Comparison between UHV and ambient KPFM

In comparison to the results obtained by means of ambient KPFM, the SRAM cell has also been investigated with UHV KPFM at the Helmholtz-Zentrum Berlin. The conventional SRAM cell has been chosen for the comparative measurements because the lateral KPFM bias variation probed during the ambient KPFM measurements does not depend on the operation frequency (Sect. 6.1.1.2) or on the applied ac-bias (Sect. 6.1.1.3) which are different for the UHV and the ambient KPFM system (Sect. 3.2).

The ambient and UHV KPFM measurements have been performed across the n⁺pn⁺pn⁺ junction, which was used to demonstrate quantitative dopant profiling in Sect. 6.1.1.1 (Fig.

6.1). For direct comparison, the results of the ambient KPFM measurement from Fig. 6.1 are illustrated again together with the UHV KPFM results in Fig. 6.9. The schematic doping pattern of the SRAM cell is illustrated in Fig. 6.9(a), where also the investigated section line across the n⁺pn⁺pn⁺ junction is marked as a blue line.

The results of the ambient KPFM measurements are illustrated in the left part of Fig.

6.9. The simultaneously recorded surface topography and KPFM bias are given in Fig.

6.9(b) and (c), respectively. The investigated section line across the n⁺pn⁺pn⁺ junction is marked as a blue line in the KPFM bias image. The KPFM bias section line probed across the n⁺pn⁺pn⁺ junction is plotted and compared to the calculated energy differences [E_C − E_F(n)] and [E_V − E_F(p)] in Fig. 6.9(d). A lateral KPFM bias variation of approxi-mately 120 mV is probed between the n⁺-type and the ”n-channel” regions at the ambient KPFM measurements which is in good agreement with the calculated energy difference of 130 meV [Fig. 6.9(d)]. As discussed in Sect. 6.1.1.1, the CPD model predicts an energy difference of 990 meV between the n⁺-type and the ”n-channel” regions.

The results of the UHV KPFM measurements are shown in the right part of Fig. 6.9.

Also in the UHV KPFM setup the surface topography [Fig. 6.9(e)] and the KPFM bias [Fig. 6.9(f)] are probed simultaneously. The investigated section line across the n⁺pn⁺pn⁺ junction is marked as a blue line in the KPFM bias image [Fig. 6.9(f)]. The KPFM bias probed along the section line is compared to the calculated energy differences [E_C − E_F(n)]

and [E_V − E_F(p)] in Fig. 6.9(g). Note that zero on the U_K-scale of the UHV measurements is defined by a calibration procedure performed before the UHV KPFM measurements which includes the applied cantilever. A lateral KPFM bias variation of about 160 mV is probed between the n⁺-type and the ”n-channel” regions in the UHV KPFM measurement.

This is in reasonable agreement with the calculated energy difference of 130 meV.

76 6 RESULTS

Figure 6.9: Comparison between ambient and UHV KPFM measurements performed across the n⁺pn⁺pn⁺ junction in the SRAM cell. (a) Schematic doping pattern with marked section line, (b) ambient surface topography, (c) ambient KPFM bias with marked section line, (d) ambient KPFM bias section line (averaged over 10 scan lines) compared to the calculated energy differences [EC− E_F(n)] and [EV − E_F(p)], (e) UHV surface topography, (f) UHV KPFM bias with marked section line, (g) UHV KPFM bias section line (averaged over 10 scan lines) compared to the calculated energy differences [E_C− E_F(n)] and [E_V − E_F(p)].

6.1 Silicon static and dynamic random access memory cells 77

Some important conclusions can be drawn from the presented comparison between ambient and UHV KPFM measurements.

First of all, almost the same lateral KPFM bias variation is probed across the investigated n⁺pn⁺pn⁺ junction at the ambient and the UHV KPFM measurements. Note that the lateral KPFM bias variation is independent of the applied cantilever, i.e. n⁺-type con-ductive NSC15 Si cantilevers from MikroMasch for ambient and Pt-Ir coated cantilevers from Nanosensors for UHV KPFM measurements (Sect. 3.2). The lateral KPFM bias variation probed during the ambient and the UHV KPFM measurements can be explained satisfyingly with the energy differences between Fermi energy and respective band edge [EC − EF(n)] and [EV − EF(p)], i.e. with the new KPFM model. The small deviation of the lateral KPFM bias variation between ambient and UHV KPFM may on the one hand result from minor differences between the real dopant concentration and the values pro-vided from Veeco Instruments (Sect. 3.1.1). The ambient and UHV KPFM measurements have been performed on the same SRAM sample and on a similar n⁺pn⁺pn⁺ junction, but not on the very same local position in the array of repeating doping pattern. For example, a B-concentration of 8 × 10¹⁶cm⁻³ instead of the defined 2 × 10¹⁷ cm⁻³ in the ”n-channel”

region would result in a calculated energy difference of 160 meV instead of 130 meV. On the other hand, the small deviation between ambient and UHV measurements may result from the averaging due to the larger tip-sample distance in ambient KPFM (Sect. 5.2.6).

Another important aspect is the much higher lateral resolution of UHV KPFM. The narrow n-LDD regions [Fig. 6.9(a)] containing P with a concentration of 5 × 10¹⁸ cm⁻³, which are expected at the lateral positions of approximately -1.5 µm, -0.6 µm, 0.6 µm, and 1.5 µm in Fig. 6.9(d), are not resolved in ambient KPFM. However, those narrow n-LDD regions can be weakly resolved as approximately 200 nm broad, differently coloured surrounding of the central n⁺-region in the UHV KPFM bias image [Fig. 6.9(f)]. The UHV KPFM bias section line probed across the n⁺pn⁺pn⁺ shows a continuous characteristic in which the n-LDD regions appear as weakly pronounced saddle points at the lateral positions of approximately -1.6 µm, -0.6 µm, 0.6 µm and 1.6 µm [Fig. 6.9(g)]. Therefore, it can be concluded that UHV KPFM provides a much better lateral resolution than ambient KPFM and is recommended for electrical investigations of nanostructures.

The third fundamental conclusion concerns the negligible influence of ac-bias-induced band bending (Sect. 5.2.5). The ambient KPFM measurements have been performed with an effective ac-bias of 6 V. In UHV KPFM only 0.1 V ac-bias are applied. However, independent of the different ac-bias almost the same lateral KPFM bias variation is probed in ambient and UHV KPFM. Thus, ac-bias-induced band bending may occur during the KPFM measurement but has no influence on the probed lateral KPFM bias variation.

The final important remark concerns the KPFM offset bias due to potential shielding (Sect. 5.2.4). The ambient KPFM measurements have been performed on the untreated sample surface featuring a native oxide as well as adsorbed water (Sect. 4). For reducing surface contaminations and the thin water film, the SRAM sample has been cleaned with acetone and heated for one hour at 155^◦C before the UHV KPFM measurements (Sect.

3.2.2). Only the contribution of the native oxide layer on the SRAM cell is expected to be similar during the ambient and UHV KPFM measurements. Therefore, the KPFM

78 6 RESULTS

offset bias differs during the ambient and UHV KPFM measurements. However, indepen-dent of the surface treatment almost the same lateral KPFM bias variation is probed in ambient and UHV KPFM measurements. It can be concluded that independent of the sur-face condition in ambient and UHV KPFM the probed lateral KPFM bias variation may be used for quantitative dopant profiling. This holds true as long as the signal from the asymmetric electric dipole is not completely dominated by potential shielding (Sect. 5.2.4).

Although a comparison between ambient and UHV KPFM measurements has been per-formed only for one of the semiconducting samples discussed in this work, i.e. the con-ventional SRAM cell, the observed agreement of the lateral KPFM bias variation for both measurement types is promising. Further comparative measurements are recommended.

Nevertheless, these first results show that ambient KPFM as well as UHV KPFM are suitable for the quantitative investigation of the dopant concentration in semiconductors.

6.1.2 Dynamic random access memory cell