WO2014140423A2

WO2014140423A2 - Method and apparatus for welding a semiconductor substrate with a laser

Info

Publication number: WO2014140423A2
Application number: PCT/FI2014/050175
Authority: WO
Inventors: Samuli Kivistö; Jarno Kangastupa
Original assignee: Corelase Oy
Priority date: 2013-03-14
Filing date: 2014-03-10
Publication date: 2014-09-18
Also published as: WO2014140423A3

Abstract

The present invention provides for a novel method and apparatus for welding a semiconductor substrate (10) with a laser, wherein the laser (20) has a wavelength which is transparent to the material of the semiconductor substrate (10). In the novel method the laser (20) a first series of pulses (b₁) is first subjected to the substrate, wherein the first series of pulses (bi) features at least two successive pulses (p₁, p₂, p₃) which occur with substantially equal time intervals (i₁, i₂) of a first magnitude. Then a second series of pulses (b₂) is subjected to the substrate, wherein the second series of pulses (b₂) feature at least two successive pulses (p₄, p₅, p₆) which occur with substantially equal time intervals (i₄, i₅) of the first magnitude. The last pulse (p₃) of a previous series of pulses (b₁) and the first pulse (i₄) of a subsequent second series of pulses (b₂) occur with a time interval (i₃) of a second magnitude which is unequal of at least one order of magnitude in respect to first magnitude thus establishing a burst sequence of laser pulses.

Description

METHOD AND APPARATUS FOR WELDING A SEMICONDUCTOR SUBSTRATE WITH A LASER

FIELD OF THE INVENTION

The present invention relates to laser welding a semiconductor substrate. More specifically, the invention relates to a method and apparatus according to the preamble portions of claims 1 and 11.

BACKGROUND ART

JP 2007012752 A discloses a method for sealing a component package by administering a pulse laser light having an ultralong pulse. During one pulse period, a beam spot of the pulse laser light makes scanning movement at a constant speed and at a constant laser output without standing still to avoid overlapping of beam spots.

Known laser welding devices, however, are constructed such that the substrate to be processed must be controlled in a very precise manner in order to establish correct overlap between successive pulses. Known laser welding devices are therefore typically quite complex and expensive.

It is therefore an aim of the present invention to provide an improved laser welding method and device which decrease the accuracy requirement for managing the movement of the substrate to be processed.

SUMMARY

The aim of the present invention is achieved with aid of a novel method for welding a semiconductor substrate with a laser, wherein the laser has a wavelength which is transparent to the material of the semiconductor substrate. In the novel method the laser a first series of pulses is first subjected to the substrate, wherein the first series of pulses features at least two successive pulses which occur with substantially equal time intervals of a first magnitude. Then a second series of pulses is subjected to the substrate, wherein the second series of pulses feature at least two successive pulses which occur with substantially equal time intervals of the first magnitude. The last pulse of a previous series of pulses and the first pulse of a subsequent second series of pulses occur with a time inter- val of a second magnitude which is unequal of at least one order of magnitude in respect to first magnitude thus establishing a burst sequence of laser pulses.

More specifically, the method according to the present invention is characterized by the characterizing portion of claim 1. The aim of the invention is on the other hand achieved with aid of a novel apparatus for welding a semiconductor substrate with a laser. The apparatus includes a laser pulse source and an acousto-optical modulator which is connected to the laser pulse source and configured to modify the produced pulse sequences into sequences. The apparatus also includes a laser head which is connected to the laser pulse source and configured to trans- form received signal pulse sequences into laser pulse sequences. The apparatus further includes output optics which is connected to the laser head and adapted to collimate and focus the received laser pulses into a welding beam.

According to one embodiment, the source of laser pulses is an oscillator for producing laser pulses. More specifically, the apparatus according to the present invention is characterized by the characterizing portion of claim 11.

Considerable benefits are gained with aid of the present invention.

The series of cyclic pulses, i.e. bursts, enable more precise control over the overlap of pulses, whereby it is less critical to control the movement of the substrate. This reduces the complexity of the equipment required to move the processed substrate in a controlled manner. In addition, the improved overlap control yields other benefits including easier start-up of the process, ability to use lower optical power output and pulse energy as well as possibility for increased processing speed, which all lead to improved absorption.

In other words, with aid of the invention, it is possible to more independently control the movement of the substrate and the frequency of the pulses.

Moreover, by adjusting the duration and shape of the bursts, the process may be controlled such that having triggered the process in a certain part of the substrate, the power output may be decreased while maintaining said scribing phenomenon. For example, the amplitude of the first pulse in a burst may be high, whereby the amplitude of the following pulses in the burst may decrease by pulse. Consequently, the energy of individual pulses may be reduced, whereby some thermally sensitive materials may even exhibit reduced cracking due to the reduced energy. BRIEF DESCRIPTION OF DRAWINGS

In the following, exemplary embodiments of the invention are described in greater detail with reference to the accompanying drawings in which:

Fig. 1 presents a cross-sectional view of a semiconductor substrate being processed by a laser beam, Fig. 2 presents graph of a burst sequence having three pulse series plotted into a time- power plot,

Fig. 3 presents a block diagram of a welding apparatus according to one embodiment.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

As illustrated in Fig. 1, a laser beam 20 is focused on a glass or semiconductor substrate, e.g. sapphire, quartz, or silicon substrates or similar substrate 10. The laser 20 has a wavelength which is transparent to the material of the semiconductor substrate 10. The principles behind focusing the beam in welding are known and it is common practice to focus the beam small enough to establish the required intensity and fluency which is dependent on the processed material. Power, frequency and spot size are parameters which are adjusted to establish high fluency.

The beam 20 is subjected to the substrate in burst sequences. Fig. 2 illustrates one particular burst sequence arrangement featuring three successive bursts, i.e. three consecutive series of pulses bi to b₃. In the illustrated embodiment, the first series of pulses bi features three successive pulses pi, p₂ and p₃ which are transmitted at successive moments in time t_{l s} t₂, t₃ with substantially equal time intervals ii, i₂. The first time interval ii is the elapsed time t₂ - between the first pulse pi and the second pulse p₂, while the second time interval i₂ is the elapsed time t₃ - 1₂ between the second pulse p₂ and the third pulse p₃. The time intervals ii and i₂ both have a first magnitude which in the illustrated exam- pie is in the range of 5 to 50 ns. According to a particular embodiment, the first magnitude is in the range of 10 to 35 ns. In other words, the time elapsed between the first and the second pulse pi, p₂ as well as between the second and the third pulse p₂, p₃ is in the range of 5 to 50 ns, preferably between 10 to 35 ns. The said pulses have duration of 20 - 100 ps.

After the first series of pulses bi, a second series of pulses b₂ is transmitted. The second series of pulses b₂ features three successive pulses p₄, p₅ and p₆, i.e. the fourth, fifth and sixth pulse, which are transmitted at successive moments in time t₄, ts, t₆ with substantially equal time intervals Ϊ4, i₅ similar to the first and second time interval i_{l s} i₂ in the first series of pulses bi. The time intervals i₄, i₅ between the pulses p₄, to p₆ of the second series of pulses b₂ are also of the first magnitude, e.g. 5 to 50 ns, preferably 10 to 35 ns. However, the time interval i₃ between the last pulse p₃ of the first series of pulses bi and the first pulse p₄ of the second series of pulses b₂ is of a second magnitude which is unequal of at least one order of magnitude in respect to first magnitude. In the illustrated example, the second magnitude is in the range of 50 ns to 50 ms. According to a particular embodiment, the second magnitude is in the range of 100 ns to 20 ms. In other words, the time elapsed between the last pulse p₃ of a previous series of pulses bi and the first pulse Ϊ4 of a subsequent second series of pulses b₂ is in the range of 50 ns to 50 ms, preferably between 100 ns to 20 ms. After the second series of pulses b₂, a third series of pulses b₃ is transmitted in a similar fashion. The third series of pulses b₃ also features three successive pulses p₇, ps and pg, i.e. the seventh, eighth and ninth pulse, which are transmitted at successive moments in time t₇, t₈, tg with substantially equal time intervals i₇, is similar to the first and second time interval i_{l s} i₂ in the first series of pulses bi and to the third and fourth time interval i₃, M in the second series of pulses b₂. The time interval i₆ between the last pulse p₆ of the second series of pulses b₂ and the first pulse p7 of the third series of pulses b₃ is similar to that i₃ between the last pulse p₃ of the first series of pulses bi and the first pulse p₄ of the second series of pulses b₂, wherein said time interval ¾ is of a second magnitude which is unequal of at least one order of magnitude in respect to first magnitude. Fig. 2 shows the first three bursts bi to b₃ of the laser welding sequence according to one embodiment of the invention. Naturally, the sequence would continue in a series of similar bursts transmitted with interval as explained above.

The explained method may be varied without departing from the inventive concept. For example, the pulses pi to ₃, p₄ to p₆, p₇ to pg transmitted in a burst bi, b₂, b₃, respectively, may vary in power. According to the example shown in Fig. 2, all pulses pi to p9 are transmitted at substantially similar level of power. According to an alternative embodiment (not shown), the intensity of the pulses within the burst drops with each pulse. For example, the first pulse pi in a burst bi may be transmitted at an initial level of power and the subsequent second pulse p₂ is transmitted at a lower power level than the first pulse pi and the subsequent third pulse p₃ is transmitted at a lower power level than the second pulse p₂. With aid of the decreasing power level among pulses pi to p₃ of the same burst bi it is possible to increase the accuracy and quality of the processing. The idea behind decreasing power level of successive pulses in a burst is that the first and most powerful pulse triggers the process, where after the later and less powerful pulses maintain the transformation of the solid work piece for creating the internal discontinuity. More particularly, the first pulse triggers a non-linear absorption phenomenon. Once the nonlinear absorption has been initiated, a smaller intensity is required to continue the absorption and thus the process.

Referring now to Fig. 3 which illustrates a laser welding apparatus 100 according to one embodiment of the invention. The apparatus 100 includes two main sections: a main unit 110 and a laser head 120. The main unit 110 includes laser pulse source for producing basic laser pulse sequences such as a master oscillator. There are various commercially available master oscillators which are suitable for producing the desired laser pulses. Suitable operating frequencies for a master oscillator include 10MHz, 30MHz, 50 MHz and 100 MHz, for example. Connected to the master oscillator 1 11 is a first pre-amplifier

112 which is configured to receive and amplify the pulses produced by the master oscillator 111. The first pre-amplifier 112 is configured to provide amplification of 10 to 30 dB for the pulse sequence. Connected to the first pre-amplifier 112 is a signal processor

113 which is configured to receive pre-amp lifted pulses from the first pre-amplifier 112 and to selectively let through desired pulses. In other words, the signal processor 113 acts as a pulse picker. According to the embodiment, the signal processor 113 is an acousto- optical modulator. The acousto -optical modulator 113 operates at a frequency of at least 100 MHz. Triggered with an electrical control signal, the acousto -optical modulator 113 allows the desired single or bursts of pulses to be transmitted through it and to be conse- quently amplified in the following amplifiers 114 and 121. With the electrical signal, the acousto-optical modulator 113 can be controlled to provide different shapes of pulse bursts, as an example equal, ascending or descending power levels within the pulse burst. Also the amount of pulses pi to p₃ in a burst is set using the electrical control signal of the acousto-optical modulator 113. The acousto-optical modulator 113 is connected to a second pre-amp lifter 114 which is configured to receive the processed signal from the acousto-optical modulator 1 13 and to amplify the selectively picked pulses. The second pre-amp lifter 114 is configured to provide amplification of 10 to 30 dB for the pulse sequence.

After the main unit 110 has produced and processed the signal, it is passed on to the laser head 120. More specifically the second pre-amp lifter 114 is connected to the main amplifier 121 of the laser head 120, wherein the main amplifier 121 of the laser head 120 is configured to provide amplification of 10 to 30 dB for amplifying the pulse sequence received from the main unit 1 10 to its final form. The amplified signal is then passed to the output optics 122 of the laser head 120. The output optics 122 is connected to the main amplifier 121 and configured to receive the amplified signal and to collimate and focus the beam to the semiconductor substrate 10 (cf. Fig. 1). More precisely, the semiconductor substrate 10 has two surfaces 11, 12 at a distance from each other. This distance defines the thickness of the substrate 10, whereby the laser 20 is focused in between said surfaces 11, 12 of the substrate 10. TABLE 1 : LIST OF REFERENCE NUMBERS.

Number Part

10 substrate

11 first surface

12 second surface

20 laser beam

21 focus point

100 laser welding apparatus

110 main unit

111 master oscillator

112 first pre-amplifier

113 pulse picker

114 second pre-amplifier

120 laser head

121 main amplifier

122 output optics

Claims

1. Method for welding a semiconductor substrate (10) with a laser (20) having a wavelength which is transparent to the material of the semiconductor substrate (10), in which method the laser (20) is transmitted in at least one series of cyclic pulses (pi ... P9), characterized in:

- first transmitting a first series of pulses (bi) comprising at least two successive pulses (pi, p₂, p₃) occurring with substantially equal time intervals (ii, i₂) of a first magnitude, and

- subsequently transmitting a second series of pulses (b₂) comprising at least two successive pulses (p₄, p₅, p₆) occurring with substantially equal time intervals ( , 15) of the first magnitude,

wherein the last pulse (p₃) of a previous series of pulses (bi) and the first pulse ( ) of a subsequent second series of pulses (b₂) occur with a time interval (i₃) of a second magnitude, which is unequal of at least one order of magnitude in respect to first magnitude thus establishing a burst sequence of laser pulses.

2. Method according to claim 1, wherein the second magnitude is greater than the first magnitude of at least one order of magnitude.

3. Method according to claim 1 or 2, wherein the semiconductor substrate (10) has two surfaces (11 , 12) at a distance from each other, the distance defining the thickness of the substrate (10), wherein the laser (20) is focused in between said surfaces (11, 12) of the substrate (10).

4. Method according to any of the preceding claims, wherein the first magnitude is in the range of 5 to 50 ns.

5. Method according to any of claims 1 to 3, wherein the first magnitude is in the range of 10 to 35 ns.

6. Method according to any of the preceding claims, wherein the second magnitude is in the range of 50 ns ... 50 ms.

7. Method according to any of claims 1 to 5, wherein second magnitude is in the range of 100 ns ... 20 ms.

8. Method according to any of the preceding claims, wherein the pulses (pi to p₆) have duration of 20 to 100 ps.

9. Method according to any of the preceding claims, wherein the successive pulses (pi to p3, p4 to p₆) of the series of pulses (bi, b₂) are transmitted at substantially similar level of power.

10. Method according to any of claims 1 to 8, wherein the intensity of the pulses (pi to p₃) within the series of pulses (bi) decreases with each pulse.

11. Apparatus (100) for welding a semiconductor substrate (10) with a laser (20), comprising:

- a laser pulse source (111),

- a signal processor connected to the laser pulse source (111) and configured to modify the produced pulse sequences into sequences,

- a laser head (120) connected to the laser pulse source (111) and configured to transform received signal pulse sequences into laser pulse sequences, and

- output optics (122) connected to the laser head (120) and adapted to colli- mate and focus the received the laser pulses into a welding beam, characterized in that the signal processor is an electrically controlled acousto- optical modulator (113).

12. Apparatus (100) according to claim 11, wherein the source of laser pulses (pi to p₆) is an oscillator (111) for producing laser pulses.

13. Apparatus (100) according to claim 11 or 12, wherein the acousto-optical modulator (113) is configured to produce a sequenced series of pulses (pi to p₆) as defined in claim 1.