JOURNAL OF ELECTRON MICROSCOPY TECHNIQUE 18:325-328 (1991)
Simple Method for the Preparation of InP Based Samples for TEM Investigation ARPAD BARNA AND BBLA PfiCZ Research Institute for Technical Physics of the Hungarian Academy of Sciences, H-1325 Budapest, P.O. Box 76, Hungary
KEY WORDS
Ion milling, Ar' ion bombardment, Transparent region, InP
ABSTRACT
A novel, rapid, and simple method is described for the preparation of InP based samples for investigation by transmission electron microscopy (TEM). The key feature of the technique is A r + ion bombardment in a n iodine ambient. Cross sectional micrographs of AuiInP samples are shown as a n example. The technique developed produces a large area of transparent region.
INTRODUCTION In most cases a n Art ion beam is used for ion milling of semiconductor samples. In the case of indium(1n)containing compound semiconductors, sample preparation is more difficult because Ar' ion bombardment during thinning causes the appearance of surface In dots. Since the sputtering yield for In atoms is lower than for phosphorus atoms the surface accumulation of relatively nonvolatile In is not surprising. Several papers have been published proposing solutions to this problem. The known methods are based on I' ion bombardment, applied by Chew and Cullis (1984) and Ivey and Piercy (19881, or on the bombardment with a n Arlo, gas mixture, used by Ivey and Piercy (1987).
MATERIALS AND METHODS In this paper we report a new method in which we use a n iodine vapor stream impinging on the sample surface bombarded by Art ions. We use milling equipment developed in our institute by Barna (19841,which has two independent, water cooled, high power ion sources operating between 3-10 kV. We observed both sides of the sample using a n optical microscope and by turning over the sample holder. We prepared cross sections from InP samples a s follows. Two small chips from each sample were bonded face to face, and embedded into a small aluminium disc, which had a diameter of 3 mm. For this purpose we used carbon-araldite mixture as a glue. The samples were thinned by mechanical grinding to approximately 50 pm, first using S i c abrasive paper followed by a polishing method first with 10 pm then with 1 pm grain size diamond paste. Dimpling was not applied. The mechanically prepared samples were finally thinned by ion milling using two guns bombarding the same surface of the sample; in this way we used a very small incident angle. We used Art ions a t 10 keV. The samples were milled at a n incident angle (y) of 3-5" to the specimen surface without sample cooling. The sample was rotating during the thinning, and during the last 20-30 minutes a special method proposed by Barna and Barna (1989) and Barna et a1.(1990) was
Q 1991 WILEY-LISS.
INC.
applied. In the case of layered structures consisting of different materials, inclined steps were formed due to the different sputtering rate of elements. The angles (and of course the height) of the inclined steps can be decreased rapidly using one gun only. At the same time, the sample is rocked 250" around the normal to the interfaces about a n axis normal to the plane of the sample. The ion milling of a sample about 50 pm thick took 1-1.5 hours from each side of the sample (the sample is turned over after about half of the sample is removed by thinning). During the last 5 minutes of ion milling only 3 kV of accelerating voltage for the Ar ion bombardment was applied in a n iodine ambient to remove the surface In dots. In our system (70 lisec effective pumping speed with a diffusion pump) the pressure increase due to the iodine inlet was 5 x mbar. In this way a smooth surface without segregated In dots could be obtained. Iodine is a dangerous material for vacuum components. That is why we did not apply i t through the original gas inlet of our ion milling unit. Iodine was introduced from a sublimating iodine crystal, placed in a glass capsule outside the vacuum chamber and directed to the sample surface through a copper tube. The glass capsule was connected to the tube through a simple valve made from quartz. The iodine flux could be adjusted by opening and closing the valve. The scheme of the experimental set-up is shown in Figure 1. We emphasize that during these experiments the rocking angle (0)was 250". Compared with earlier works we have solved the problem of ion milling of In-containing semiconductors without applying a n I + ion beam. We believe that iodine reacted with In-forming In-iodide, which has a higher sputtering yield than In. It should be mentioned that the optimum value of the partial pressure of iodine +
Received July 26, 1990; accepted in revised form October 22, 1990. Address reprint requests to Arpad Barna, Research Institute for Technical Physics of the Hungarian Academy of Sciences, H-1325 Budapest, P.O.Box 76, Hungary.
A. BARNA AND B. PQCZ
326
static:
UI
=O
rocked: O c I u k ~90" r o t a t e d : UJ = n.360' YJ
Fig. 1. Schematic diagram of ion milling set-up.
depends on the used vacuum pump. For example, when we use a turbomolecular pump with a pumping speed of 250 lisec the necessary pressure increase because of the iodine inlet was about 3 x mbar. The thinned samples were examined using JEOL 1OOU and a JEOL 1OOC transmission electron microscopes.
RESULTS AND DISCUSSION The thinned InP substrates are shown in Figure 2. Indium can be seen on the surface of the sample thinned by using only Ar ion bombardment (Fig. 2A). Applying the above described method the sample shown in Figure 2B was obtained. As a n example the cross section of a Au (100 nm)/InP(111) sample can be seen in Figure 3. To demonstrate our results we show here a low magnification bright field image of large transparent area of the sample. The iodine raises the sputtering yield of InP, but it does not raise the yield of gold. When we require a cross section of a metallized sample (e.g., for high resolution electron microscopy) we thin it using only Ari ion bombardment to obtain the required thickness. The milling process should be finished by as short a bombardment as possible in a n iodine ambient to remove the surface In. Higher magnification images of the AulInP sample are shown in the inserts of Figure 3A, D. The sample was annealed a t 375°C for 10 minutes in flowing forming gas. During the heat treatment elongated pits grew into the InP substrate. Using electron diffraction we identified AuJn, and Au,P, phases, which formed during the interaction of gold with InP.
Fig. 2. Bright field images of the surfaces of InP. A Without iodine treatment; €3: with iodine treatment.
To summarize our results, we have proposed here a simple and rapid method for the preparation of In-containing compound semiconductor samples for TEM. The method produces a large transparent area. Cross sectional micrographs of AulInP samples are shown as a n example.
ACKNOWLEDGMENTS The authors are grateful to P.B. Barna and G. Radnoczi for helpful discussions. Technical assistance by E. Szabo is also acknowledged.
InP BASED SAMPLES FOR TEM
Fig. 3. Cross sections of Au(100nm)/InP(111)samples annealed at 375°C. A-D: The low magnification images represent a continuously thin area of the interface (marked by arrows) with a total length of about 50 IJ-m.The inserts in A and D show two parts of the sample a t higher magnification.
327
A. BARNA AND B. PGCZ
328
REFERENCES
ment of large area surface topograhy during ion etching. Vacuum.
--".
An.115-13n I"_AAy
Barna, A. (1984) A new type of ion milling equipment for sample preparation. In: Proceedings of the 8th Eurpean Congress of Electron Microscopy, Budapest, Vol.1. A.Csanady, P. Rohlih, and D. Szabo, eds., pp. 107-108. Barna, A,, and Barna, P.B. (1989) Model considerations of ion beam thinning for preparing TEM samples. In: Proceedings of the 3rd Balkan Congress on Electron Microscopy, Athens. Lukas H. Margaritis, ed., pp. 246-249. Barna, A,, Barna, P.B., and Zalar, A. (1990) Analysis of the develop-
Chew, N.G., and Cullis, A.G. (1984) Iodine ion milling of indiumcontaining compound semiconductors. Appl. Phys. Lett., 44:142144. Ivey, D.G., and Piercy, G.R. (1987) Ion milling of InP specimens with AriO, for transmission electron microscopy. J. Electron Microsc. Tech., 7:107-108. Ivey, D.G., and Piercy, G.R. (1988) Cross-sectional TEM specimens of metal contacts to semiconductors.J. Electron Microsc. Tech., 8233235.
Simple Method for the Preparation of InP Based Samples for TEM Investigation ARPAD BARNA AND BBLA PfiCZ Research Institute for Technical Physics of the Hungarian Academy of Sciences, H-1325 Budapest, P.O. Box 76, Hungary
KEY WORDS
Ion milling, Ar' ion bombardment, Transparent region, InP
ABSTRACT
A novel, rapid, and simple method is described for the preparation of InP based samples for investigation by transmission electron microscopy (TEM). The key feature of the technique is A r + ion bombardment in a n iodine ambient. Cross sectional micrographs of AuiInP samples are shown as a n example. The technique developed produces a large area of transparent region.
INTRODUCTION In most cases a n Art ion beam is used for ion milling of semiconductor samples. In the case of indium(1n)containing compound semiconductors, sample preparation is more difficult because Ar' ion bombardment during thinning causes the appearance of surface In dots. Since the sputtering yield for In atoms is lower than for phosphorus atoms the surface accumulation of relatively nonvolatile In is not surprising. Several papers have been published proposing solutions to this problem. The known methods are based on I' ion bombardment, applied by Chew and Cullis (1984) and Ivey and Piercy (19881, or on the bombardment with a n Arlo, gas mixture, used by Ivey and Piercy (1987).
MATERIALS AND METHODS In this paper we report a new method in which we use a n iodine vapor stream impinging on the sample surface bombarded by Art ions. We use milling equipment developed in our institute by Barna (19841,which has two independent, water cooled, high power ion sources operating between 3-10 kV. We observed both sides of the sample using a n optical microscope and by turning over the sample holder. We prepared cross sections from InP samples a s follows. Two small chips from each sample were bonded face to face, and embedded into a small aluminium disc, which had a diameter of 3 mm. For this purpose we used carbon-araldite mixture as a glue. The samples were thinned by mechanical grinding to approximately 50 pm, first using S i c abrasive paper followed by a polishing method first with 10 pm then with 1 pm grain size diamond paste. Dimpling was not applied. The mechanically prepared samples were finally thinned by ion milling using two guns bombarding the same surface of the sample; in this way we used a very small incident angle. We used Art ions a t 10 keV. The samples were milled at a n incident angle (y) of 3-5" to the specimen surface without sample cooling. The sample was rotating during the thinning, and during the last 20-30 minutes a special method proposed by Barna and Barna (1989) and Barna et a1.(1990) was
Q 1991 WILEY-LISS.
INC.
applied. In the case of layered structures consisting of different materials, inclined steps were formed due to the different sputtering rate of elements. The angles (and of course the height) of the inclined steps can be decreased rapidly using one gun only. At the same time, the sample is rocked 250" around the normal to the interfaces about a n axis normal to the plane of the sample. The ion milling of a sample about 50 pm thick took 1-1.5 hours from each side of the sample (the sample is turned over after about half of the sample is removed by thinning). During the last 5 minutes of ion milling only 3 kV of accelerating voltage for the Ar ion bombardment was applied in a n iodine ambient to remove the surface In dots. In our system (70 lisec effective pumping speed with a diffusion pump) the pressure increase due to the iodine inlet was 5 x mbar. In this way a smooth surface without segregated In dots could be obtained. Iodine is a dangerous material for vacuum components. That is why we did not apply i t through the original gas inlet of our ion milling unit. Iodine was introduced from a sublimating iodine crystal, placed in a glass capsule outside the vacuum chamber and directed to the sample surface through a copper tube. The glass capsule was connected to the tube through a simple valve made from quartz. The iodine flux could be adjusted by opening and closing the valve. The scheme of the experimental set-up is shown in Figure 1. We emphasize that during these experiments the rocking angle (0)was 250". Compared with earlier works we have solved the problem of ion milling of In-containing semiconductors without applying a n I + ion beam. We believe that iodine reacted with In-forming In-iodide, which has a higher sputtering yield than In. It should be mentioned that the optimum value of the partial pressure of iodine +
Received July 26, 1990; accepted in revised form October 22, 1990. Address reprint requests to Arpad Barna, Research Institute for Technical Physics of the Hungarian Academy of Sciences, H-1325 Budapest, P.O.Box 76, Hungary.
A. BARNA AND B. PQCZ
326
static:
UI
=O
rocked: O c I u k ~90" r o t a t e d : UJ = n.360' YJ
Fig. 1. Schematic diagram of ion milling set-up.
depends on the used vacuum pump. For example, when we use a turbomolecular pump with a pumping speed of 250 lisec the necessary pressure increase because of the iodine inlet was about 3 x mbar. The thinned samples were examined using JEOL 1OOU and a JEOL 1OOC transmission electron microscopes.
RESULTS AND DISCUSSION The thinned InP substrates are shown in Figure 2. Indium can be seen on the surface of the sample thinned by using only Ar ion bombardment (Fig. 2A). Applying the above described method the sample shown in Figure 2B was obtained. As a n example the cross section of a Au (100 nm)/InP(111) sample can be seen in Figure 3. To demonstrate our results we show here a low magnification bright field image of large transparent area of the sample. The iodine raises the sputtering yield of InP, but it does not raise the yield of gold. When we require a cross section of a metallized sample (e.g., for high resolution electron microscopy) we thin it using only Ari ion bombardment to obtain the required thickness. The milling process should be finished by as short a bombardment as possible in a n iodine ambient to remove the surface In. Higher magnification images of the AulInP sample are shown in the inserts of Figure 3A, D. The sample was annealed a t 375°C for 10 minutes in flowing forming gas. During the heat treatment elongated pits grew into the InP substrate. Using electron diffraction we identified AuJn, and Au,P, phases, which formed during the interaction of gold with InP.
Fig. 2. Bright field images of the surfaces of InP. A Without iodine treatment; €3: with iodine treatment.
To summarize our results, we have proposed here a simple and rapid method for the preparation of In-containing compound semiconductor samples for TEM. The method produces a large transparent area. Cross sectional micrographs of AulInP samples are shown as a n example.
ACKNOWLEDGMENTS The authors are grateful to P.B. Barna and G. Radnoczi for helpful discussions. Technical assistance by E. Szabo is also acknowledged.
InP BASED SAMPLES FOR TEM
Fig. 3. Cross sections of Au(100nm)/InP(111)samples annealed at 375°C. A-D: The low magnification images represent a continuously thin area of the interface (marked by arrows) with a total length of about 50 IJ-m.The inserts in A and D show two parts of the sample a t higher magnification.
327
A. BARNA AND B. PGCZ
328
REFERENCES
ment of large area surface topograhy during ion etching. Vacuum.
--".
An.115-13n I"_AAy
Barna, A. (1984) A new type of ion milling equipment for sample preparation. In: Proceedings of the 8th Eurpean Congress of Electron Microscopy, Budapest, Vol.1. A.Csanady, P. Rohlih, and D. Szabo, eds., pp. 107-108. Barna, A,, and Barna, P.B. (1989) Model considerations of ion beam thinning for preparing TEM samples. In: Proceedings of the 3rd Balkan Congress on Electron Microscopy, Athens. Lukas H. Margaritis, ed., pp. 246-249. Barna, A,, Barna, P.B., and Zalar, A. (1990) Analysis of the develop-
Chew, N.G., and Cullis, A.G. (1984) Iodine ion milling of indiumcontaining compound semiconductors. Appl. Phys. Lett., 44:142144. Ivey, D.G., and Piercy, G.R. (1987) Ion milling of InP specimens with AriO, for transmission electron microscopy. J. Electron Microsc. Tech., 7:107-108. Ivey, D.G., and Piercy, G.R. (1988) Cross-sectional TEM specimens of metal contacts to semiconductors.J. Electron Microsc. Tech., 8233235.
