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A new method of specimen preparation for FIM and FEM
Surface Science 67 (1977) 605-610 0 North-Holland Publishing Company
A NEW METHOD OF SPECIMENPREPARATION FOR FIM AND FEM Received 17 February 1977; manuscript received in final form 6 June 1977
Metal tips for field ion microscopy (FIM) and field emission microscopy (FEM) usually have been prepared from a thin wire with a diameter of about 0.1 mm by electrolytic or chemical polishing. It is often desirable, however, to prepare tips from a thin sheet or even from inside of a block because this would extend the versatility of FIM and FEM by allowing us to obtain tips from metals of any shape. This has been realized for a thin sheet [ 1,2] by applying a photoetching method, and recently also for a block [3] by applying the photoetching method in combination with the non-strain acid-cut method [4,5]. In this paper the above techniques for obtaining sample needles from a sheet or a block are described in some detail, taking three actual cases as examples. (a) Tungsten needles from a 0.2 mm thick sheet (i) Spreading the photoresist: A sheet of tungsten, 15 X 20 X 0.2 mm, is boiled in a concentrated NaOH solution for 30 min to remove oils and fats which may prevent the photoresist from adhering well to the surface. The sheet is rinsed with water repeatedly and then dried. It is next immersed in a suitably diluted photoresist solution (Kodak micro resist 752, Eastman Kodak Co.) and taken out gently, and dried in a drying oven at 100°C for 30 min. This spreading and drying is repeated once again in order to minimize undesirable effects due to tiny air bubbles or dust which may be present in the solution and cause the occurrence of tine pinholes in the photoresist on the metal surface. (ii) Exposing to light through a photomask: After the photoresist is dried, photomasks are placed on both the surfaces of the sheet. The photomask is actually a sheet of photographic film with a pattern as shown schematically in fig. la. As the photoresist with these masks on it is exposed to light, a latent image of the pattern, narrow strips with a width 0.4 mm, each 0.4 mm apart; is made in the photoresist. The two photomasks on both sides of the sheet should coincide with each other as shown in fig. 1b. Two minutes of exposure to light from a high pressure mercury arc lamp (Toshiba SHLlOOUV, Tokyo Shibaura Denki Co.) set about 14 cm away from the specimen are enough for setting the resist. (iii) Developing: After the photomasks are removed, the sheet is soaked in a developing solution, which takes away that part of the photoresist not exposed to light, leaving the other part of the photoresist on the surface as an image of the transparent pattern. Two minutes of gentle agitation are sufficient. The sheet is then rinsed with a rinse solution and dried. 605
Y. Yashiro, T. Terao 1 Specimen preparation for FIM and FEM
606
u.v.llght
I
(a)
a
photomask
(c)
developing
(e)
removing
(b)
masking
(d)
etching
and
I
exposing
photoresist
1. A schematic diagram illustrating the procedure for making needles from a thin (0.2 mm) metal sheet. M: photomask, R: photoresist, S: sample sheet. Fig.
(iv) Hardening: The sheet is heated at 140°C for 30 min in order to harden the photoresist. (v) Etching: The sheet is then subjected to electrolytic etching at room temperature. A 7 : 1 mixture of CHsOH and concentrated Hz SO4 is used as an etching solution and the current density is 10 A/dm2. Molybdenum needles can also be prepared in just the same manner as described above. (b) Stainless steel needles from a I mm thick sheet A cleaned sheet of stainless steel, 15 X20 X 1 mm, is spread with a photoresist (OSPR, Oriental Sangyo Co.) and dried at 50°C for 30 min. The photomask pattern and the relative positions of the masks on the both surfaces of the sheet are as shown in fig. 2. The relative positions in this case differ from those in the preceding case so as to give metal needles as thin as possible from a rather thick metal sheet.
m
u.v.light (a)
masking
and
exposing
(c)
(b)
removing
etching
photoresist
Fig. 2. Making needles from a thick (1 mm) metal sheet. The relative positions of the photomasks are different from those in fig. lb. Letters S, R, and M are defined in fig. 1.
Y, Y4sW0,
T. Term
/
Specimen preparation for FIM and FEM
607
After 2 min exposure to light, it is soaked in a d~ve~op~~ solution for two minutes, rinsed with water, dried at 100°C for 30 min and then heated at 150°C for about an hour. The condition of electrolytic etching is the same as is described in example (c) below. (e) ~ru~~~e~s&Z preedfes fiotrz a 2 mnt rhick skeet This method can be applied to a metal block of any shape because we can first cut it to obtain a flat surface using the acid-cut method [4]. In this example, a stainless steel sheet with an area of 20 X40 mm and a thickness of 2 mm is used, and two needles are taken out from one surface, the other surface covered with the photoresist being left untouched. After spreading the photoresist (OSPR), two areas of 1 X 20 mm, 10 mm apart, are exposed to light as shown in fig_3. Electrolytic etching is performed using concentrated &PC, saturated with 00s (chromium trioxide) as an etching solution. The current density is 20-30 A/dm2, and the etching temperature is 65--70°C in
B
[b) masking
Dhotomask
and
exposing
(d) etching
developing
u.v.light adhesive
&’
positioning cellophane adhesive
tapes
(g) removing adhesive and etching
tape5
(h)
wmoving
photoreSist
Fjg. 3. A schematic diagram showing the procedure of producing needles from a flat surface.
Fig. 4. Photographs showing the end of stage (d) in fig. 3. this case. About 10 h of etching produce a pair of hillocks, whose tops are covered with photoresist, protruding from the flat plane as shown in fig. 4. The width of the ridge is about 0.2 mm and the height is about 0.5 mm. The etch factor E, whose definition is given in fig. 5, is 1.3-l .5 which seems to decrease with the lowering of etching temperature. After the sample sheet is dried, strips of cellophane adhesive tapes (width 0.2 mm, thickness 0.05 mm) are stuck on both of the feet of each hill-
Y. Yashiro, T. Terao / Specimen preparation for HM and FEM
609
&s-Q R
sample Fig. 5.
metal
Definition of the etch factor e.
ock, as shown in fig. 3e. The sheet is then dipped in the photoresist solution and dried. After exposure to light, the adhesive tapes are peeled off. This leaves narrow gaps in the photoresist which expose the naked parts of the stainless steel to the etchant in the next procedure. Further electrolytic etching detaches the needles from the sample sheet. The residual photoresist covering each needle is removed by immersion in a hot concentrated alkaline solution. A needle thus obtained is shown in fig. 6. The essential feature of the above method is that, in comb~ation with other non-strain cutting methods, any part of a metal block can be removed as a needle without straining the metal. The needles thus obtained may then be used for making specimens for observation in FIM and FEM.
610
Y. Yoshiro, T. Terao f Specimen prepararion for FIM and FEM
The authors are indebted to Dr. H. Morikawa and Mr. M. Kozakai for performing a part of this experiment. They also gratefully acknowledge Dr. A.J. Melmed of NBS for improving this manuscript. Yuzo YASHIRO and Terumi TERAO
Department of Coordinated Science, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466, Japan
References f l] Y. Yashiro and H. Ishihara, Oyo Buturi 36 (1967) 809 (in Japanese). [2] [3] [4] [S]
H. Ishihara, Bull. Nagoya Inst. of Tech. 19 (1967) 161 (in Japanese). Y. YashIro and T. Terao, 23rd Intern. Field Emission Symp., Pennsylvania, 1976. T. Imura, J. Phys. Sot. Japan 24 (1969) 468 (in Japanese). T. Tono and ‘I’.Imura, J. Cryst. Sot. Japan 18 (1967) 252 (in Japanese).
A NEW METHOD OF SPECIMENPREPARATION FOR FIM AND FEM Received 17 February 1977; manuscript received in final form 6 June 1977
Metal tips for field ion microscopy (FIM) and field emission microscopy (FEM) usually have been prepared from a thin wire with a diameter of about 0.1 mm by electrolytic or chemical polishing. It is often desirable, however, to prepare tips from a thin sheet or even from inside of a block because this would extend the versatility of FIM and FEM by allowing us to obtain tips from metals of any shape. This has been realized for a thin sheet [ 1,2] by applying a photoetching method, and recently also for a block [3] by applying the photoetching method in combination with the non-strain acid-cut method [4,5]. In this paper the above techniques for obtaining sample needles from a sheet or a block are described in some detail, taking three actual cases as examples. (a) Tungsten needles from a 0.2 mm thick sheet (i) Spreading the photoresist: A sheet of tungsten, 15 X 20 X 0.2 mm, is boiled in a concentrated NaOH solution for 30 min to remove oils and fats which may prevent the photoresist from adhering well to the surface. The sheet is rinsed with water repeatedly and then dried. It is next immersed in a suitably diluted photoresist solution (Kodak micro resist 752, Eastman Kodak Co.) and taken out gently, and dried in a drying oven at 100°C for 30 min. This spreading and drying is repeated once again in order to minimize undesirable effects due to tiny air bubbles or dust which may be present in the solution and cause the occurrence of tine pinholes in the photoresist on the metal surface. (ii) Exposing to light through a photomask: After the photoresist is dried, photomasks are placed on both the surfaces of the sheet. The photomask is actually a sheet of photographic film with a pattern as shown schematically in fig. la. As the photoresist with these masks on it is exposed to light, a latent image of the pattern, narrow strips with a width 0.4 mm, each 0.4 mm apart; is made in the photoresist. The two photomasks on both sides of the sheet should coincide with each other as shown in fig. 1b. Two minutes of exposure to light from a high pressure mercury arc lamp (Toshiba SHLlOOUV, Tokyo Shibaura Denki Co.) set about 14 cm away from the specimen are enough for setting the resist. (iii) Developing: After the photomasks are removed, the sheet is soaked in a developing solution, which takes away that part of the photoresist not exposed to light, leaving the other part of the photoresist on the surface as an image of the transparent pattern. Two minutes of gentle agitation are sufficient. The sheet is then rinsed with a rinse solution and dried. 605
Y. Yashiro, T. Terao 1 Specimen preparation for FIM and FEM
606
u.v.llght
I
(a)
a
photomask
(c)
developing
(e)
removing
(b)
masking
(d)
etching
and
I
exposing
photoresist
1. A schematic diagram illustrating the procedure for making needles from a thin (0.2 mm) metal sheet. M: photomask, R: photoresist, S: sample sheet. Fig.
(iv) Hardening: The sheet is heated at 140°C for 30 min in order to harden the photoresist. (v) Etching: The sheet is then subjected to electrolytic etching at room temperature. A 7 : 1 mixture of CHsOH and concentrated Hz SO4 is used as an etching solution and the current density is 10 A/dm2. Molybdenum needles can also be prepared in just the same manner as described above. (b) Stainless steel needles from a I mm thick sheet A cleaned sheet of stainless steel, 15 X20 X 1 mm, is spread with a photoresist (OSPR, Oriental Sangyo Co.) and dried at 50°C for 30 min. The photomask pattern and the relative positions of the masks on the both surfaces of the sheet are as shown in fig. 2. The relative positions in this case differ from those in the preceding case so as to give metal needles as thin as possible from a rather thick metal sheet.
m
u.v.light (a)
masking
and
exposing
(c)
(b)
removing
etching
photoresist
Fig. 2. Making needles from a thick (1 mm) metal sheet. The relative positions of the photomasks are different from those in fig. lb. Letters S, R, and M are defined in fig. 1.
Y, Y4sW0,
T. Term
/
Specimen preparation for FIM and FEM
607
After 2 min exposure to light, it is soaked in a d~ve~op~~ solution for two minutes, rinsed with water, dried at 100°C for 30 min and then heated at 150°C for about an hour. The condition of electrolytic etching is the same as is described in example (c) below. (e) ~ru~~~e~s&Z preedfes fiotrz a 2 mnt rhick skeet This method can be applied to a metal block of any shape because we can first cut it to obtain a flat surface using the acid-cut method [4]. In this example, a stainless steel sheet with an area of 20 X40 mm and a thickness of 2 mm is used, and two needles are taken out from one surface, the other surface covered with the photoresist being left untouched. After spreading the photoresist (OSPR), two areas of 1 X 20 mm, 10 mm apart, are exposed to light as shown in fig_3. Electrolytic etching is performed using concentrated &PC, saturated with 00s (chromium trioxide) as an etching solution. The current density is 20-30 A/dm2, and the etching temperature is 65--70°C in
B
[b) masking
Dhotomask
and
exposing
(d) etching
developing
u.v.light adhesive
&’
positioning cellophane adhesive
tapes
(g) removing adhesive and etching
tape5
(h)
wmoving
photoreSist
Fjg. 3. A schematic diagram showing the procedure of producing needles from a flat surface.
Fig. 4. Photographs showing the end of stage (d) in fig. 3. this case. About 10 h of etching produce a pair of hillocks, whose tops are covered with photoresist, protruding from the flat plane as shown in fig. 4. The width of the ridge is about 0.2 mm and the height is about 0.5 mm. The etch factor E, whose definition is given in fig. 5, is 1.3-l .5 which seems to decrease with the lowering of etching temperature. After the sample sheet is dried, strips of cellophane adhesive tapes (width 0.2 mm, thickness 0.05 mm) are stuck on both of the feet of each hill-
Y. Yashiro, T. Terao / Specimen preparation for HM and FEM
609
&s-Q R
sample Fig. 5.
metal
Definition of the etch factor e.
ock, as shown in fig. 3e. The sheet is then dipped in the photoresist solution and dried. After exposure to light, the adhesive tapes are peeled off. This leaves narrow gaps in the photoresist which expose the naked parts of the stainless steel to the etchant in the next procedure. Further electrolytic etching detaches the needles from the sample sheet. The residual photoresist covering each needle is removed by immersion in a hot concentrated alkaline solution. A needle thus obtained is shown in fig. 6. The essential feature of the above method is that, in comb~ation with other non-strain cutting methods, any part of a metal block can be removed as a needle without straining the metal. The needles thus obtained may then be used for making specimens for observation in FIM and FEM.
610
Y. Yoshiro, T. Terao f Specimen prepararion for FIM and FEM
The authors are indebted to Dr. H. Morikawa and Mr. M. Kozakai for performing a part of this experiment. They also gratefully acknowledge Dr. A.J. Melmed of NBS for improving this manuscript. Yuzo YASHIRO and Terumi TERAO
Department of Coordinated Science, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466, Japan
References f l] Y. Yashiro and H. Ishihara, Oyo Buturi 36 (1967) 809 (in Japanese). [2] [3] [4] [S]
H. Ishihara, Bull. Nagoya Inst. of Tech. 19 (1967) 161 (in Japanese). Y. YashIro and T. Terao, 23rd Intern. Field Emission Symp., Pennsylvania, 1976. T. Imura, J. Phys. Sot. Japan 24 (1969) 468 (in Japanese). T. Tono and ‘I’.Imura, J. Cryst. Sot. Japan 18 (1967) 252 (in Japanese).