysis results of infinitesimal bulk metal contamination will be presented from those methods. In the case of Cu, it was detected like slightly increasing on the surface of epitaxial layer in the initial contamination. Copper contamination on the surface of substrate ascended little with the surface of the epitaxial layer during growth but was detected at the interface more than it was at the surface and in the bulk [16, 17]. Metal impurities contaminated intentionally on the surface of substrate were investigated by surface metal analysis and BV test. Fe was not detected on the surface and didn’t affect the BV failure rate. Cu was detected very little on the surface and did not affect the BV failure rate, but Ni was detected in proportion to the initial contamination and affected the BV failure rate, which means that the criti- cal metal on the surface of an epi substrate is Ni. In Cu and Fe, there was little effect on BV failure regardless of the contamination on the surface of the epi substrate up to 5 × 10¹¹ atom/cm².

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