JP3148626B2 - Sample preparation method for transmission electron microscope - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of processing a material having irregularities on its surface into a cross-sectional sample for observation with a transmission electron microscope.

[0002]

2. Description of the Related Art LSI or Ga formed on a silicon substrate
2. Description of the Related Art A transmission electron microscope is used to evaluate a multilayer structure and an interface state of a semiconductor element such as an IC formed on an As substrate. In order to enable observation and analysis of a region near the surface of a semiconductor element by a transmission electron microscope, it is necessary to thin a sample in a cross-sectional direction so that an electron beam can be transmitted.

Conventionally, in the preparation of a cross section sample for a transmission electron microscope for a semiconductor element, two sample pieces having a size of 2 to 3 mm are cut out from a target portion of the semiconductor element, and the surfaces of the two sample pieces are cut out. They are bonded to each other with an adhesive resin, and then thinned in the cross-sectional direction by mechanical polishing and ion milling.
I "1992, 186-191).

According to this method, when the surface of the semiconductor element is flat, the thickness of the adhesive resin on the bonding surface is reduced to several to several tens of μm.
m, but when a finished semiconductor device is used as a sample, Au bumps or Au balls with a height of several tens to several hundreds of μm are present on the surface of the semiconductor device. And a thick resin layer is formed between the two sample pieces.

[0005] In ion milling, the sputter etching rate by an ion beam differs greatly between a semiconductor material and an adhesive resin. If the adhesive resin layer between the bonding surfaces is thick, the resin layer is etched earlier and a hole is formed. Evil. Since the ion beam irradiates the sample surface at a relatively shallow angle, once a hole is formed in the resin layer, the ion beam hits the inner wall surface of the hole and acts to widen the hole. The hole is widened, and the semiconductor element portion to be observed disappears before being thinned.

Further, since thermosetting resin is generally used as an adhesive resin for bonding the sample pieces, if the adhesive resin layer on the bonding surface is thick, air bubbles are generated in the resin layer during resin curing. A cavity may be formed on the bonding surface. Also in this case, at the time of ion milling, the element portion to be observed disappears in a form in which the hole is widened before the semiconductor element portion becomes thin.

In particular, MMIC (Monolithic Microwave I)
This is a problem when a large number of gold wires having a thickness of about 20 μm are formed on the substrate surface as in the case of an integrated circuit.
Therefore, for a completed semiconductor device, a cross-sectional thin film sample for a transmission electron microscope has been conventionally manufactured by the following methods (1), (2), and (3).

(1) A method of mechanically removing protrusions on the surface. In this method, after a protrusion on a semiconductor element surface such as an Au bump or an Au ball is mechanically removed, a thin film sample is prepared by conventional bonding, mechanical polishing, and ion milling. The mechanical removal of Au bumps and Au balls is L
It is generally performed by shaving under a stereoscopic microscope using a prober tip or the like used for the SI test.

(2) A method of removing protrusions on the surface by chemical treatment. In this method, the protrusions on the surface of the semiconductor element are removed by a chemical treatment, and then the conventional bonding,
Perform thin film sample preparation by mechanical polishing and ion milling.
When the protrusion is an Au bump or an Au ball, the removal is performed by immersing the semiconductor element in aqua regia.

(3) A method in which only the portion to be observed is thinned. In this method, the sample is processed using dicing and a focused ion beam apparatus without using the methods of lamination, mechanical polishing, and ion milling, so that only the portion to be observed is thinned while avoiding Au bumps and Au balls. (Japanese Patent Laid-Open No. 5
-302876).

[0011]

The above-mentioned conventional methods for preparing a thin film sample have the following problems.
According to the method of mechanically removing a projection on the surface of a semiconductor element, a large stress is applied to an element portion connected to the projection when removing the projection. Therefore, when the observation target exists under the Au bump or Au ball, the Au bump or Au
There is a high possibility that the observation target portion will be damaged when the ball is removed. Further, since a compound semiconductor substrate of GaAs or the like is brittle, if the Au bumps and Au balls protruding from the surface of the compound semiconductor element are mechanically removed, the periphery thereof will be destroyed.

In the method of removing protrusions on the surface of a semiconductor element by a chemical treatment, a strong acid such as aqua regia is used. Such a strong acid often affects the semiconductor material to be observed, and it is not possible to selectively remove only protrusions such as Au balls and Au bumps. In the method of thinning only the portion to be observed using a focused ion beam device, the depth that can be processed by the focused ion beam is 10 to 10.
It is up to 20 μm. On the other hand, Au bumps and Au balls have a height of 40 to 100 μm, and this method cannot be used when there is an observation target underneath.

The present invention has been made in view of such problems of the prior art, and has as its object to provide a method for producing a cross-sectional thin film sample for a transmission electron microscope from a semiconductor device or the like having an uneven surface. Aim.

[0014]

According to the present invention, for example, when a cross-sectional thin film sample of a semiconductor element is prepared, bonding is performed by mixing fine particles of the same material as the semiconductor element or a material having similar hardness to the semiconductor element. The above object is achieved by bonding semiconductor elements to each other or a semiconductor element and a dummy semiconductor substrate with a chemical resin, and then performing the conventional steps of mechanical polishing and ion milling.

That is, according to the present invention, a sample obtained by bonding another material piece to the surface of a sample piece having an uneven surface is thinned in a direction substantially parallel to the surface by a step including ion milling and transmitted. In a sample manufacturing method for manufacturing a sample for an electron microscope, the bonding is performed using an adhesive mixed with fine particles of a material having the same or substantially the same hardness as the sample piece. It is desirable that the fine particles mixed into the adhesive have a maximum diameter of 3/4 or less of the surface unevenness dimension.

When the sample piece is a silicon semiconductor device, the fine particles mixed into the adhesive can be fine particles obtained by pulverizing a silicon semiconductor substrate or silica powder. According to the present invention, at the time of bonding samples, such as between semiconductor elements or a semiconductor element and a semiconductor substrate, fine particles having a hardness almost equal to the hardness of the sample are mixed in the adhesive resin for bonding, so that The formation of a cavity and the prevention of resin detachment at the beginning of ion milling can be prevented, and a sample in which the surface of a semiconductor element or the like is thinned can be manufactured.

[0017]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 2 is a schematic diagram of a Si-LSI semiconductor device having Au bumps on its surface. A semiconductor device 14 is formed on a surface layer of the silicon substrate 11. A pad 12 is formed on the substrate surface, and a hemispherical Au bump 13 is formed thereon. Au bump 1
The height of 3 is about 50 μm.

FIG. 1 is an explanatory view showing a step-by-step example of manufacturing a cross-sectional sample for a transmission electron microscope according to the present invention from a Si-LSI semiconductor device having an Au bump on the surface shown in FIG. . First, a target portion of a semiconductor element is cut out as a sample piece 15 having a size of about 1.5 × 2.0 mm using a diamond cutter or the like as shown in FIG. Subsequently, an adhesive containing fine particles is prepared by mixing fine particles 16 of the same material or a material having a similar hardness with the substrate 11 of the semiconductor element with a thermosetting resin, and as shown in FIG. The sample piece 15 cut out earlier using the adhesive 17 and the dummy silicon substrate 18 are attached to each other to produce a bonded sample 20. At this time,
If there is no inconvenience such that the gaps between the sample surfaces are excessively wide due to the bumps of the surfaces such as the Au bumps 13 abutting each other, two sample pieces cut out from the same semiconductor element without using a dummy silicon substrate You may paste them facing each other. As the adhesive, a two-part araldite adhesive was used.

The preparation of the adhesive containing fine particles was carried out as follows. As the fine particles 16 to be mixed into the adhesive, fine particles of the same material as the substrate 11 on which the semiconductor element is formed or a material having a hardness similar thereto are used. Most simply, a crushed substrate 11 of the semiconductor element to be observed can be used. That is, when a thin film sample of a semiconductor device formed on a silicon substrate is manufactured, a crushed silicon substrate is used, and when a thin film sample of a semiconductor device formed on a GaAs substrate is manufactured, a thinned GaAs substrate is used. Used. Of course, any material other than the actual substrate may be used as long as the material has a similar hardness. For example, a commercially available silica powder or metal particles having a hardness close to that of silicon may be used instead of the silicon substrate.

After the fine particles are crushed, they are classified by sieving with a mesh having a controlled hole size.
Large particles of about 1/2 to 3/4 of the height of 3 and 10 μm
Particles having a small diameter of about m or less were mixed and used. The reason why the size of the large-diameter particles is set to be about ２〜 to ／ of the height of the Au bump 13 is that if particles having a size larger than the height of the Au bump are used, the semiconductor element sample piece 15 and the silicon substrate Since the distance between the bonding surfaces is wider than the height of the Au bump 13, it is difficult to reduce the thickness of the cross section of the semiconductor element. Conversely, if only the particles whose dimensions are smaller than half the height of the Au bump are used, the space between the bonding surfaces cannot be sufficiently filled. Also,
The gap between the particles increases, the resin between the particles comes off during the ion milling, the particles come off, and the effect of mixing the fine particles into the adhesive cannot be sufficiently expected. Also, when the amount of resin in the thermosetting resin layer is reduced by mixing the resin and the particles, holes are formed due to the particles coming off before the film is thinned, and the effect of mixing the particles cannot be expected sufficiently. is there. However, a large gap is formed between the particles only with the large diameter particles.
m and smaller diameter particles are mixed and used.

The step of bonding the sample piece 15 and the dummy substrate 18 and curing the resin to form a bonded sample 20 includes:
Using a jig shown in FIG. 3, the pressing was performed with a spring.
The jig includes fixed plates 31, 32 connected by two guide rods 33, 34, and a movable plate 35 movable along the guide rods 33, 34. The movable plate 35 is fixed by a spring 36 to the fixed plate 31. Is being urged towards. The sample piece 15 and the dummy substrate 18 were bonded to each other with an adhesive mixed with fine particles, then inserted between the fixed plate 31 and the movable plate 35, and the adhesive was cured while applying a load by a spring 36. As described above, the adhesive layer of the bonded sample 20 can be thinned by curing while applying a load. Note that the bonded sample 20 may be manufactured by first bonding a dummy substrate to the surface of a semiconductor element, and then cutting out a target portion using a diamond cutter or the like.

Next, as shown in FIG. 4, the bonded sample 20 is fixed to a polishing jig 41 using a heat-soluble resin, and the surface of the sample is mechanically polished. First, as shown in FIG. 4A, both surfaces of the sample 20 are roughly polished in a cross-sectional direction with a rotary polishing plate 42 using a diamond abrasive (rough abrasive) 44 having a particle diameter of about 9 μm ejected from a nozzle 43. To a thickness of about 80 μm. Subsequently, as shown in FIG. 4B, a cross section of one side of the bonded sample 20 is plane mirror-polished using an alumina abrasive (mirror abrasive) 46 having a particle diameter of 1 μm or less dropped from a dropper 45. The sample thus obtained is shown in FIG.

Next, using the dimple grinder shown in FIG. 5, as shown in FIG.
The central portion 21 of the surface of the side 0 which is not mirror-polished is polished in a mortar shape. The dimple grinder puts the sample 20 on the turntable 51 and brings the polishing plate 52 having a rotation axis orthogonal to the rotation axis of the turntable 51 into contact with the sample 20 with a polishing plate 52 having a diameter of about 1 cm. Polish. In the dimple grinder polishing, the mortar-shaped portion 21 is finally mirror-finished using two types of abrasives in the same manner as the polishing using the flat polishing plate.

Finally, the sample is further thinned by an ion milling device. That is, a sample whose central portion was polished in a mortar shape was bonded to a single-hole mesh such that the center of the sample was located at the center of the single-hole mesh, and as shown in FIG.
Argon ion beams 54 and 55 of several kV are incident on both sides of the sample 20 at a shallow angle of 10 to 15 ° in an ion milling apparatus, and are thinned by sputter etching until an electron beam can be transmitted.

FIG. 6 shows a state in which a hole has been formed in a part of the sample by processing by the ion milling apparatus. (A) is a plan view, and (b) is an AA sectional view thereof. The central portion of the sample, which is concave in a mortar shape, becomes gradually thinner by the irradiation of the argon ion beam, and a hole 61 is formed. When the hole 61 is formed, the irradiation of the argon ion beam is stopped, and the area around the hole is observed with an electron microscope to check whether or not the target place is sufficiently thin. When the sample is not thin enough, the sample is returned to the ion milling apparatus and the sputter etching by the ion beam is repeated. Observation of the sample with a transmission electron microscope
As shown in (b), the process is performed at a position 62 with a thickness of about several hundred to 1 μm beside the hole 61. When an EDX (energy dispersive X-ray detector) is used as a detector, composition analysis can be performed in addition to morphological observation using a transmission electron microscope.

An experiment was conducted on the mixing amount of the adhesive and the fine particles using a silicon substrate semiconductor device and silica powder. As a result, the mixing ratio is about 5% by weight of the thermosetting resin.
0%, fine particles are 1 / of Au bump height (about 50 μm)
By setting 25 to 30% of particles having a size of 2/3/4 and 25 to 20% of particles having a size of 10 μm or less, the bonded surface could be filled with the finest particles at the highest density, and the transmission electron microscope was used. , Good observation results were obtained.

At the time of bonding the sample piece 15 and the dummy substrate, a glass plate having a thickness of several hundred μm is used instead of the semiconductor substrate 18 as the dummy substrate, and silica is used as fine particles mixed with the thermosetting resin. Then, the polishing position can be confirmed through the transparent glass and silica in the next plane polishing step, and a cross-sectional sample at a specific position can be prepared.

[0028]

According to the present invention, a cross-sectional sample for a transmission electron microscope of a semiconductor element having an uneven structure on the surface can be manufactured. Therefore, it is possible to perform an observation and analysis by assembling the LSI for performing the characteristic evaluation, fabricating an LSI on which an Au ball is formed, or a cross-sectional sample of an LSI with a bump, and observe the correlation between the electrical characteristics of the semiconductor element and the structure. Precise evaluation becomes possible. This enables a useful evaluation for improving the performance of ICs and LSIs.

[Brief description of the drawings]

FIG. 1 is a view for explaining an example of producing a cross-sectional sample for a transmission electron microscope according to the method of the present invention.

FIG. 2 is a schematic view of a Si-LSI semiconductor device having Au bumps on its surface.

FIG. 3 is a schematic view of a jig used for producing a bonded sample.

FIG. 4 is a view for explaining mechanical polishing of a sample surface;
(A) is explanatory drawing of a plane rough polishing, (b) is a plane mirror polishing.

FIG. 5 is a diagram illustrating polishing by a dimple grinder.

FIGS. 6A and 6B are diagrams showing a state in which a part of a sample is perforated by processing by an ion milling device, wherein FIG. 6A is a plan view and FIG.

[Explanation of symbols]

11 ... substrate, 12 ... pad, 13 ... Au bump, 14 ...
Semiconductor device, 15: specimen, 16: fine particles, 17
... adhesive, 18 ... dummy substrate, 20 ... bonded sample,
21 31, 32: fixed plate, 33, 34: guide rod, 35: movable plate, 36: spring, 41: polishing jig, 42: rotary polishing plate, 43: nozzle, 44: coarse abrasive, 45: dropper,
46: mirror polishing agent, 51: turntable, 52: polishing plate, 5
4, 55: ion beam, 61: hole, 62: observation position

Continuation of the front page (56) References JP-A-7-167760 (JP, A) JP-A-63-19834 (JP, A) JP-A-5-302876 (JP, A) JP-A-7-297676 (JP, A) , A) Real opening 1986-123958 (JP, U) (58) Field surveyed (Int. Cl. ⁷ , DB name) G01N 1/00-1/34 JICST file (JOIS)

Claims (3)

(57) [Claims] 1. A sample for a transmission electron microscope, wherein a sample obtained by bonding another material piece to the surface of a sample piece having an irregular shape on its surface is thinned in a direction substantially parallel to the surface by a step including ion milling. In the method of preparing a sample, the bonding is performed using an adhesive mixed with fine particles of a material having the same or substantially the same hardness as the sample piece. 2. The method according to claim 1, wherein the fine particles have a maximum diameter of not more than 3/4 of the uneven size. 3. The method according to claim 1, wherein the sample piece is a silicon semiconductor device, and the fine particles are fine particles obtained by pulverizing a silicon semiconductor substrate or silica powder.