JP3417038B2 - Silicon substrate etching method - 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 for etching a silicon substrate, and more particularly, to a method for etching a silicon substrate used for forming a diaphragm in a semiconductor pressure sensor.

[0002]

2. Description of the Related Art Conventionally, when manufacturing a semiconductor pressure sensor,
When forming a thin diaphragm on a part of a silicon substrate using the (110) plane, anisotropic etching with KOH was performed. That is, the (110) plane was etched by time-controlled KOH. However, since it is the (110) plane, the etched surface becomes jagged, which causes variations in sensor characteristics. Therefore, it is necessary to flatten the etching surface.

Therefore, as disclosed in Nippon Denso Public Technical Report 88-002, an SiO ₂ (SP-SiO ₂ ) film formed by a sputtering method is provided on the etching surface after the diaphragm is formed.

[0004]

However, when the SP-SiO ₂ film is provided in this way, a member different from the silicon substrate is required and the number of steps is increased. Further, in order to etch the (110) plane, (10
Even if electrochemical etching is used like etching of the (0) surface, the etched surface is notched.

Therefore, an object of the present invention is to provide a method for etching a silicon substrate which can easily flatten the etching surface of (110) plane silicon.

[0006]

According to a first aspect of the present invention, a PN junction is formed when electrochemically etching a (110) -faced silicon substrate having a PN junction with an alkali anisotropic etching solution. start the etching from one side, the anodized film formed on the etched surface of the PN junction and its oxide film etching it in equilibrium
The gist is a method of etching a silicon substrate in which voltage application is terminated when a predetermined time has elapsed from the time when the etching surface has irregularities .

According to a second aspect of the present invention, the time at which the equilibrium state is reached in the first aspect of the invention is an inflection point at which a current flowing is detected and a peak of the current is reached to a constant current. The method of etching a silicon substrate will be summarized.

According to a third aspect of the present invention, there is provided a method for etching a silicon substrate, wherein the inflection point in the second aspect of the invention is considered to be when a predetermined time has elapsed after detection of a peak current. The summary will be given.

A fourth aspect of the present invention has as its gist a method of etching a silicon substrate, wherein the predetermined time is 5 minutes or more in the first aspect of the invention. The invention according to claim 5 is the silicon of the (110) plane having a PN junction.
The substrate is electrochemically etched using an alkaline anisotropic etching solution.
From the one side that forms the PN junction,
Start etching, etching surface at PN junction
Balance between anodic oxide film formation and oxide film etching
Voltage is applied when a predetermined time has passed from the time when
So as to terminate the voltage application at a predetermined time after the timing when said equilibrium as its gist the etching method of the silicon substrate is by alternating wave.

According to a sixth aspect of the invention, the period of the AC wave in the fifth aspect of the invention is the time required for completely removing the anodic oxide film on the etching surface by the alkali anisotropic etching solution. The method of etching a silicon substrate will be summarized.

[0011]

According to the first aspect of the invention, the etching is started from one surface forming the PN junction, and the formation of the anodic oxide film on the etched surface of the PN junction and the etching of the oxide film are in an equilibrium state. The voltage application is terminated when a predetermined time (for example, the time indicated by T in FIG. 4) has elapsed from the point of time. As a result, when the formation of the anodic oxide film on the etching surface of the PN junction and the etching of the oxide film are in an equilibrium state, the etching surface has irregularities, but the etching surface is flattened by the subsequent voltage application. . It is considered that this is because it is difficult to etch the anodic oxide film of the concave portion in the uneven portion of the etching surface and the anodic oxide film of the convex portion is etched, and the silicon of the convex portion is removed.

In addition to the operation of the invention described in claim 1, the invention described in claim 2 detects an energized current at the time of reaching an equilibrium state, and changes the current into a constant current after a peak of the current. It is said to be a point.

According to the invention described in claim 3, in addition to the effect of the invention described in claim 2, the inflection point has passed a predetermined time (for example, the time indicated by Ta in FIG. 4) after the peak current is detected. Considered to be time.

According to the invention described in claim 4, in addition to the operation of the invention described in claim 1, the predetermined time is set to 5 minutes or more. The invention according to claim 5, the voltage at the predetermined time from the time when a equilibrium state is applied by the AC wave, flattening of the etching surface is promoted. It is considered that this is because the anodic oxide film on the convex portion of the etching surface is selectively etched and the silicon on the convex portion is removed.

According to the invention of claim 6, in addition to the action of the invention of claim 5, the period of the AC wave is such that all the anodic oxide film on the etching surface is removed by the alkali anisotropic etching solution. It is said to take time.

[0016]

【Example】

(First Embodiment) A first embodiment of the present invention will be described below with reference to the drawings.

This embodiment is embodied in a semiconductor pressure sensor using a piezoresistive layer. FIG. 1 shows a schematic view of an electrochemical etching apparatus for forming a diaphragm on a silicon wafer 1. First, the silicon wafer 1 will be described. An N type epitaxial layer 3 is formed on one surface of a P type (110) plane oriented silicon substrate 2. The N-type epitaxial layer 3 is formed a P ⁺ -type impurity diffusion layer 4, the P ⁺ -type impurity diffusion layer 4 is piezoresistive for sensing distortion. Further, the N ⁺ type impurity diffusion layer 5 is formed in the N type epitaxial layer 3, and the N ⁺ type impurity diffusion layer 5 makes ohmic contact with the N type epitaxial layer 3. Furthermore, a silicon oxide film 6 is formed on the surface of the N-type epitaxial layer 3. The P ⁺ -type impurity diffusion layer 4 and the N ⁺ -type impurity diffusion layer 5 are made of aluminum 7,
At 8, the surface of the silicon oxide film 6 is electrically extracted.

On the surface of the silicon substrate 2 where the N-type epitaxial layer 3 is not formed, a mask material 9 is formed in a region where the diaphragm is not formed. Such a silicon wafer 1 is prepared. The silicon wafer 1 is fixed to the ceramic support substrate 11 with the platinum electrode 12 sandwiched therebetween. Surface of silicon wafer 1 not etched (N-type epitaxial layer 3 formation surface)
Is protected by a resin wax 10. The platinum electrode 12 is in contact with the aluminum 8. That is, the platinum electrode 12 is brought into electrical contact with the N-type epitaxial layer 3 through the aluminum 8 and the N ⁺ -type impurity diffusion layer 5, and electrochemical stop etching is performed.

On the other hand, in the container 13, a KOH aqueous solution (33 wt%, 82 ° C.) as an alkali anisotropic etching solution is used.
14 are filled. KOH aqueous solution 14 in container 13
The above-mentioned silicon wafer 1 is dipped in and the platinum electrode plate 15 is arranged so as to face the silicon wafer 1.

Then, the platinum electrode 12 of the silicon wafer 1
Between the platinum electrode plate 15 and the platinum electrode plate 15 (2V)
6, the ammeter 17, and the contact 18 are connected in series.
Further, the controller 19 includes a start switch 20 and an ammeter 1.
7 and the contact 18 are connected. Then, by closing the contact 18, a potential difference is applied between the silicon wafer 1 and the platinum electrode plate 15 by the constant voltage power supply (2 volts) 16. At this time, the ammeter 17 detects the current flowing from the silicon wafer 1 to the platinum electrode plate 15. The controller 19 detects the start of etching by the signal from the start switch 20 and also detects the energizing current by the signal from the ammeter 17. Further, the controller 19 drives the contacts 18 to open and close. The controller 19 is mainly composed of a microcomputer.

The controller 19 executes the processing shown in FIGS. This processing will be described based on the time chart of FIG. The vertical axis in FIG. 4 represents the current value and the open / closed state of the contact 18.

First, when the etching start signal is input from the start switch 20, the controller 19 activates the process shown in FIG. The controller 19 closes the contact 18 in step 101, and in steps 102, flags F1 and F2.
To "0". Further, the controller 19 reads the current value I _i from the ammeter 17 in step 103, and in step 104 the difference ΔI _i (= I _i −I) between the current value I _i and the previous current value I _{i-1. i-1} ) is calculated.

Then, the controller 19 executes the step 1
At 05, it is determined whether the rate of change ΔI _i of the energizing current is inverted from positive to negative. That is, in FIG.
Then, it is judged whether or not the timing (tp timing) has come. The controller 19, when the change rate [Delta] I _i of the electric current is not inverted from positive to negative the process returns to step 103, the flag F1 in step 106 if the rate of change [Delta] I _i of the electric current is inverted from positive to negative to "1" Set.

Further, the controller 19 executes the interrupt processing of FIG. 3 every predetermined time. The controller 19 determines in step 201 whether or not the flag F2 is "0", and initially F
Since 2 = 0, the process proceeds to step 202. The controller 19 determines in step 202 whether or not the flag F1 is "1", and initially F1 = 0, so the process returns.

Then, the controller 19 shifts from step 202 to step 203 when the energization current value reaches a peak (timing of tp in FIG. 4) and F1 = 1.
Then, the controller 19 determines whether the rate of change [Delta] I _i of the electric current in step 203 becomes "0," and returns since [Delta] I _i ≠ 0 becomes [Delta] I _i is negative, during a period tp -t2 in Fig.

After that, the controller 19 sets the flag F2 to "1" at step 204 when the rate of change ΔI _i of the energizing current becomes "0" (timing t2 in FIG. 4). Further, the controller 19 starts a timer in step 205, and whether a predetermined time T (10 minutes in this embodiment) has elapsed since the rate of change ΔI _i of the energizing current by this timer became “0”. Determine whether or not. Initially, the controller 19 returns because the timer has not elapsed the predetermined time T. In the subsequent processing, F2 = 1, so step 201 → 20.
Go to 5.

When the predetermined time T has elapsed in step 205 (timing t3 in FIG. 4), the controller 19 proceeds to step 206 to open the contact 18 in FIG. Immediately upon completion of this energization, KOH
The silicon wafer 1 is taken out of the aqueous solution 14 and washed with water. This completes the electrochemical etching.

The behavior of the energizing current value of FIG. 4 will be described. In the first region after the energization is started, the P-type silicon substrate 2 is etched by the chemical reaction between KOH and silicon. This is because the voltage is supplied to aluminum 8, but P
This is because no current is supplied to the silicon wafer 1 by the PN junction formed by the type silicon substrate 2 and the N type epitaxial layer 3. In the second region, which has a peak current, in the same region, the P-type silicon substrate 2 is used.
Anodic oxidation progresses by the electrochemical reaction of. This is because the silicon substrate 2 is etched and the PN depletion layer comes into contact with the KOH aqueous solution 14, so that a current flows and the silicon is oxidized. Further, the gentle peak in the second region is due to the surface distribution (thickness variation) of the thickness of the silicon wafer 1.

At the timing of t2 in the next third region, the anodic oxide film is also etched even though the etching rate is slow, so that etching and generation of the anodic oxide film are in equilibrium. At this point, as shown in FIG. 5, irregularities are formed on the etching surface. Further, in this embodiment, overetching is performed by continuing voltage application in the KOH aqueous solution for 10 minutes after the time t2. As a result, as shown in FIGS. 6 and 7, the slope of the convex portion in the unevenness formed on the etching surface becomes gentle, and the etching surface of the diaphragm gradually becomes smooth. This state change is confirmed by a cross-sectional photograph of the etched part.

It is considered that this is due to the phenomenon (mechanism) described below. As shown in FIG. 5, unevenness is formed on the etching surface, but the film quality of the anodic oxide film on the convex portion is different from that of the anodic oxide film on the concave portion. The etching rate of the oxide film is faster. It is considered that this is because etching of the anodic oxide film on the convex portion progresses and silicon of the convex portion is exposed and etching progresses.

In this way, after the peak of the energization current value in the second region of FIG. 4, overetching is performed from the inflection point (timing of t2) to the equilibrium current by the anodic oxide film in the third region, and a predetermined value is obtained. When the time T (= 10 minutes) has elapsed, the etching is completed.

FIG. 8 shows the relationship between the over-etching time and the etching surface roughness. Without overetching, there was a roughness of 5 μm, but overetching was 5
If it is performed for more than 10 minutes, the roughness becomes 2 μm or less, and if it is performed for 10 minutes or more, the roughness becomes 1 μm or less. This over-etching time is 10 minutes in this embodiment, but 5 minutes or more,
It is preferable to perform the treatment for 10 to 60 minutes.

In the anisotropic etching with KOH,
The roughness of the etching surface changes depending on the concentration and temperature of the KOH aqueous solution, the impurity concentration of the silicon wafer, and the like, which makes it difficult to control the smoothing of the etching surface. However, in this embodiment, it is only necessary to set the overetching time. It is possible to smooth the etched surface to a certain roughness or less. In other words, the KOH aqueous solution can be easily managed because the etching surface becomes smooth without being affected by the concentration and temperature of the KOH aqueous solution, the impurity concentration of the silicon wafer, and the like. That is, the KOH aqueous solution may not be 33 wt% and 82 ° C.

Further, the diaphragm thickness is determined by (thickness of the N-type epitaxial layer 3 + thickness of the depletion layer extending to the P-type silicon substrate 2 side) as shown in FIG.
If the thickness and impurity concentration of the N-type epitaxial layer 3 are specified, the diaphragm thickness can be adjusted by adjusting the applied voltage value.

If the etching opening area changes, the current values in the first region and the second region in FIG. 4 change, but the waveform in the relationship between the current and the etching time does not change. Therefore, t2 in FIG. 4, that is, ΔI / Δt = 0
When the overetching time T is set by detecting when the above condition occurs, a uniformly smooth etched surface can be obtained within the silicon wafer surface.

In the semiconductor pressure sensor in which the diaphragm is formed in this manner, the diaphragm surface is smooth, so that the linearity between the pressure and the sensor output is excellent. In other words, if the etching surface has roughness, the diaphragm thickness is not uniform, so that the pressure distribution varies and the linearity between the pressure and the sensor output deteriorates.

As described above, in this embodiment, when the (110) -faced silicon wafer 1 having the PN junction is electrochemically etched using the KOH aqueous solution, the etching is started from one face forming the PN junction, and the PN junction is formed. The voltage application is terminated when a predetermined time T (10 minutes) elapses from the time when the formation of the anodic oxide film on the etching surface of the junction and the etching of the oxide film are in equilibrium (timing t2 in FIG. 4). I did it. As a result, when the formation of the anodic oxide film on the etching surface of the PN junction and the etching of the oxide film are in an equilibrium state, the etching surface has irregularities, but the etching surface is flattened by the subsequent voltage application. . In this way, Nippon Denso Public Technical Report 88-0
It is possible to easily flatten the etched surface of the silicon of the (110) plane by using electrochemical etching without providing the SP-SiO ₂ film as in 02.

Further, since the time point when the equilibrium state is reached is the inflection point to the constant current after the peak of the current value is detected by the energization current, the time point when the equilibrium state is reached can be accurately detected.
Further, since the predetermined time T (overetching time) is set to 5 minutes or more, the surface roughness of the etching surface can be surely flattened to 2 μm or less.

Next, an application example of this embodiment will be described. Figure 2
In step 106, the voltage application may be terminated after the elapse of To from the peak time of the energizing current by judging whether the predetermined time To (shown in FIG. 4) has elapsed. That is, the time Ta that is an inflection point to a constant current after the energization current peak is experimentally obtained in advance, and the time when the predetermined time Ta after the peak current is detected is regarded as the inflection point. Then, when the time To (= Ta + T) elapses after the energization current peak, the voltage application is terminated.

By doing so, it is not necessary to detect an inflection point to a constant current after the energization current peak.
The calculation processing in the controller 19 can be reduced. That is, the inflection point detection processing to the constant current in step 203 of FIG. 3 is not necessary, and the flags F1 and F2 are also unnecessary, and the calculation load of the controller 19 can be reduced. (Second Embodiment) Next, the second embodiment will be described focusing on the differences from the first embodiment.

In this embodiment, a pulse voltage is applied as a voltage application method during overetching. This will be described below with reference to FIGS. FIG. 9 corresponds to FIG. 4 of the first embodiment, and FIG. 10 corresponds to FIG. 3 of the first embodiment.

In FIG. 10, when the anodic oxide film formation and the anodic oxide film etching reach an equilibrium state in step 203, the controller 19 proceeds to step 207 via steps 204 and 205. The controller 19 sets the opening / closing operation mode of the contact 18 in step 207.
In this opening / closing operation mode, the controller 19 opens / closes the contact 18, as shown in FIG. At this time, the opening time tx and the closing time ty of the contact 18 are both 2.5 seconds, and the cycle tz is 5 seconds.

By doing so, as shown in FIG. 11A, the etching surface has triangular irregularities, and when the anodic oxide film formation and the anodic oxide film etching are in an equilibrium state, FIG. ), An oxide film is formed on the triangular irregularities on the etched surface. And
At the opening time tx of the contact 18, as shown in FIG. 11 (c), only the anodic oxide film on the convex portion is removed by the KOH aqueous solution to expose the silicon of the convex portion, and as shown in FIG. 11 (d), The exposed convex silicon is etched.
Further, an anodic oxide film is formed on both the convex portion and the concave portion during the closing time ty of the contact 18.

By repeating this operation, as shown in FIG.
As shown in, a smooth etching surface is obtained. By opening / closing the contact 18 during the over-etching as described above, the etching surface can be flattened faster than in the state where the contact 18 is kept closed as in the first embodiment. That is, in the first embodiment, the overetching time T is set to 1
Although it was set to 0 minutes, it is set to 5 minutes in this embodiment.

Here, the reason why only the convex portion of the etching surface is selectively etched by using the pulse wave as the applied voltage during the over-etching will be described. When the anodic oxide film formation and anodic oxide film etching are in equilibrium, the time required for the anodic oxide film (thickness of about 10 Å) to be completely removed by the KOH aqueous solution in a state where no voltage is applied is t. Then, by opening the contact 18 at about t / 2, the anodic oxide film on the convex portion on the etching surface can be selectively etched. That is, the film quality of the anodic oxide film on the convex portion is different from that of the anodic oxide film on the concave portion, and the anodic oxide film on the convex portion has a higher etching rate than the anodic oxide film on the concave portion. Therefore, the contact opening time tx
It is considered that (when no voltage is applied), the protrusion having the anodic oxide film having a high etching rate is selectively etched by the KOH aqueous solution.

As described above, in this embodiment, a pulse wave (AC wave) is applied as voltage application for a predetermined time T from the time when the anodic oxide film formation and the anodic oxide film etching are in equilibrium (timing t2 in FIG. 9). ) Was added. Therefore, flattening on the etching surface is promoted. That is, the anodic oxide film on the convex portion of the etching surface is selectively etched, and the silicon on the convex portion is removed. As a result, the etching surface can be flattened faster than in the case where a constant voltage is continuously applied during overetching as in the first embodiment.

Further, the period tz of the pulse wave (AC wave)
Is the time required for the anodic oxide film on the etching surface to be completely removed by the KOH aqueous solution, so that planarization can be reliably promoted.

Next, an application example of this second embodiment is shown in FIG.
13 shows. During overetching after the anodic oxide film formation and the anodic oxide film etching are in equilibrium, FIG.
As shown in 2, a sine wave is applied. That is, as shown in FIG. 13, the constant voltage power supply (2 volts) 16 and the sine wave generating power supply 21 are connected in parallel, and the controller 19 switches the changeover switch 22 during overetching to supply the constant voltage until then. Switch from the state to the sine wave supply state.

In addition to the sine wave, a sawtooth wave or a triangular wave may be used as the voltage applied during overetching.
The present invention is not limited to the above-described embodiments, and for example, the etching solution is not limited to the KOH aqueous solution, and the tetramethylammonium hydroxide aqueous solution (TMA) is used.
Other alkaline anisotropic etching solutions such as H: (CH ₃ ) ₄ NOH) and ethylenediamine may be used.

Although the applied voltage is 2 volts in the above-mentioned embodiments, it may be 0.7 volts or more. further,
In each of the above-described embodiments, the case where the diaphragm of the semiconductor pressure sensor is formed has been described, but it can be used when forming the thin portion (beam portion) of the semiconductor acceleration sensor.

[0051]

As described above in detail, according to the first aspect of the invention, the etching surface of the (110) plane of silicon can be easily flattened. According to the invention described in claim 2, in addition to the effect of the invention described in claim 1, it is possible to accurately detect the time point when the equilibrium state is reached. Further, claim 3
According to the invention described in (1), in addition to the effect of the invention described in (2), it is not necessary to detect an inflection point to a constant current, so that the processing can be reduced. Further, according to the invention described in claim 4, in addition to the effect of the invention described in claim 1, the surface roughness of the etching surface can be set to 2 μm or less.
Further, according to the invention described in claim 5, it is possible to more quickly flatten et etching surface. Further, according to the invention described in claim 6, in addition to the effect of the invention described in claim 5, it is possible to surely promote the flattening.

[Brief description of drawings]

FIG. 1 is a schematic view of an electrochemical etching apparatus of an example.

FIG. 2 is a flowchart for explaining an electrochemical etching operation.

FIG. 3 is a flowchart for explaining an electrochemical etching operation.

FIG. 4 is a time chart for explaining an electrochemical etching operation.

FIG. 5 is a cross-sectional view for explaining an electrochemical etching operation.

FIG. 6 is a cross-sectional view for explaining an electrochemical etching operation.

FIG. 7 is a cross-sectional view for explaining an electrochemical etching operation.

FIG. 8 is a graph showing the relationship between overetching time and roughness of an etched surface.

FIG. 9 is a time chart for explaining the electrochemical etching operation of the second embodiment.

FIG. 10 is a flow chart for explaining the electrochemical etching operation of the second embodiment.

FIG. 11 is a cross-sectional view of the etching surface of the second embodiment, in which (a) to (e) are cross-sectional views showing changes in the etching surface.

FIG. 12 is a waveform diagram of an applied voltage in an application example of the second embodiment.

FIG. 13 is a schematic diagram of an electrochemical etching apparatus in an application example of the second embodiment.

[Explanation of symbols]

1 Silicon Wafer 2 P-Type Silicon Substrate 3 N-Type Epitaxial Layer 14 KOH Aqueous Solution as Alkaline Anisotropic Etching Solution 17 Ammeter

─────────────────────────────────────────────────── ─── Continuation of the front page (56) Reference JP-A-1-318234 (JP, A) JP-A-5-90244 (JP, A) JP-A-4-267524 (JP, A) (58) Field (Int.Cl. ⁷ , DB name) H01L 21 / 306,21 / 3063,21 / 308

Claims (6)

(57) [Claims] 1. When electrochemically etching a (110) -faced silicon substrate having a PN junction with an alkaline anisotropic etching solution, the etching is started from one face forming the PN junction,
The formation of the anodic oxide film on the etching surface of the PN junction and the etching of the oxide film are in equilibrium, and the etching is performed.
A method for etching a silicon substrate, characterized in that the voltage application is terminated when a predetermined time elapses from the time when there is unevenness on the ring surface . 2. The method for etching a silicon substrate according to claim 1, wherein the time when the equilibrium state is reached is an inflection point to a constant current after a current is detected and a peak of the current is detected. 3. The method for etching a silicon substrate according to claim 2, wherein the inflection point is regarded as a time when a predetermined time has elapsed after the peak current was detected. 4. The predetermined time is set to 5 minutes or more.
The method for etching a silicon substrate according to. 5. A (110) plane silico having a PN junction.
Electrolyte substrate using alkaline anisotropic etching solution
When etching , start the etching from one side to form the PN junction,
Anodic oxide film formation on the etching surface in PN junction
And when the oxide film etching is in equilibrium
Stop the voltage application when a predetermined time has passed
And, it features and be Resid silicon substrate etching method in that the voltage applied at a given time from the time when a state of equilibrium is due to the AC waves. 6. The cycle of the alternating current wave is a time required to remove all the anodic oxide film on the etching surface by the alkali anisotropic etching solution.
The method for etching a silicon substrate according to.